3d analytic model for testing numerical tidal models a 3d analytic solution is presented for tides in channels with arbitrary lateral depth variation the solution is valid for narrow channels in which the lateral variation of the amplitude of tidal elevation is small the error introduced by the solution is on the order of a few percent in a tidal channel of a few kilometers in width the solution allows an arbitrary lateral depth variation and thus provides a wide choice of depth functions especially those with large bottom slopes the largest amplitude of the along estuary velocity appears on the surface in the deepest water the depth averaged velocity is the largest in the deepest water the time of flood ebb in deep water lags that in shallow water the time of flood ebb on the surface lags that at the bottom since this solution is simple and allows arbitrary lateral depth variations it can be used to demonstrate the first order tidal flow in narrow tidal channels of variable depth and to test high resolution numerical models with large depth gradients

on the movement and low level structure of cold fronts abstract this paper presents a review of theoretical and observational studies relating to the low level structure of cold fronts and explores the factors that are pertinent to frontal motion observational studies have shown that in some cases surface cold fronts move at speeds faster than the normal component of the wind at all levels in the lower troposphere and therefore propagate other case studies have shown that the low level flow immediately behind the front and normal to it is faster than the front and that the front has the local structure of a gravity current its speed of movement being well determined by the gravity current speed equation them different types of behavior are related to results of recent theoretical studies and the mechanism by which fronts can propagate is elucidated it is shown that a necessary requirement for propagation is the existence of an alongfront temperature gradient we question the relevance of the gravity current speed equation in general despite its appar

seasonal climatic forcing of alpine glaciers revealed with orbital synthetic aperture radar the evolution of four dynamic radar glacier zones at the surface of an alpine icefield in british columbia is monitored using a time series of 35 first european remote sensing satellite ers 1 synthetic aperture radar sar images acquired from 1992 to 1994 these zones result from changing wetness and textural properties and appear to represent 1 cold snow with no liquid water present 2 an initial melt front with an upper boundary near the elevation of the 0 isotherm 3 metamorphosed rapidly melting first year snow with a rough or pitted surface and 4 bare ice this interpretation is aided by temperature and runoff data air photographs and field measurements of snowpack properties acquired simultaneously with two ers i sar scenes ice surface elevations derived from 1 50 000 topographic maps and simulations of radar backscatter from a geometric optics model of surface scattering meltwater production is affected by the development of zones 2 3 and 4 which form migrate up elevation and disappear each year between april and september

the role of snow sublimation in frontogenesis recent work on the influence of moist processes at synoptic fronts has shown how condensation heating can be a strong and even a dominant factor in their development at mesoscale lines of convection the heating is again dominant but it is thought that cooling by the evaporation of precipitation is also a crucial factor in their development and propagation although most simple models of synoptic fronts do not attempt to simulate diabatic cooling accurately data from the fronts 87 experiment have identified evidence of a strong mesoscale downdraught below a synoptic frontal surface there has been speculation that such a downdraught is forced by the sublimation of falling snow since snow is more efficient at cooling the atmosphere for a given precipitation rate than evaporation of rain rainfall parametrizations are likely to underestimate the influence of such cooling in this paper a suitable parametrization of diabatic cooling based on the assumption of small stability to slantwise convection is used to simulate a band of snow sublimation in developing fronts the semi geostrophic equations are employed in the study of fronts which are formed by deformation or horizontal shear of the geostrophic winds it is found that the cooling has little influence on the large scales of motion and does not greatly accelerate the formation of frontal singularity however the cross frontal flows in the vicinity of the cooling are modified strongly with the formation of a narrow downdraught of comparable dimensions and intensity to that suggested by the experimental data the cooling has a weak influence on all the fields at a larger distance from the front this is discussed in the context of the large scale or geostrophic response to lines of convection

500 2000 mhz brightness temperature spectra of the northwestern greenland ice sheet an ultra wideband radiometer has been developed to measure subsurface properties of the cryosphere including ice sheets and sea ice the radiometer measures brightness temperature spectra from 0 5 to 2 ghz using 12 channels each of which measures scene brightness temperatures over an 88 mhz bandwidth resolved into 0 24 mhz intervals the instrument was flown over northwestern greenland in september 2016 and acquired the first wideband low frequency brightness temperature spectra over the ice sheet and coastal region the results reveal strong spatial and spectral variations that correlate well with the physical properties of the surface encountered along the flight path which started over ocean then passed the rock near the coast and then up onto the ablation wet percolation and dry snow zones of the interior ice sheet in particular strong spectral responses in percolation and dry snow zones are observed and plausibly explained by varying the distribution of horizontal density layers and isolated icy bodies in the upper portion of the firn the success of the airborne deployment of the instrument and subsequent implementation of algorithms to limit radio frequency interference in unprotected bands is motivating continued airborne investigations as well as stimulating research into the feasibility of a spaceborne instrument

influence of strong gradients of wind and humidity on frontal ageostrophic circulations the impact is studied of strong gradients of wind and humidity on the ageostrophic circulation developed around a conceptual cold frontal system the simulated basic wind and temperature fields are chosen to best represent the frontal observations made during the fronts 87 field experiment and in particular to take account of the presence of strong low level jets the numerical solution of the diagnostic sawyer eliassen equation in both its adiabatic and diabatic forms permitted us to test the impact of the wind and humidity structure on the corresponding ageostrophic circulation in particular strong negative shear above the low level jet core enhanced subsiding motions on the warm side of the front which restricted the horizontal extent of the cloud cover on the other hand the presence of well marked horizontal humidity gradients aloft at the rear edge of the cloud cover enhanced the ascending motions ahead of the frontal surface creating locally a secondary maximum of upward motions the degree of stability within the prefrontal boundary layer was found to be of major importance with regard to the intensification of the upright convection near the leading edge of the front moreover the incorporation of a parametrization scheme representing the ekman pumping at the top of the boundary layer enhanced the diagnosed circulation ahead of the front a comparison with observations made during two case studies of fronts 87 seems to confirm these results

coastal lee waves on ers 1 sar images alternative dark bright patterns on ers 1 synthetic aperture radar sar images of the west side of the taiwan strait taken on december 8 1994 were recognized to be the sea surface signature of a coastal lee wave such waves are called coastal lee waves because they occur along the lee side of the coast the coastal lee waves appeared in the form of a wave packet distributed within an offshore band 20 40 km wide the first packet which occurred in the northern portion of the observed area contained six waves with variable wavelengths defined as the spatial separation between two waves from 1 7 to 2 7 km the second packet in the middle contained 10 waves with a relatively uniform wavelength of 4 2 km the third packet in the southern portion contained 17 waves with an average wavelength of 2 0 km the crest lengths were from 20 to 80 km local meteorologic parameters observed simultaneously at fuzhou china close to the imaged area showed an offshore wind of 1 5 3 5 m s and a land surface air temperature of 19 c which was 4 c lower than the sea surface temperature sst thus the lower atmospheric boundary conditions at imaging time were very favorable both for generating the land breeze circulation and small wind waves on the sea surface which are in the bragg scattering wavelength band of the c band ers 1 sar a physical model of a three layer atmosphere was developed in order to explain how the land breeze circulation can generate the coastal lee waves the results showed that the vertical velocity disturbance caused by the wind convergence at the land breeze frontal zone is of vital importance for the generation of coastal lee waves and the model gave very good estimates of the processes observed the sar imaging mechanisms of the waves were analyzed in detail the differences between coastal lee waves and ocean internal waves which appear as similar alternative dark bright patterns on sar images were also discussed

multifrequency polarimetric synthetic aperture radar observations of sea ice the first known fully polarimetric airborne synthetic aperture radar sar data set of sea ice is introduced images were acquired in the beaufort bering and chukchi seas in march 1988 during a campaign for validation of defense meteorological satellite program special sensor microwave imager radiometer ice products statistics of the magnitude phase and polarization of complex backscattered signals recorded by the jet propulsion laboratory three frequency sar are examined in detail for various scenes with different ice characteristics the full stokes matrix information generated from c l and p band data characterize the scattering behavior of different ice types polarization ratios and phase differences between linear copolarized returns are used for discrimination between particular image features and mechanisms are suggested for the observed polarimetric characteristics results indicate that combinations of frequency and polarization enhance current capability to distinguish ice of different properties using single frequency fixed polarization micro wave radar a specific example is the polarimetric identification of new ice formation which may not be easily distinguished in ers 1 5 3ghz vv polarization sar data such findings are consistent with theoretical model simulations of scattering characteristics of sea ice overall these preliminary results demonstrate that radar polarimetry will likely add a new dimension to our current capability for extracting geophysically important ice variables using radar remote sensing methods

importance of diabatic processes on ageostrophic circulations observed during the fronts 87 experiment the impact of condensation and evaporation on real cross frontal circulations is studied using data from the fronts 87 field experiment the sawyer eliassen equation is solved numerically in its primitive equation form for one intensive observing period of fronts 87 three different schemes of parametrization of condensation heating are used comparison between diagnosed and observed ageostrophic circulations shows that latent heat release plays an important role in enhancing the ascending branch of the ageostrophic circulation ahead of the frontal zone in particular when a parametrization scheme for the low level moisture convergence is applied a cell of strong ascending motion is formed ahead of the frontal zone in good agreement with the observations further a simple parametrization of evaporative cooling below the frontal discontinuity leads to a reinforcement of downdraughts in the cold sector of the front as observed in regions where precipitation originating from the strong line convection evaporates in comparison with their adiabatic counterparts the current solutions show strong similarities with the observed circulation and indicate the predominance of diabatic effects in the frontal zone

observational evidence of slantwise convective adjustment abstract attempts were made during two field experiments to fly instrumented aircraft along absolute momentum m surfaces as a means of accurately determining slantwise convective stability the application of this technique appears to have been quite successful we present the results of four such efforts conducted in the ascent regions of midlatitude cyclones observed during the new england winter storms and genesis of atlantic lows experiments in three of the four cases the atmosphere was almost exactly neutral to slantwise ascent while being quite stable to vertical displacements in the fourth case the atmosphere departed from neutrality but was also substantially drier evidently due to subsidence we find excellent agreement between assessments of stability based on the m surface flights and on cross sections constructed from rawinsonde observations on the basis of these results i hypothesize that slantwise convective neutrality is characteristic of the ascent regions of baroclinic cyclones and

an investigation of the influence of latent heat release on cold frontal motion abstract the effects of condensational heating on cold frontal translation speed are explored through the use of potential vorticity pv diagnostics and model sensitivity experiments it is hypothesized that condensational heating can lead to faster frontal translation speeds in the presence of vertical shear because of the horizontal propagation of the positive pv anomaly associated with the front a case study of a cold front with an evolving precipitation structure is presented a positive correlation existed between the position of condensational heating relative to the frontal zone and frontal translation speed with faster frontal movement occurring when condensational heating was present in the prefrontal zone this front was numerically simulated to see if the hypothesized mechanism for frontal movement was active through the use of a pv budget it was confirmed that condensational heating did contribute to the forward propagation of the cold frontal pv band numerical experiments were performed

a preliminary investigation of the influence of basal and surface topography on supraglacial lake distribution near jakobshavn isbrae western greenland the spatial distribution of supraglacial lakes has been hypothesized to be dominantly controlled by the component of surface roughness influenced by basal topography basal topography and surface roughness profiles within the jakobshavn isbrae drainage basin in western greenland acquired from an ice penetrating radar echo sounder were analysed through fourier and wavelet decompositions spectral analyses of basal to surface transfer under a range of ice thickness were compared with spatial distribution of lakes mapped from high resolution landsat imagery fourier analysis identifies dominant signals in both the basal and surface profiles to be ranging between wavelengths of 1 25 12 5 km the strongest peaks of transfer of basal signals to the surface were identified at wavelengths 11 5 km wavelet analysis identifies these peaks with thicker 1200 1400 m and thinner 500 700 m ice respectively and also identifies surface frequencies not present in the basal signal indicating some influence from other factors spatial autocorrelation analysis of supraglacial lake distribution indentifies high correlations at 1 9 5 6 11 24 and 30 km lags lags at 5 6 and 11 km correspond to dominant frequencies present in the basal and surface profiles over thinner and thicker ice respectively these 5 and 11 km frequency components present in the basal topography surface roughness and lake distribution are within the theoretical transfer window of basal to surface transfer as a function of ice thickness the transect analysed in this study does not contain wavelengths in the surface structure that are less than the ice thickness even though there are relatively higher frequencies present in the surface profile between 0 and 15 km that have less power in the basal profile copyright 2011 john wiley sons ltd

the parcel method in a baroclinic atmosphere abstract the parcel method of investigating the susceptibility of the atmosphere to particular disturbances involves postulating parcel displacements in an undisturbed environment and deducing the likelihood of such a displacement by estimating the kinetic energy change that would result on the convective scale such a method has provided realistic and practical criteria for the existence and intensity of such motions on the larger scale theoreticians tend to reject such arguments as heuristic at best this paper is an attempt to reconsider the role of parcel theory on different length scales and in particular in the application of symmetric instability ideas to frontal rainbands it is concluded that parcel theory has to be applied with care but that in some situations it can provide a useful though partial insight into atmospheric disturbances

airborne doppler radar and sounding analysis of an oceanic cold front abstract an analysis of a cold front over the eastern atlantic ocean based on airborne doppler wind syntheses and dropsonde data is presented the focus and unique aspect of this study is a segment of the front that was near the center of the cyclone the dual doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of 30 km covers a total distance of 450 km in the cross frontal direction the finescale resolution and areal coverage of the dataset are believed to be unprecedented the cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient the latter was most intense aloft and not at the surface in contrast to the classical paradigm of surface cold fronts the shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone this observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface the surface convergence within the frontal zon

simulations of the potential vorticity structure and budget of fronts 87 iop8 dropsonde observations in the cold fronts sampled during the joint british french fronts 87 experiment along the west coast of europe have shown that the dry and moist potential vorticity fields q d and q e respectively have significant anomalies on the mesoscale the processes associated with these anomalies during a 12 h period encompassing the observation time have been examined by simulating the q d and q e structures observed near the cold front of iop8 using the meteo france mesoscale peridot model and by developing and using an eulerian potential vorticity budget module high resolution dropsonde rawinsonde and doppler radar observations are used as verification of the simulated potential vorticity structures initial and boundary conditions are obtained from three different analysis systems the simulated potential vorticity structures are sensitive to the initial conditions because the analyses over the data sparse atlantic ocean vary significantly from one analysis system to another of the seven persistent mesoscale potential vorticity features identified in the observations five were simulated reasonably well using at least one of the initial analyses and examined with the potential vorticity budget the low level pre frontal positive q d anomaly along the length of the cold front is maintained through a balance between lagrangian source sink terms dominated by differential diabatic heating and vertical advection within the main frontal updraught this balance maintains the q d anomaly in a region of near zero q d tendency between areas of q d generation near the surface and q d destruction in the middle troposphere a mid tropospheric region of negative q e directly behind the surface cold front appears to be produced by a weak but persistent generation by the solenoid term in the region of strong humidity gradients along the back edge of the mid tropospheric cloud shield this position and the model evolution suggest that the solenoid created region of negative q e can lead to enhanced mid tropospheric updraughts through conditional symmetric instability or a combination of conditional symmetric instability and large scale forcing the magnitudes of regions of negative q d and q e on the anticyclonic side of the upper level jet decrease throughout the simulation due to horizontal momentum diffusion while a post frontal low level negative q e region is generated principally by temperature and moisture diffusion

mesoscale dynamics of a deepening secondary cyclone in fastex iop16 three dimensional structure retrieved from dropsonde data the mesoscale dynamics of a secondary cyclone sampled during the field phase of the fronts and atlantic storm track experiment fastex intensive observation period iop 16 are documented using airborne dropsonde data the method proposed to recover the mesoscale analytical three dimensional 3 d dynamic fields from the dropsonde measurements is evaluated using simulated fields airborne in situ measurements and airborne doppler radar measurements these simulations and comparisons indicate that the method is able to resolve the mesoscale structures within the studied secondary cyclone both in the regions of clear air and precipitation it appears however that the most accurate description of the 3 d dynamic fields in both these regions would be obtained by combining doppler and dropsonde measurements the studied secondary cyclone is a fast moving frontal wave that developed on the trailing cold front of a parent low situated over greenland using the dropsonde derived 3 d fields the mesoscale flows involved in the organization of the secondary cyclone are described the potential vorticity and equivalent potential vorticity fields deduced from the basic analytical fields indicate that the atmosphere is roughly neutral to moist slantwise instability this configuration is known to significantly promote frontogenesis the vertical configuration of the jets and circulations within this secondary cyclone are compared with the coupling uncoupling scheme of shapiro a slight coupling is found between the thermally indirect circulation and the ageostrophic circulation the obtained coupling is however not as developed as in the ideal case described by shapiro which may explain the small upward velocities in our case airborne doppler radar and dropsonde measurements are combined so as to infer the multi scale processes involved in the mesoscale and convective scale organization of the atlantic secondary cyclones and to evaluate the new emerging theoretical interpretations of secondary cyclogenesis

estuarine plume a case study by satellite sar observations and in situ measurements estuary plumes on the western louisiana continental shelf in the gulf of mexico outside of constricted channels of the sabine and calcasieu lakes were observed on two radarsat 1 synthetic aperture radar sar images the in situ data showed a change in salinity of 2 3 psus across the front on the northwest edge of the plume corresponding to locations of vertical salinity variation of 5 5 7 5 psu velocity magnitude outside of the inlet reached more than 2 3 ms mathrm 1 seaward and a brackish lens in the upper 4 5 m of the water column the spread of which contributed to the development of a plume the volume of brackish water within the plume was estimated an order of magnitude larger than the volume output from the calcasieu river within an entire ebb tidal period this implies that the plume was formed during multiple tidal cycles and it sustained during the flood tides because only weak diurnal tides exist both satellite and in situ observations showed an interesting western intensification of the plume an analysis to the momentum vorticity equations suggests that the western intensification is due to the asymmetry of the vorticity divergence relationship under the coriolis force this can also be enhanced by an ambient coastal current that strengthens the convergence at the front estuary plumes are shown as bright and dark features in two sar images results from a radar model simulation and in situ water temperature data showed that the sar observed plume features were dominated by the water temperature differences

analyses of internal solitary waves generated at the columbia river plume front using sar imagery 1 internal solitons generated at the columbia river plume front during ebb tide are displayed on a synthetic aperture radar sar image taken on 26 july 2004 at 14 24 22 utc scale analyses suggest that these internal solitons belong to the finite depth category a theoretical model relating the radar backscatter cross section to dynamic parameters of internal solitons in the continuous stratification is developed using the model and background stratification data collected by the river influences on shelf ecosystem rise project in july 2004 we extract the soliton dynamic parameters of the half width amplitude phase speed and average energy flux per crest length along a typical section across the internal solitons the observed stratification is simulated by using a theoretical model parameterized by pycnocline depth thickness and maximum buoyancy frequency the half width amplitude phase speed and energy flux of the leading soliton at the section are 119 3 m 9 3 m 0 69 m s 1 and 417 4 w m 1 with the uncertainty ranges of 14 7 m 2 6 m 0 10 m s 1 and 281 6 w m 1 respectively sensitivity analyses reveal that the extraction of the soliton half width from the sar image is not affected by the stratification the amplitude is less influenced by the pycnocline thickness than the depth and is independent of the maximum buoyancy frequency

sub mesoscale ocean vortex trains in the luzon strait 1 this study uses the argos satellite tracked surface drifter trajectory data and envisat european satellite asar advanced synthetic aperture radar images to illustrate the ocean vortex trains ovt in the luzon strait two cases that occurred in the northwest of babuyan island are observed the first train of three cyclonic vortices showed up on drifter trajectories from 20 to 20 5 n and from 120 to 121 e and the second consisting of five pairs of cyclonic anticyclonic vortices occurred on the upstream side of the first one from 19 5 to 20 0 n and from 121 0 to 122 0 e and showed up on the asar images acquired on 19 november 2006 the total length of the vortex train axis reaches about 250 km all vortices propagate northwestward 315 tn the mean angular velocity is 2 07 0 18 10 5 s 1 theoretical models of ocean vortex radar image derived from radar imaging theories are used to extract dynamical parameters from asar imagery signatures which include the distance between two consecutive vortices and that between two rows of vortices of 22 6 1 9 km and 8 2 1 2 km respectively the maximum rotational velocity radius as 4 70 km and the vortex rotational angular velocity 3 10 5s 1 dynamical analyses give the mean velocity of the current of 0 65 ms 1 and the propagation velocity of the vortex 0 58 ms 1 the vortex shedding rate is estimated as 2 57 10 5 s 1 the reynolds number is estimated as 50 to 500 for the individual vortex and the vortex train the rossby numbers are o 0 4 and o 0 5 respectively implying that both vortex and vortex train observed in the luzon strait have a sub mesoscale nature this study also reveals a strong current with an average surface current velocity of around 0 7 ms 1 and the direction of around 315 tn flowing directly from the pacific to scs passing through the southern luzon strait the mean flow velocity can be calculated using methods developed in this study and ovt dynamical models this information may provide more insight to the circulation systems in the area including the origin of kuroshio

a new look at the super outbreak of tornadoes on 3 4 april 1974 abstract the outbreak of tornadoes from the mississippi river to just east of the appalachian mountains on 2 5 april 1974 is analyzed using conventional techniques and the pennsylvania state university national center for atmospheric research fifth generation mesoscale model mm5 the mm5 was run for 48 h using the ncep ncar reanalysis dataset for initial conditions it is suggested that the first damaging squall line within the storm of 2 5 april 1974 herein referred to as the super outbreak storm was initiated by updrafts associated with an undular bore the bore resulted from the forward advance of a pacific cold front into a stable air mass the second major squall line within the super outbreak storm which produced the strongest and most numerous tornadoes was directly connected with the lifting associated with a cold front aloft this second squall line was located along the farthest forward protrusion of a pacific cold front as it occluded with a lee trough dryline an important factor in the

satellite synthetic aperture radar detection of delaware bay plumes jet like feature analysis current structure sech 2 h constitutes a determining factor for the radar image the modeled image intensity depends on the axial velocity decreasing as x 1 along the jet axis and is sensitive to the wind direction with respect to the jet axis the model was used to interpret synthetic aperture radar sar images of delaware bay plumes taken by the radarsat 1 canadian radar satellite and ers 2 european remote sensing satellite satellites during a period from summer 1996 to spring 1997 in all seasons the low salinity plumes appear as relatively bright jet like patterns on sar images from a morphological interpretation of the image features both summer and winter plumes can be divided into three sections along the axis source jet and dispersion region along the transverse direction sar image interpretations indicate that the plumes have a twin jet structure which conforms to decomposition of field measurements in summer the typical axial velocity is estimated at 6 10 1 ms 1 with a reynolds number of 18 at about 5 km downstream from the source the plume behaves like a turbulent jet and beyond that range down to about 10 km downstream it behaves like a laminar jet in winter the plumes become weaker than in summer the typical axial velocity is estimated at 4 10 1 ms 1 with a reynolds number of 3 the jet behaves like the laminar jet off the source down to about 4 km downstream in both summer and winter cases the sar images were taken at maximum flood tide and the plumes appeared as jets in the spring case the sar image was taken at early flood tide the plume appeared as an integrated body with relatively uniform bright tunes in all cases the plume disperses within about 25 km downstream index terms 4235 oceanography general estuarine processes 4528 oceanography physical fronts and jets 4546 oceanography physical nearshore processes 4568 oceanography physical turbulence diffusion and mixing processes keywords delaware bay estuarine plume radar image

greenland ice sheet surface properties and ice dynamics from ers 1 sar imagery c band synthetic aperture radar sar imagery from the european space agency s ers 1 satellite reveals the basic zonation of the surface of the greenland ice sheet the zones have backscatter signatures related to the structure of the snowpack which varies with the balance of accumulation and melt at various elevations the boundaries of zones can be accurately located with the use of this high resolution imagery the images also reveal a large flow feature in northeast greenland that is similar to ice streams in antarctica and may play a major role in the discharge of ice from the ice sheet

dispersal of the pearl river plume over continental shelf in summer abstract satellite images of turbidity were used to study the climatological monthly and typical snapshot distributions of the pearl river plume over the shelf in summer from 2003 to 2016 these images show that the plume spreads offshore over the eastern shelf and is trapped near the coast over the western shelf eastward extension of the plume retreats from june to august monthly spatial variations of the plume are characterized by eastward spreading westward spreading or both time series of monthly plume area was quantified by applying the k mean clustering method to identify the turbid plume water decomposition of the 14 year monthly turbidity data by the empirical orthogonal function eof analysis isolated the 1st mode in both the eastward and westward spreading pattern as the time series closely related to the pearl river discharge and the 2nd mode with out of phase turbidity anomalies over the eastern and western shelves that is associated with the prevailing wind direction eight typical plume types were detected from the satellite snapshots they are characterized by coastal jet eastward offshore spreading westward spreading bidirectional spreading bulge isolated patch offshore branch and offshore filaments respectively their possible mechanisms are discussed

modeling multifrequency pol insar data from the percolation zone of the greenland ice sheet the analysis of data from an airborne synthetic aperture radar sar campaign in the percolation zone of greenland revealed an interferometric coherence undulation behavior with respect to vertical wavenumber which cannot be explained with existing models we propose a model extension that accounts for scattering from distinct layers below the surface simulations show that the periodicity of the coherence undulation is mainly driven by the vertical distance between dominant subsurface layers while the amplitude of the undulation is determined by the ratio between scattering from distinct layers and scattering from the firn volume we use the model to interpret quad pol sar data at x c s l and p bands the inferred layer depths match layer detections in ground based radar data and in situ measurements we conclude that in the percolation zone scattering from subsurface layers has to be taken into account to correctly interpret sar data and demonstrate the potential to retrieve geophysical information about the vertical subsurface structure

a wide cold frontal rainband and its relationship to frontal topography a wide cold frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high resolution doppler radar it was found that the movement of the rainband was closely related to the velocity of the frontal surface and that the substructural features of the rainband corresponded closely with the topographical features of that surface the evolution of the topography of the frontal surface is described and some processes that might affect frontal topography and therefore the structure and movement of rainbands are examined

a three dimensional model of airflow and orographic rainfall enhancement a three dimensional model of airflow over hills has been used in conjunction with a two dimensional model of orographic rainfall enhancement this development has allowed the accurate production of rainfall contour maps in areas of complex terrain a comparison has been made using a variety of atmospheric stability profiles between the wind fields and rainfall distributions over a bell shaped hill and those over a two dimensional ridge of similar height the results show that in conditions of either stably stratified air or with an inversion layer aloft three dimensional airflow effects can lead to much reduced rainfall over a bell shaped hill it is shown that in such situations the use of a two dimensional approximation can introduce significant errors an example is presented of the rainfall distribution in complex hill terrain over south kintyre on the western coast of scotland

sar imaging and hydrodynamic analysis of ocean bottom topographic waves 1 the satellite synthetic aperture radar sar images display wave like patterns of the ocean bottom topographic features at the south outlet of taiwan strait ts field measurements indicate that the most ts water body is vertically stratified however sar imaging models available were developed for homogeneous waters hence explaining sar imaging mechanisms of bottom features in a stratified ocean is beyond the scope of those models in order to explore these mechanisms and to determine the quantitative relations between the sar imagery and the bottom features a two dimensional three layer ocean model with sinusoidal bottom topographic features is developed analytical solutions and inferences of the momentum equations of the ocean model lead to the following conditions 1 in the lower layer the topography induced waves topographic waves hereafter exist in the form of stationary waves which satisfy a lower boundary resonance condition kc0 here is an angular frequency of the stationary waves k is a wavenumber of bottom topographic corrugation and c0 is a background current speed 2 as internal waves the topographic waves may propagate vertically to the upper layer with an unchanged wavenumber k if a frequency relation n3 n2 is satisfied here n2 and n3 are the brunt waisala frequencies of middle layer and upper layer respectively 3 the topographic waves are extremely amplified if an upper layer resonance condition is satisfied the sar image of topographic waves is derived on the basis of current modulated small wave spectra the results indicate that the topographic waves on sar images have the same wavelength of bottom topographic corrugation and the imagery brightness peaks are either inphase or antiphase with respect to the topographic corrugation depending on a sign of a coupling factor these theoretical predictions are verified by field observations the results of this study provide a physical basis for quantitative interpretation of sar images of bottom topographic waves in the stratified ocean

low level jet streams associated with atmospheric cold fronts seven case studies from the fronts 87 experiment sounding data collected during the european experiment fronts87 are used to investigate the dynamic structure of mid latitude low level jets associated with atmospheric cold fronts these jets are identified as strong currents of relative warm and moist air confined in the lower layers of the atmosphere within the first two kilometers ahead of cold fronts the importance of the low level jets lies on the fact that they can be responsible for the occurrence of strong winds near the surface which sometimes can be disastrous especially near coasts where frictional deceleration due to the land is minimum furthermore they are often associated with a narrow region of ascending motions just ahead of the frontal surface which can account for strong precipitation at ground the data to be discussed in this paper are provided from the special upper air sounding stations deployed at brittany france during fronts87 a comparison of seven case studies permitted to give a detailed picture of the jets characteristics and to better understand their dynamic structure

frontogenesis at the boundary between air masses of different potential vorticity theoretical models of frontogenesis using the semi geostrophic approximation have imagined the process to occur between air masses whose only difference is their respective surface potential temperature this is equivalent to assuming the ertel potential vorticity pv to be constant in the troposphere and that there is a horizontal temperature gradient at the surface for many cases this is a good approximation and furthermore the constraint on the dynamics of such uniform pv is a fundamental one in some locations and synoptic situations however the air masses not only have a different surface potential temperature but also have significantly different static stability in this case the pv as well as the boundary potential temperature has a horizontal variation here we take the variation in pv to be entirely due to the static stability holding f constant although in reality there is a contribution due to the variation of f with latitude examples of this type of frontogenesis are coastal fronts where the air over the ocean has a different stability to that over the land some mesoscale polar vortices which form at the ice margin and cold fronts with polar air moving southwards over a warmer ocean giving enhanced convection and hence lower stability results are presented from a two dimensional semi geostrophic model of frontogenesis between two rigid horizontal surfaces to illustrate the effects of variations in pv in many cases the conclusions can be deduced from the geometrical aspects of the problem if the cold air mass has the larger stability the frontogenesis at the lower boundary proceeds at an increased rate compared to that at the upper boundary indeed it is possible for a particular stability difference between the air masses to eliminate the upper frontogenesis completely producing a purely lower tropospheric phenomenon the geometrically converse conclusions apply if the cold air mass has the smaller stability the results also allow the role of the mean value of the pv or stability of the two air masses to be described

mapping of ice layer extent and snow accumulation in the percolation zone of the greenland ice sheet 1 the greenland ice sheet underwent record extensive melt in 2002 and prolonged melt in 2003 the severe melting created a significant and extensive ice layer over the greenland ice sheet an innovative approach is developed to detect the ice layer formation using data acquired by the seawinds scatterometer on the quikscat satellite quikscat backscatter together with in situ data from automatic weather stations of the greenland climate network are used to map the extent of ice layer formation the results reveal areas of extensive ice layer formed by the 2002 melt which is consistent with the maximum melt extent in 2002 moreover during freezing seasons quikscat data show a linear decrease in backscatter in decibels or db that is related to the amount of snow accumulation in the ice layer formation region this snow accumulation signature is caused by the attenuation of radar waves in the snow layer accumulating since the last major melt event whose thickness appears as an exponential function in relation to the backscatter signature we use the greenland climate network data to calibrate the quikscat accumulation rate in order to estimate and map snow accumulation quikscat results capture the extreme snowfall in mid april 2003 which deposited more than 0 5 m of snow in a day as measured by the automated weather station at the nasa south east site large scale quikscat results show an anomalous increase of snow accumulation over the southeast region of greenland during the 2002 2003 freezing season

laboratory measurements of sea ice connections to microwave remote sensing the connections between laboratory measurements and remote sensing observations of sea ice are explored the focus of this paper is on thin ice which is more easily simulated in a laboratory environment the authors summarize results of c band scatterometer measurements and discuss how they may help in the interpretation of remote sensing data they compare the measurements with observations of thin ice from ers and airborne radar data sets they suggest that laboratory backscatter signatures should serve as bounds on the interpretation of remote sensing data they examine these bounds from the perspective of thin ice signatures the effect of temperature and surface processes such as frost flowers and slush on these signatures controlled experiments also suggest new directions in remote sensing measurements the potential of polarimetric radar measurements in the retrieval of thickness of thin ice is discussed in addition to the radar results the authors discuss the importance of low frequency passive measurements with respect to the thickness of thin ice

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

passage of a tracer through frontal zones a model for the formation of forward sloping cold fronts it is shown that a forward sloping cold front may form in a simple two dimensional model of an atmospheric frontal zone through differential advection of dry and wet bulb potential temperature as such two frontal zones may coexist with opposing slopes this behaviour is manifest in the neutral eady edge wave which exhibits kata cold front behaviour while the unstable eady wave with an ana cold front does not generate a forward sloping cold front these results come about as a consequence of the wave like behaviour of the fronts in the model that is the ability of a front to propagate through the air locally as such they highlight the fact that a frontal zone need not be a barrier to airflow and that any air properties from water vapour to atmospheric pollution may pass through a front

a study of mesoscale convective bands behind cold fronts part i mesoscale organization this paper reports observations made over the north atlantic of a system of convective cloud bands found within the cold air behind an occluded front the observations were made from an instrumented aircraft which additionally released a pattern of dropsondes measuring temperature relative humidity and wind as functions of pressure these data are analysed to establish some of the mesoscale cloud scale and microphysical properties of the system here in part i the mesoscale structure is described and compared with the predictions of various theoretical models the convective cloud structure emerges as a consequence of the mesoscale organization but appears to play only a minor role in its creation and maintenance one of the theoretical models considered conditional symmetric instability provides a piausible explanation of the mesoscale observations

snow and ice facies variability and ice layer formation on canadian arctic ice caps 1999 2005 1 time series of enhanced resolution data from the seawinds scatterometer aboard quikscat were used to map the distribution of snow and ice surface facies and ice layer formation in the percolation zone on ice caps in the queen elizabeth islands during the period 1999 2005 iterative self organizing data analysis classification of the mean postfreeze up biweekly average 0 signal for the 7 year period resulted in the delineation of four snow and ice surface facies interpreted as the percolation saturation superimposed ice and glacier ice zones analysis with national centers for environmental prediction national center for atmospheric research reanalysis reveals that changes in geopotential height in the troposphere 700 500 and 300 hpa and air temperature 700 hpa level are positively negatively correlated with area changes in the glacier ice percolation and saturation zones and changes in facies boundary elevation the change in biweekly averaged backscatter following the freeze up periods between successive autumns was used to map changes in the distribution of ice layers formed by meltwater percolation and refreezing in the snowpack within the percolation zone strongly positive air temperature anomalies at the 700 hpa level in 2001 and 2005 are consistent with extensive ice layer formation in the percolation zones of all ice caps such large interannual changes in ice layer formation are likely associated with large changes to the density profile of the snowpack and may be associated with surface elevation changes that are unrelated to changes in surface mass balance

four dimensional variational analyses of fastex situations using special observations this paper focuses on intensive observation period 17 of the fronts and atlantic storm track experiment fastex the fastex special soundings obtained during that period are added to the conventional dataset and assimilated with a 4 dimensional variational 4d var scheme under development at meteo france results show a consistent use of the high density fastex ship soundings by the 4d var formulation and an improvement brought by this new scheme over 3d var when using only the vertical profiles synchronous with synoptic time the application of the 4d var analysis to dropsondes launched by a system relative flight designed by the joint centre for mesoscale meteorology also produces a fine scale description of sub structures of the mature system

airborne observations of a front near a col during fastex abstract an analysis of an oceanic front situated near a col defined by the surface pressure field is presented there have been few observational examples of this type of front presented in the literature the primary source of information for this study was data recorded by an aircraft equipped with a doppler radar the front was approximately two dimensional and the cross frontal scale at low levels was 30 40 km a prefrontal low level jet was identified in the high resolution analyses and was shown to be supergeostrophic surface pressure measurements and the horizontal temperature gradients were used to calculate the geostrophic wind and the thermal wind imbalance twi in the alongfront direction large negative values of twi the vertical shear is less than predicted for the given horizontal temperature gradient were located near a region of frontogenesis the strong ageostrophic component of the wind parallel to the front suggests that the alongfrontal component of the wind may not have been in g

quantifying vulnerability of antarctic ice shelves to hydrofracture using microwave scattering properties abstract recent ice shelf disintegrations on the antarctic peninsula and subsequent increases in ice sheet mass loss have highlighted the importance of tracking ice shelf stability with respect to surface melt ponding and hydrofracture in this study we use active microwave scatterometry in time series to estimate melt season duration and winter backscatter levels as a proxy for relative concentration of refrozen ice lenses in antarctic ice shelf firn we demonstrate a physical relationship between melt days and firn ice backscatter using scatterometry and field data from greenland and apply the observed relationship to derive and map a vulnerability index for antarctica s ice shelves the index reveals that some remaining antarctic peninsula ice shelves have already reached a firn state that is vulnerable to hydrofracture we also show that the progression of an ice shelf towards vulnerability is affected by many factors such as surface mass balance internal stresses and ice shelf geometry

the role of the environmental flow in the development of secondary frontal cyclones the impact of the environmental flow on the development of secondary frontal cyclones is investigated several case studies are examined as examples of secondary frontal cyclone events observed in the north atlantic western europe sector a simple measure of growth is defined to chart their development the vorticity attribution technique of bishop is utilized to calculate the action of the large scale environmental flow on the fronts in particular the environmental along front stretching shown to be important in theoretical models of frontal instabilities is calculated the role of the environmental deformation appears to be crucial as part of a baroclinic life cycle stretching deformation acts to build up a front but suppress along front waves if the stretching rate diminishes barotropic instabilities may then break out diagnostics are examined to try to ascertain the growth mechanisms at work in each frontal cyclone case a range of values for the commonly prescribed deformation frontogenesis and shearing frontogenesis parameters are calculated

scale interactions involved in the initiation structure and evolution of the 15 december 1992 mcs observed during toga coare part i synoptic scale processes this paper the first of a series examines the synoptic scale mechanisms involved in the initiation structure and evolution of a mesoscale convective system observed during toga coare this study relies upon the use of the japanese geosynchronous meteorological satellite 4 imagery and ecmwf model outputs from which diagnostic parameters are derived and interpreted this mesoscale convective system consists initially of two groups of convective entities that progressively move toward each other and merge it is shown that the synoptic scale flow creates a favorable environment for the formation of this large convective system through the production of convective available potential energy cape by horizontal advection and the enhancement of low level convergence in the region where the convective system formed moreover the general evolution of the system is found to be governed by the synoptic scale circulation and more precisely by the temporal evolution of cape and low level convergence the mechanisms leading to the initiation and general evolution of the system are examined the easterly equatorial jet at 500 hpa triggered positive potential vorticity areas that propagated westward and generated an anticyclonic circulation this anticyclonic circulation was enhanced during the development phase of the convective system through vortex stretching and tilting which accelerated the low level westerlies corresponding to the southern branch of this circulation and enhanced the low level convergence associated with the studied convective system in a companion paper the mesoscale and convective scale processes involved in the internal organization of this mesoscale convective system are examined using the airborne doppler radar dataset collected within the system from 1700 to 2100 utc the downscale interactions i e from synoptic scale to mesoscale and convective scale are scrutinized using both the synoptic scale context described in this paper and the mesoscale and convective scale characteristics derived from the airborne radar observations

warm and occluded fronts in two dimensional moist baroclinic instability the two dimensional eady mode of baroclinic instability exhibits a strong cold front structure after a period of development using semi geostrophic equations excluding moist processes indeed the surface wave has a single centre of cyclonic vorticity on the cold side of the warm air anomaly here results are presented from a semi geostrophic numerical model including moist effects which predicts an eady wave with a cold and a warm front associated with two centres of surface vorticity located on either side of the warm anomaly the ascent regions are taken to be saturated and close to moist neutral which in this baroclinic environment corresponds to a condition of zero equivalent potential vorticity the descent is taken to be unsaturated this development of a structure resembling a warm front occurs only when the moist effects become dominant within a pre existing weak dry eady mode using the same condensation representation starting from an extremely small and random amplitude disturbance a pure moist eady mode develops with a single vorticity maximum in the form of a cold front in nature it is likely that often the moisture becomes dominant after a period albeit sometimes a short period of essentially unsaturated growth although observational evidence is limited therefore the process of warm frontogenesis described here may be a contributory factor to those minority of mid latitude cyclones which exhibit a strong warm front the model indicates that the warm front moves more slowly than the cold front resulting in a cold occlusion process these investigations require a detailed examination of the initial growth of small amplitude disturbances near the limit of neutral moist ascent i e pure moist modes results of previous such calculations using an analytical 2 level model are confirmed here using a numerical multi level model the more accurate estimates obtained here show that the finite growth rate in the limit of zero equivalent potential vorticity has a value of 1 9 times the dry value compared with the 2 4 fold increase given by the 2 level model

secondary frontal waves in the north atlantic region a dynamical perspective of current ideas the current understanding of the dynamics of secondary cyclogenesis on frontal systems in the north atlantic region is reviewed these secondary cyclones are often poorly forecast and in extreme cases can have damaging consequences but because of their small scales and potential for rapid growth detailed observations and analyses are limited in this paper recent work is described and compared it is argued that on the synoptic scale the development of primary baroclinic waves is well understood but on the subscales of these systems where the secondaries develop there is a wider variety of mechanisms which may cause or modify cyclone growth this variety may be reflected in a wider range of resulting systems various dynamical models have been postulated secondary frontal waves are seen as the instability of a low level potential vorticity strip or warm band at a front with possible finite amplitude triggering by upper level features latent heating is thought to provide a suitable unstable potential vorticity strip but the degree to which the latent heating contributes to the instability itself is a matter of dissent secondary systems are often shallow so boundary layer processes are important but the influence of these is not well understood laboratory experiments have suggested that the boundary layer will act to suppress instability in general frontogenetic strain flow while acting to intensify the basic state front suppresses wave development in contrast recent work suggests that a frontolytic strain flow may be important in enhancing the development of one class of waves the fastex experiment an international field study of north atlantic secondary cyclogensis has recently attempted to observe these systems in detail it is hoped that the data which have been obtained will help to resolve some of the outstanding issues in this field

the extratropical transitions of hurricanes felix and iris in 1995 abstract the extratropical transitions of hurricanes felix and iris in 1995 are examined and compared both systems affected northwest europe but only iris developed significantly as an extratropical system in both cases the hurricane interacts with a preexisting extratropical system over the western atlantic the remnants of the exhurricanes can be identified and tracked across the atlantic as separate low level potential vorticity pv anomalies the nature of the baroclinic wave involved in the extratropical transition is described from a pv perspective and shown to differ significantly between the two cases the role of vertical shear in modifying the hurricane structure during the early phase of the transition is investigated iris moved into a region of strong shear the high pv tower of iris developed a marked downshear tilt felix moved into a vertically sheared environment also but the shear was weaker than for iris and the pv tower of felix did not tilt much iris maintained its warm core struc

mesoscale dynamics of cold fronts structures described by dropsoundings in fronts 87 the fronts 87 project was a european experiment to make mesoscale observations to deduce the dynamics of active cold fronts approaching north west europe with a minimum of orographic influences in this paper data from dropsoundings made by the meteorological office s c 130 research aircraft are analysed the aircraft flew a pattern with 4 or 5 runs oriented approximately at right angles to the front but with each run displaced in the along front direction by about 100 km the dropsondes gave soundings to a height of approximately 7 km with a cross frontal spacing of 20 km at best and 60 km on average between 30 and 50 soundings were made in each event over an area of 500 500 km2 these data are unique in their mesoscale resolution over a synoptic scale region this was made possible by the ability to track up to 5 sondes in the air at any time several new aspects of cold front dynamics have been identified from these data the approach here is to treat each run as an instantaneous cross frontal section the discussion is centred on the hypotheses postulated before the experiment these were intended to allow verification or refutation of aspects of models of frontal dynamics such as those embodied in semi geostrophic theory a minimal and purely objective analysis has been performed on the data presented here but the high resolution of the dropsonde observations permits evaluation of differentiated quantities such as potential vorticity pv with some confidence issues raised here include the degree of thermal wind balance the structure of conserved variables such as equivalent potential temperature and absolute momentum the cross frontal circulation and the role of diabatic processes such as the evaporation of snow and the potential vorticity structure on the mesoscale and its interpretation the intention is to present an integrated view of the dynamical structure of fronts in the light of theoretical rather than conceptual or airflow models

distinguishing the cold conveyor belt and sting jet airstreams in an intense extratropical cyclone abstractstrong winds equatorward and rearward of a cyclone core have often been associated with two phenomena the cold conveyor belt ccb jet and sting jets here detailed observations of the mesoscale structure in this region of an intense cyclone are analyzed the in situ and dropsonde observations were obtained during two research flights through the cyclone during the diabatic influences on mesoscale structures in extratropical storms diamet field campaign a numerical weather prediction model is used to link the strong wind regions with three types of airstreams or coherent ensembles of trajectories two types are identified with the ccb hooking around the cyclone center while the third is identified with a sting jet descending from the cloud head to the west of the cyclone chemical tracer observations show for the first time that the ccb and sting jet airstreams are distinct air masses even when the associated low level wind maxima are not spatially distinct in the model the ccb experie

observations of the mesoscale sub structure in the cold air of a developing frontal cyclone observations from 58 dropwindsondes released in a mesoscale array during the fronts 92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern north atlantic a conceptual model involving the intertwining of dry intrusion and cloud head flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour in the cold air two distinct dry intrusions were responsible for two cold fronts trailing south westwards from the tip of the cloud head both were surface features at the beginning of our study but the leading one evolved into an upper front with mid level convection as the dry intrusion responsible for it overran the warm sector areas of both high and low potential vorticity were indicated within the dry intrusions upon encountering a critical level of zero system relative velocity at the top of the moist boundary layer the dry intrusions arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front normal direction with a slope of typically 1 in 60 although most parts of the laminae were subsaturated their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud lines within the boundary layer these formed as an extension of the south western tip of the cloud head associated with the developing cyclone possible mechanisms for generating the observed structures are discussed

numerical simulations of maritime frontogenesis abstract a hydrostatic primitive equation model initialized in a highly baroclinically unstable state was used to simulate maritime cyclogenesis and frontogenesis in order to identify boundary layer physical processes important in maritime frontogenesis several different simulations were performed in an effort to isolate impacts due solely to the boundary layer moist processes were not included an adiabatic and inviscid simulation provided the control for these experiments two different boundary layer parameterizations were used a k theory parameterization featuring richardson number dependent eddy diffusivity and a second order closure scheme with prognostic equations for the turbulence quantities results indicated that strong warm and cold fronts formed in the adiabatic and inviscid case but that the vertical motion fields were weak in the k theory simulation the results were somewhat more realistic with stronger vertical motion in both the k theory and second order closure simulations the b

theoretical expression for an ocean internal soliton synthetic aperture radar image and determination of the soliton characteristic half width this paper deals with the development of techniques for satellite synthetic aperture radar sar ocean image interpretation we derived a theoretical model of a radar image for a korteweg de vries type ocean internal soliton and validated the model using ocean internal wave signals taken from ers 1 sar and radarsat sar images the results indicate that the model perfectly simulates ocean internal soliton signatures with double sign variations of radar backscatter on the basis of the model we developed the curve fitting method and the peak to peak method for determining the internal soliton characteristic half widths which then were used to calculate the internal soliton amplitudes the test results indicate that ocean internal soliton amplitudes derived by the two methods agree with in situ data acquired on the portuguese continental shelf and in the south china sea with reasonable accuracy the role that wind fields play in ocean radar remote sensing was also analyzed finally the modulation ratio of ocean internal waves on radar images was quantitatively estimated

dynamics of a low level jet observed during the fronts 87 experiment a study of the internal dynamics of a low level jet its role on the frontal discontinuity evolution and its relevance to the organization of convection at mesoscale and small scale is presented the low level jet studied is associated with an active cold front that was observed during the french british co operative field experiment fronts 87 the originality of the present low level jet description lies in the fact that it is observed not only by means of soundings but also by means of a new tool for mesoscale meteorology observations a wind profiler stratospheric tropospheric uhf radar the high resolution of the wind profiler data in both time and height allows one to assess in precipitating as well as in clear sky areas a detailed image of the low level jet and the associated mesoscale and convective scale circulations the comparison between observations and predictions of various theories provides information on the physical mechanisms responsible for the initiation and maintenance of the observed mesoscale features the flow resulting from the presence of the low level jet produces inertial acceleration and baroclinic symmetric instability which lead to a narrow mesoscale line of sloping motions moreover the interaction of the cold surface outflow with the low level vertical shear below the jet maximum is found to be crucial in the intensification and maintenance of the observed narrow cold frontal rainband

dynamical effects of ice sublimation in a frontal wave the dynamical role of ice sublimation in weather systems is briefly reviewed observations are presented from the fronts and atlantic storm track experiment fastex intensive observation period 16 that show variations of static stability humidity and mesoscale circulation corresponding to those associated theoretically with sublimation of ice precipitation it is thus suggested that the observations display the mechanism proposed by clough and franks in which forward mesoscale flows are associated with moist adiabatic descent supported by the sublimation cooling this mechanism was suggested as an important stage in the evolution of many mesoscale rain bands a set of three model simulations of the event has been made with versions of the mel office s unified model of these a mesoscale model integration with 11 km resolution and 45 levels clearly displays the symptoms and is diagnosed to demonstrate its consistency with the clough franks mechanism an integration omitting the cooling due to sublimation differs significantly from the full model experiment in the structure of low level wind fields frontal troughs and mesoscale precipitation distribution il is also demonstrated that the static stability transition mesoscale circulation and mid tropospheric potential vorticity perturbations are substantially weakened in this integration thus confirming that the clough franks mechanism is also operating in the numerical weather prediction nwp model we deduce from these studies that ice precipitation and its sublimation has a major role in determining mesoscale circulation and structure in mid latitude weather systems affecting stratification and the formation of features such as fronts and rain bands these are substantially affected by the fall and evaporation of ice crystals which are both important to temperature and moisture transports and the behaviour of nwp models on time scales of hours to days in our integrations dynamical feedback due to sublimation cooling coincided with extreme negative potential vorticity values and potentially conditional symmetric instability hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case in moist warm sectors a substantial role for sublimation may be anticipated more generally particularly for air trajectories receiving most ice precipitation we suggest that the described phenomenon be referred to as sublimation enhanced descent or sed it is concluded that in view of tins demonstrated sensitivity substantial attention should be given to refining microphysical parametrizations in nwp models and that radar and sounding observations from the fastex experiment provide a suitable basis for validating these schemes

boundary layer mean dynamic characteristics of cold fronts over brittany the european mesoscale frontal dynamic project fronts 87 lasted from 18 october 1987 to 13 january 1988 its aim was to study the dynamics of activ cold fronts observed over the area centred on the english channel between england and france in this paper the mean dynamic characteristics derived from the five intensive observation phases during which the acoustic sodar at brest was operating are presented observations of wind at ground level sodar wind in the surface and the boundary layers sodar friction velocity and radiosonde wind measurements are combined to determine the dynamics of a mean cold front in the boundary layer in spite of the important variability of the frontal phenomena mean frontal characteristics over brittany in autumn can be identified the analysis shows that for all the cases except for the dry cold front the variations of the dynamical parameters at the frontal passage are closely related variations of the friction velocity and the horizontal wind in the surface and the boundary layers are linearly related

dual doppler radar analysis of a midlatitude squall line with a trailing region of stratiform rain abstract the mesoscale structure of a squall line system that passed over oklahoma on 22 may 1976 is investigated by dual doppler radar analysis the mature storm consisted of a leading line of deep convection which exhibited organized multicellular structure trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level these contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels which has been termed the transition zone while conventional and single doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm the dual doppler analysis demonstrates the pervasiveness of these features over much of the squall line s length the structure of a midtropospheric maximum of rearward system relative flow crossing the system is particularly well described by the dual doppler data this mesoscale current originat

measurements of snow and glacier covered areas with single polarization sar climatological and hydrological investigations in mountainous regions frequently require delineation of the areas covered by snow and glacial ice active microwave sensors can discriminate snow and glacier from other targets are effective under all weather conditions and have a spatial resolution compatible with the topographic variation in alpine regions using data acquired with the nasa airsar which operates at three frequencies and four combinations of polarization we examine the usage of synthetic aperture radar sar to map snow and glacier covered areas in order to assess the available sars that operate from satellites we analyze single frequency single polarization data and we compare our results with images from the landsat thematic mapper obtained under clear conditions only a few days after the sar flight single polarization c band 5 3 ghz sar data can discriminate between areas covered by wet snow from those that are ice free but do not easily separate glacier ice from snow and rock l band sar 1 25 ghz data cannot discriminate between snow and glacier ice the accuracy 74 of sar is high enough to justify its use as the data source in areas that are too cloud covered to obtain reliable data from the thematic mapper

frontal instability generated by tropospheric potential vorticity anomalies the development of secondary disturbances on a two dimensional front is described using a linear stability analysis a two stage process is envisaged in which first strong frontogenesis produces condensation at the front this leads to the generation of a lower tropospheric zone of high potential vorticity pv lying along the front as frontogenesis weakens the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the pv anomaly or equivalently the frontal rainband thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day 1 the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of pv anomalies the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks useful insight is gained from an analysis of the energetics of the modes this shows that for narrow frontal pv anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux recent observations suggest that frontal waves are indeed characterized by such a heat flux as most frontal zones involve significant condensation and therefore are likely to exhibit a lower tropospheric pv maximum it is stressed that studies of frontal waves need to include this aspect of the frontal structure

non hydrostatic simulations of a cold front observed during the fronts 87 experiment a two dimensional non hydrostatic cloud model is used to simulate a frontal system observed on 11 november 1987 during the fronts 87 experiment the cloud resolving model is able to reproduce a large variety of frontal bands starting from only the meso scale fields produced by a hydrostatic forecast model a narrow cold frontal rainband is identified at the surface cold front two classes of wide bands are also simulated a wide cold frontal rainband and warm sector wide rainbands the vertical velocity fields exhibit similar banded features in both the simulations and the doppler radar observations with bands having the same position intensity and width several sensitivity experiments are presented one of them shows that the initial knowledge of the ageostrophic circulation is not necessary to give a successful simulation other experiments show that the along front baroclinicity drives the wide band formation whereas the along front deformation is not important the heat moisture and horizontal momentum budgets are also investigated in order to determine the rainband effects on larger scales

numerical simulations of an observed narrow cold frontal rainband abstract a cold frontal rainband which occurred during the afternoon of 28 december 1988 is numerically simulated using the penn state ncar three dimensional mm5 modeling system this case is characterized by a line of severe convection that has a gravity current like structure along the leading edge of a strong surface cold front the authors test whether this gravity current like structure is associated with the cold air outflow boundary generated by the evaporation of hydrometeors from the postfrontal precipitation system it is found that the neglect of moist processes in the model did not significantly affect the gravity current like structure of the front pbl planetary boundary layer frictional processes which produce cross frontal low level wind shear uz play an important role the role of this cross frontal low level wind shear is to generate low level convergence and to steepen the frontal slope thus the observed intense ncfr narrow cold frontal rainband is closely related to frictio

global climatologies of fronts airmass boundaries and airstream boundaries why the definition of front matters abstractclimatologies of fronts airmass boundaries and airstream boundaries can be calculated using automated approaches on gridded data such approaches may require choices to define a front in

frontogenesis in the presence of small stability to slantwise convection abstract it is often observed that despite the existence of near neutrality to slantwise convection rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases this is probably due to the presence of active frontogenesis which as shown here maintains such structures in order to investigate this process we use two dimensional semigeostrophic theory to solve the deformation forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive in this case numerical simulation is necessary to determine the evolution of the front several previous studies have described the role of diabatic forcing in modifying the cross frontal circulation by use of the sawyer eliassen equation in particular analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis numeri

vhf st radar observations of an upper level front using vertical and oblique beam c2n measurements abstract mesoscale aspects of an upper level front that moved over brittany france during the mesoscale frontal dynamical project 1987 experiment are investigated using very high frequency stratospheric tropospheric vhf st radar data and european centre for medium range weather forecasts ecmwf analyses the synoptic study shows that the upper level front moved without significant change over the radar during the observation period from 0000 utc 14 november 1987 to 0000 utc 15 november 1987 this allows radar data height time cross sections to be interpreted as representing the corresponding spatial data distributions a radar technique of stable airmass detection using the measurement of the aspect ratio that is the vertical to oblique power ratio and the turbulence that is the structure constant of the refractive index fluctuations c2n is developed this method is applied to the case study of the upper level frontal passage which allows the associated stable air masses above the radar site

airborne radar observations of a cold front during fastex1 abstract a detailed analysis using airborne doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented the precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core relative winds in contrast to previous schematic models it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation core the southern section of the core can be accompanied by an abrupt discontinuity and lighter precipitation while the northern section can be associated with more slowly changing variables but heavier precipitation the peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical vorticity in the boundary layer the overall motion of the cold front was not well predicted using density current theory even though the kinematic structure of the front resembled cla

interpretation of sar imagery of the greenland ice sheet using coregistered tm imagery abstract landsat thematic mapper tm images are coregistered with a seasat synthetic aperture radar sar image of the margin of the greenland ice sheet and used in the interpretation of features in the sar data a winter tm image is used to provide topographic information while a summer tm image provides information on the pattern of melt and the location of lakes many previous hypotheses are confirmed and some are rejected it is shown that 1 the snow line is easily d detectable in sar imagery 2 areas of higher radar backscatter in the bare ice zone most often correspond to areas full of crevasses too small to resolve in the sar image 3 subsurface scattering causes significantlblurring of features in sar imagery 4 the band of surface lakes occurring above the snow line corresponds to at least a portion of the wet snow zone 5 water is the cause of reduced radar backscatter above the snow line and 6 the fact that water causes reduced radar backscatter above the snow line permits the use of backscatter variations as a proxy indicator of surface topography in snow covered regions experiencing melt

sting jets in extratropical cyclones a review this paper reviews the current state of knowledge of sting jets in extratropical cyclones sting jets were formally identified in 2004 by the pioneering work of keith browning reviewing this and subsequent studies we define the sting jet as a coherent air flow that descends from mid levels inside the cloud head into the frontal fracture region of a shapiro keyser cyclone over a period of a few hours leading to a distinct region of near surface stronger winds it therefore lies above the cold conveyor belt during some stage its life but at least in some cases descends to reach the top of boundary layer ahead of the cold conveyor belt it is not attributed to a specific mechanism in this definition we conclude that it is likely that a continuum of sting jet descent and speed up mechanisms exists at one extreme is balanced descent partly associated with frontolysis in the frontal fracture region more horizontally small scale and stronger frontolytic descent may occur associated with weak stability to slantwise convective downdraughts instability to slantwise convective downdraughts may occur in some systems leading to multiple slantwise convective downdraughts associated with the release of conditional symmetric instability and even possibly symmetric instability the global climatology of sting jets and the interaction between the atmospheric boundary layer and sting jets are revealed as specific areas where more research is needed finally we describe eight myths and misunderstandings that exist in the current literature with the aim of guiding future research into the sting jet phenomenon

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the

discrimination of glacier facies using multi temporal sar data glacier facies from the greenland ice sheet and the wrangell st elias mountains alaska are analyzed using multi temporal synthetic aperture radar sar data from the european space agency ers 1 satellite distinct zones and facies are visible in multi temporal sar data including the dry snow facies the combined percolation and wet snow facies the ice facies transient melt areas and moraine in greenland and south central alaska very similar multi temporal signatures are evident for the same facies although these facies are found at lower altitude in west greenland where the equilibrium line appears to be found at sea level at 71 30 n during the year analyzed 1992 93 probably because of the cooling effect of the eruption of mount pinatubo in greenland both the percolation and dry snow facies are excellent distributed targets for sensor calibration with backscatter coefficients stable to within 0 2 db however the percolation facies near the top of mount wrangell are more complex and less easily delineated than in greenland and at high altitude the glacier facies have a multi temporal signature which depends sensitively on slope orientation