evolution of an axisymmetric tropical cyclone before reaching slantwise moist neutrality abstractin a previous study the authors showed that the intensification process of a numerically simulated axisymmetric tropical cyclone tc can be divided into two periods denoted by phase i a

distribution of helicity cape and shear in tropical cyclones abstract the previous study of helicity cape and shear in hurricane bonnie 1998 was extended to all eight tropical cyclones sampled by nasa during the convection and moisture experiments camex storms were categorized as having large or small ambient vertical wind shear with 10 m s 1 as the dividing line in strongly sheared storms the downshear mean helicity exceeded the upshear mean by a factor of 4 as in the previous study the helicity differences resulted directly from the tropical cyclone response to ambient shear with enhanced in up out flow and veering of the wind with height present downshear cape in strongly sheared storms was 60 larger downshear mean inflow near the surface and the depth of the inflow layer each were 4 times larger downshear at more than 30 of observation points outside the 100 km radius in the downshear right quadrant midlatitude empirical parameters indicated a strong likelihood of supercells no such points existed upshear in highly sheared storms much small

spiral gravity waves radiating from tropical cyclones internal gravity waves are continuously generated by deep moist convection around the globe satellite images suggest that tropical cyclones produce short wavelength high frequency waves that radiate outward with the wave fronts wrapped into tight spirals by the large differential advection of the sheared tangential flow this letter presents new in situ observations of such waves from two sources flight level data from research aircraft that show radial wavelengths of 2 10 km and vertical velocity magnitudes from 0 1 to 1 0 ms 1 and surface observations from a research buoy in the pacific that indicate the passage of gravity waves overhead as tropical cyclones pass by at distances of 100 to 300 km numerical simulations are used to interpret these observations and to understand the broader horizontal and vertical structures of the radiating waves the simulations suggest a correlation between wave amplitude and cyclone intensity which could be used to make remote estimates of peak wind speeds

revisiting azimuthally asymmetric moist instability in the outer core of sheared tropical cyclones abstractthis study revisits the characteristics and physical processes of the azimuthally asymmetric distribution of moist instability in the outer core of vertically sheared tropical cyclones tcs

rewriting the climatology of the tropical north atlantic and caribbean sea atmosphere abstract the jordan mean tropical sounding has provided a benchmark reference for representing the climatology of the tropical north atlantic and caribbean sea atmosphere for over 50 years however recent observations and studies have suggested that during the months of the north atlantic hurricane season this region of the world is affected by multiple air masses with very distinct thermodynamic and kinematic characteristics this study examined 6000 rawinsonde observations from the caribbean sea region taken during the core months july october of the 1995 2002 hurricane seasons it was found that single mean soundings created from this new dataset were very similar to c l jordan s 1958 sounding work however recently developed multispectral satellite imagery that can track low to midlevel dry air masses indicated that the 1995 2002 hurricane season dataset and likely jordan s dataset as well was dominated by three distinct air masses moist tropical mt saharan air layer sal and midlatit

a reexamination of the jordan mean tropical sounding based on awareness of the saharan air layer results from 2002 abstract the jordan mean tropical sounding has provided a benchmark for representing the climatology of the tropical north atlantic and caribbean sea since 1958 however recent studies of the saharan air layer sal have suggested that the tropical atmosphere in these oceanic regions may contain two distinct soundings sal and non sal with differing thermodynamic and kinematic structures and that a single mean sounding like jordan s does not effectively represent these differences this work addresses this possibility by examining over 750 rawinsondes from the tropical north atlantic ocean and caribbean sea during the 2002 hurricane season it was found that a two peak bimodal moisture distribution dry sal and moist non sal exists in this region and that the jordan sounding does not represent either distribution particularly well additionally sal soundings exhibited higher values of geopotential height unique temperature profiles and stronger winds with an enhanced easterly component compared to

observations and predictability of a nondeveloping tropical disturbance over the eastern atlantic abstracta strong african easterly wave aew left the west african coast in early september 2014 and operational global numerical forecasts suggested a potential for rapid tropical cyclogenesis of this disturbance in the eastern atlantic despite the presence of a large region of dry air northwest of the disturbance analysis and in situ observations show that after leaving the coast the closed circulation associated with the aew trough was not well aligned vertically and therefore low level or midlevel dry air was advected below or above respectively areas of closed circulation gps dropwindsonde observations highlight the dry air undercutting the midlevel recirculation region in the southwestern quadrant this advection of dry air constrains the spatial extent of deep convection within the aew trough leading to the vortex decaying as the column continues to be displaced horizontally losing vertical alignment this enables increased horizontal advection of dry air into the system further limiting

understanding atypical midlevel wind speed maxima in hurricane eyewalls abstractin tropical cyclones tcs the peak wind speed is typically found near the top of the boundary layer approximately 0 5 1 km recently it was shown that in a few observed tcs the wind s

a theory for buoyancy and velocity scales in deep moist convection buoyancy and velocity scales for dry convection in statistical equilibrium were derived in the early twentieth century by prandtl but the scaling of convective velocity and buoyancy as well as the fractional area coverage of convective clouds is still unresolved for moist convection in this paper high resolution simulations of an atmosphere in radiative convective equilibrium are performed using the weather research and forecasting wrf model a three dimensional nonhydrostatic convection resolving limited area model the velocity and buoyancy scales for moist convection in statistical equilibrium are characterized by prescribing different constant cooling rates to the system it is shown that the spatiotemporal properties of deep moist convection and buoyancy and velocity scales at equilibriumdependontheterminalvelocityofraindrops andahypothesisisdevelopedtoexplainthisbehavior this hypothesis is evaluated and discussed in the context of the numerical results provided by the wrf model the influence of domain size on radiative convective equilibrium statistics is also assessed the dependence of finescale spatiotemporal properties of convective structures on numerical and physical details is investigated

extratropical impacts on atlantic tropical cyclone activity abstractwith warm sea surface temperature sst anomalies in the tropical atlantic and cold sst anomalies in the east pacific the unusually quiet hurricane season in 2013 was a surprise to the hurricane community the authors analyses suggest that the substantially suppressed atlantic tropical cyclone tc activity in august 2013 can be attributed to frequent breaking of midlatitude rossby waves which led to the equatorward intrusion of cold and dry extratropical air the resultant mid to upper tropospheric dryness and strong vertical wind shear hindered tc development using the empirical orthogonal function analysis the active rossby wave breaking in august 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the north atlantic which represents the variability of the intensity and zonal extent of the jet this mode is significantly correlated with atlantic hurricane frequency the correlation coefficient is comparable to the correlation of atlantic hurricane fre

the evolution of vortex tilt and vertical motion of tropical cyclones in directional shear flows abstractrecent studies have demonstrated the importance of moist dynamics on the intensification variability of tropical cyclones tcs in directional shear flows here we propose that dry dynamic

a unified view of tropical cyclogenesis and intensification idealized high resolution numerical simulations of tropical cyclogenesis are presented in a model that represents deep convection by a warm rain process only starting with an initially weak cloud free axisymmetric warm cored vortex maximum wind speed 5 m s 1 at a radius of 100 km rapid vortex intensification begins after a gestation period on the order of 2 days from a three dimensional perspective the genesis process is similar to that in the rotating convection paradigm for vortex intensification starting with a much stronger initial vortex vmax 15 m s 1 the patterns of deep convection and convectively amplified cyclonic relative vorticity are far from axisymmetric during the genesis period moreover the organization of the cyclonic relative vorticity into a monopole structure occurs at relatively low wind speeds before the maximum local wind speed has increased appreciably barotropic processes are shown to play an important role in helping to consolidate a single signed vorticity monopole within a few hours near the intensification begin time the rotating convection paradigm appears adequate to explain the basic genesis process within the weak initial vortex providing strong support for a hypothesis of montgomery and smith that the genesis process is not fundamentally different from that of vortex intensification in particular genesis does not require a trigger and does not depend on the prior existence of a mid level vortex

orographic influence on basic flow and cyclone circulation and their impacts on track deflection of an idealized tropical cyclone abstracta series of idealized numerical experiments and vorticity budget analyses is performed to examine several mechanisms proposed in previous studies to help understand the orographic influence on track deflection over a mesoscale mountain range when an idealized tropical cyclone tc is embedded in a uniform easterly flow and passes over a mountain with a moderate froude number it is deflected to the south upstream moves over the mountain anticyclonically and then resumes its westward movement the vorticity budget analysis indicates that the tc movement can be predicted by the maximum vorticity tendency vt the orographic effects on the above tc track deflection are explained by the following 1 upstream of the mountain the easterly basic flow is decelerated as a result of orographic blocking that causes the flow to become subgeostrophic which advects the tc to the southwest analogous to the advection of a point vortex embedded in a flow the vt is primarily dominated by the horizontal vo

an idealized numerical study of shear relative low level mean flow on tropical cyclone intensity and size abstractgiven comparable background vertical wind shear vws magnitudes the initially imposed shear relative low level mean flow lmf is hypothesized to modify the structure and convective featu

a study of the onset of melt over the arctic ocean in radarsat synthetic aperture radar data 1 the regional melt onset signal of lagrangian elements of arctic sea ice in radarsat synthetic aperture radar sar data during the spring of 1998 is examined the melt signal is clearly detectable not only in backscatter changes over multiyear ice but also first year ice and mixtures of these ice types this allows a more complete mapping of the progression of melt over the sea ice cover for the spring of 1998 the onset dates range between 10 may and 29 june the spatial pattern of melt onset is characterized by sharp boundaries delineating distinct regions with relatively uniform onset dates this pattern appears to be associated with moisture and warm air brought in by a lowpressure trough and the timing of a rain event reported at the surface heat budget of the arctic ocean sheba camp measurements from the sheba camp show good correspondence between the timing of melt in sar imagery and the onset of albedo drop off associated with the beginning of summer onset dates are compared with the timing of the zero crossing of the temperature records from drifting buoys and the onset dates derived from satellite passive microwave brightness temperature fields the timing of our estimates is within 1 2 days of the zero crossing of the buoy temperature records onset dates derived from passive microwave observations appear to be biased toward a later stage of melt comparison with results derived from a sar data set from the spring of 1992 show that the onset of melt in 1998 derived from radarsat in the beaufort sea occurred 2 weeks earlier index terms 4540 oceanography physical ice mechanics and air sea ice exchange processes 4207 oceanography general arctic and antarctic oceanography 4227 oceanography general diurnal seasonal and annual cycles 4275 oceanography general remote sensing and electromagnetic processes 0689 keywords melt onset arctic ice cover radar remote sensing lagrangian ice motion

an investigation of spiral gravity waves radiating from tropical cyclones using a linear nonhydrostatic model abstracta recent study showed observational and numerical evidence for small scale gravity waves that radiate outward from tropical cyclones these waves are wrapped into tight spirals by the radia

relationship between passive microwave derived snowmelt and surface measured discharge wheaton river yukon territory canada snow volume and melt timing are major factors influencing the water cycle at northern high altitudes and latitudes yet both are hard to quantify or monitor in remote mountainous regions twice daily special sensor microwave imager ssm i passive microwave observations of seasonal snow melt onset in the wheaton river basin yukon territory canada 60 08 05 n 134 53 45 w are used to test the idea that melt onset date and duration of snowpack melt refreeze fluctuations control the timing of the early hydrograph peaks with predictable lags this work uses the ssm i satellite data from 1988 to 2002 to evaluate the chronology of melt and runoff patterns in the upper yukon river basin the wheaton river is a small 875 km2 tributary to the yukon and is a subarctic partly glacierized heterogeneous basin with near continuous hydrographic records dating back to 1966 ssm i pixels are sensitive to melt onset due to the strong increase in snow emissivity and have a robust signal in spite of coarse 25 25 km2 pixel resolution and varied terrain results show that wheaton river peak flows closely follow the end of large daily variations in brightness temperature of pixels covering the wheaton river but the magnitude of flow is highly variable as might be expected from interannual snow mass variability spring rise in the hydrograph follows the end of high diurnal brightness temperature tb amplitude variations dav by 0 to 5 days approximately 90 of the time for this basin subsequent work will compare these findings for a larger 7250 km2 unglacierized tributary the ross river which is farther northeast 61 59 40 n 132 22 40 w in the yukon territory these techniques will also be used to try to determine the improvement in melt detection and runoff prediction from the higher resolution 15 15 km2 advanced microwave scanning radiometer for eos amsr e sensor copyright 2006 john wiley sons ltd

low richardson number in the tropical cyclone outflow layer abstractdropsondes from the noaa g iv aircraft were used to examine the presence of low bulk richardson numbers rb in tropical cyclones at least one 400 m layer above z 7 5 km exhibited rb 1 in 96 of the sondes and rb 0 25 in 35 of the sondes the latter represent almost certain turbulence sondes from major hurricane ivan 2004 were examined in detail turbulent layers fell into three broad groups the first was found below cloud base near the edge of the central dense overcast cdo where relative humidity fell below 40 near zero static stability existed within the turbulent layer with stability and shear maxima above it this structure strongly resembled that seen previously from sublimation of precipitation beneath cloud base the second type of turbulent layer was located within cdo clouds in the upper troposphere and was due almost entirely to near zero static stability this most likely arose as a result of cooling via longwave flux divergence below cdo top the third type of turbulent

evaluation of the assumptions in the steady state tropical cyclone self stratified outflow using three dimensional convection allowing simulations abstractthe criteria and assumptions that were used to derive the steady state tropical cyclone intensity and structure theory of emanuel and rotunno are assessed using three dimensional convection

some aspects of hurricane inner core dynamics and energetics abstract the energy cycle of the mature hurricane resides in the secondary circulation that passes through the storm s eyewall by equating the generation of energy in this cycle to boundary layer dissipation an upper bound on wind speed is derived this bound depends on the degree of thermodynamic disequilibrium between the tropical ocean and atmosphere on the difference between sea surface and outflow absolute temperatures and also on the ratio between the enthalpy exchange and surface drag coefficients such a bound proves to be an excellent predictor of maximum wind speeds in two different axisymmetric numerical models and does not appear to depend on the existence of the hurricane eye but further consideration of the detailed dynamics of the eye and eyewall show that the intensification of hurricanes is accelerated by feedbacks associated with a component of eye subsidence forced by radial turbulent diffusion of momentum this radial momentum diffusion is an inevitable by product of the strong fr

on the characteristic height scales of the hurricane boundary layer in this study data from 794 gps dropsondes deployed by research aircraft in 13 hurricanes are analyzed to studythecharacteristicheightscalesofthehurricaneboundarylayer theheightscalesaredefinedinavariety of ways the height of the maximum total wind speed the inflow layer depth and the mixed layer depth the height of the maximumwind speed and the inflow layerdepth are referred to asthe dynamical boundarylayer heights while the mixed layer depth is referred to as the thermodynamical boundary layer height the data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights consistent with previous studies on the boundary layer structure in individual storms the dynamical boundary layer height is found to decrease with decreasing radius to the storm center the thermodynamic boundary layer height which is much shallower than the dynamical boundary layer height is also found to decrease with decreasing radius to the storm center the results also suggest that using the traditional critical richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer these different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations

tidal flow through the straits of dover abstract results are presented from the first long term deployment of the mark ii oscr high frequency radar system this new system measures surface currents at 700 preselected locations every 20 minutes at a range up to 25 km offshore tidal analysis in 11 monthly blocks of measurements made in the straits of dover indicate consistent results for seven major constituents m2 s2 n2 o1 k1 m4 and ms4 synthesized current ellipse distributions for these constituents are compared with corresponding values from a 2 5 km grid numerical model the synthesis involved both spatial and temporal averaging with quality checks on the associated standard deviations a simple point model of vertical current structure is used to extrapolate the surface observations to provide depth integrated flows for horizontal integration to compute net fluxes spatial harmonic functions are used to fit streamline distributions to the array of point measurements by these means the net residual transport arising from nonli

the importance of diurnal processes for the seasonal cycle of sea ice microwave brightness temperatures during early summer in the weddell sea antarctica over the perennial sea ice in the western and central weddell sea antarctica the onset of summer is accompanied by a significant decrease of sea ice brightness temperatures tb as observed by passive microwave radiometers such as the special sensor microwave imager ssm i the summer specific tb drop is the dominant feature in the seasonal cycle of tb data and represents a conspicuous difference to most arctic sea ice regions where the onset of summer is mostly marked by a rise in tb data from a 5 week drift station through the western weddell sea in the 2004 05 austral summer ice station polarstern ispol helped with identifying the characteristic processes for antarctic sea ice in situ glaciological and meteorological data in combination with ssm i swath satellite data indicate that the cycle of repeated diurnal thawing and refreezing of snow freeze thaw cycles is the dominant process in the summer season with the absence of complete snow wetting the resulting metamorphous snow with increased grain size as well as the formation of ice layers leads to decreasing emissivity enhanced volume scattering and increased backscatter this causes the summer tb drop

spatial and temporal variability in snowmelt onset over arctic sea ice climate models suggest surface warming in the arctic will be rapid and pronounced implying substantial changes in snowmelt onset are likely this research therefore examines spatial and temporal variability in passive microwave derived snowmelt onset dates over arctic sea ice the objectives are to understand better the regional characteristics of snowmelt and to document whether the snowmelt onset record shows signs of climate change snowmelt onset dates are derived with scanning multichannel microwave radiometer and special sensor microwave imager brightness temperature data and they are subsequently stratified into 13 regions to analyze spatial and temporal variability results illustrate significant spatial variability in snowmelt onset with the median annual snowmelt onset date in one region of the arctic typically being statistically different from most other regions the examination of temporal variability also shows large interannual differences in the median snowmelt onset date in most regions additionally trends towards earlier snowmelt onset are documented in the vvest central arctic lincoln sea beaufort sea and canadian arctic archipelago regions

the role of equatorial rossby waves in tropical cyclogenesis part ii idealized simulations in a monsoon trough environment abstract this is the second of two papers examining the role of equatorial rossby er waves in tropical cyclone tc genesis based on results from part i it is hypothesized that genesis resulting from the circulation of an er wave alone is uncommon and that the majority of er wave related genesis events occur when a sufficiently intense er wave interacts with a favorable background flow environment this paper examines this contention by performing a series of simulations in which er waves are imposed upon idealized background flows the background flows are designed to resemble a region of a monsoon trough mt a flow feature observed at certain times of the year in all of the tc basins and most dramatically in the western north pacific basin it is believed that er wave interactions with the mt may speed up the internal breakdown genesis mechanism of the mt or even result in genesis when the mt is too weak to breakdown from in situ processes alone the latter scenario is examined here when just

self stratification of tropical cyclone outflow part ii implications for storm intensification tropical cyclones intensify and are maintained by surface enthalpy fluxes that result from the thermodynamics disequilibrium that exists between the tropical oceans and atmosphere while this general result has beenknownfor at least a half century the detailednatureof feedbacksbetweenthermodynamic anddynamic processes in tropical cyclones remains poorly understood in particular the spatial relationship between surface fluxes and the radial entropy distribution apparently does not act to amplify the entropy gradient and therefore the surface winds in previous work this problem was addressed by accounting for the radial distribution of convective fluxes of entropy out of the boundary layer this led to the conclusion that a radial gradient of such convective fluxes is necessary for intensification part i showed that the assumption of constant outflow temperature is incorrect and argued that the thermal stratification of the outflow is set by small scale turbulence that limits the richardson number the assumption of richardson number criticality of the outflow allows one to derive an equation for the variation of outflow temperature with angular momentum this in turn leads to predictions of vortex structure and intensity that agree well with tropical cyclones simulated using a full physics axisymmetric model here it is shown that the variation of outflow temperature with angular momentum also permits the vortex to intensify with time even in the absence of radial gradients of entrainment into the boundary layer an equation is derived for the rate of intensity change and compared to simple models and to simulations using a full physics model

on the warm core of a tropical cyclone formed near the tropopause abstracton the basis of numerical results of a three dimensional model diagnosed using balance dynamics a mechanism by which the upper level warm core of tropical cyclones tcs forms is proposed the numerical results reveal that an upper level warm core develops when tcs intensify just prior to reaching the mature stage potential temperature budget analysis reveals that for the tendency of potential temperature the azimuthal mean component of advection is dominant at the upper level of the eye at the mature stage sawyer eliassen diagnosis shows that tendencies due to forced flow by diabatic heating and diffusion of tangential wind are dominant in the eye and are negatively correlated to each other the distributions of the diabatic heating in the simulated tc are not peculiar therefore it is unlikely that the heating distribution itself is the primary cause of the flow from the lower stratosphere the analyses of forced circulations of idealized vortices show that the upper level subsidence is enh

how does the eye warm part ii sensitivity to vertical wind shear and a trajectory analysis abstractin part ii of this study idealized simulations of tropical cyclones are used to investigate the influence of vertical wind shear on the structure of warming and descent in the eye results are compared with the no shear simulation that was analyzed in part i during intensification of a tropical cyclone in a quiescent environment time averaged eye descent is maximized at 12 13 km height warming is not generally maximized at these levels however because the static stability is at a minimum consequently the perturbation temperature is maximized at midlevels each of the above results remains valid for sheared tropical cyclones and therefore shear does not systematically alter the height of the warm core an analysis of over 90 000 parcel trajectories yields further insight into the mechanisms of eye warming and addresses several outstanding questions regarding the character of eye descent the rate at which parcels are stirred from the eye into the eyewall is a strong function of intensity w

impacts of radiation and cold pools on the intensity and vortex tilt of weak tropical cyclones interacting with vertical wind shear abstractidealized numerical simulations of weak tropical cyclones e g tropical depressions and tropical storms in sheared environments indicate that vortex tilt reduction and convective symmetr

self stratification of tropical cyclone outflow part i implications for storm structure extant theoretical work on the steady state structure and intensity of idealized axisymmetric tropical cyclones relies on the assumption that isentropic surfaces in the storm outflow match those of the unperturbed environment at large distances from the storm s core these isentropic surfaces generally lie just above the tropopause wheretheverticaltemperaturestructureisapproximatelyisothermal soithas beenassumedthat the absolute temperature of the outflow is nearly constant here it is shown that this assumption is not justified at least when applied to storms simulated by a convection resolving axisymmetric numerical model in whichmuchofthe outflowoccursbelowtheambienttropopauseanddevelopsits ownstratification unrelated to that of the unperturbed environment the authors propose that this stratification is set in the storm s core by the requirement that the richardson number remain near its critical value for the onset of small scale turbulence this ansatz is tested by calculating the richardson number in numerically simulated storms and then showing that the assumption of constant richardson number determines the variation of the outflow temperature with angular momentum or entropy and thereby sets the low level radial structure of the storm outside its radius of maximum surface winds part ii will show that allowing the outflow temperature to vary also allows one to discard an empirical factor that was introduced in previous work on the intensification of tropical cyclones

surface to subsurface velocity projection for shallow water currents sea surface currents in coastal oceans are accessible to continuous direct observations by shore based high frequency doppler radar systems inferring current structure in shallow water from such surface current observations is attempted the approach assumes frictionally dominated flow and vertically varying current velocity on the scale of the ekman boundary layer the approximation of the velocity variation with depth is consequently derivable in terms of orthogonal basis functions from the sea surface kinematic and dynamic boundary conditions specifically the viscous momentum and shear equations evaluated at the sea surface the inference procedure developed is demonstrated with sea surface data obtained in the coastal high resolution remote sensing experiment on the continental shelf off cape hatteras despite uncertainties in the surface measurements qualitative agreement is obtained between the inferred subsurface current and the current measured in situ the sensitivity of the inference to the measurement uncertainties as well as to the model assumptions is investigated and the inferred result is found to be generally robust

evolution of microwave sea ice signatures during early summer and midsummer in the marginal ice zone emissivities at frequencies from 5 to 94 ghz and backscatter at frequencies from 1 to 17 ghz were measured from sea ice in fram strait during the marginal ice zone experiment in june and july of 1983 and 1984 the ice observed was primarily multiyear the remainder first year ice was often deformed results from this active and passive microwave study include the description of the evolution of the sea ice during early summer and midsummer the absorption properties of summer snow the interrelationship between ice thickness and the state and thickness of snow and the modulation of the microwave signature especially at the highest frequencies by the freezing of the upper few centimeters of the ice

a numerical study of convective scale structures in the outer cores of sheared tropical cyclones 1 updraft traits in different vertical wind shear magnitudes

a numerical study of outer rainband formation in a sheared tropical cyclone abstractthe dynamical process of outer rainband formation in a sheared tropical cyclone tc is examined in this study using the fully compressible nonhydrostatic tc model after the easterly vertical wind shear of 10 m s 1 was imposed upon an intensifying strong tc an outer rainband characterized by a wavenumber 1 structure formed as a typical principal rainband downshear further analysis indicates that the outer rainband formation was closely connected to the activity of the inner rainband previously formed downshear moving radially outward the inner rainband tended to be filamented owing to the strong radial gradient of angular velocity as the inner rainband approached the outer boundary of the inner core convection in its middle and upwind segments reinvigorated and nascent convective cells formed upwind of the rainband caused mainly by the decreased filamentation and stabilization subsequently the rainband reorganized into a typical outer rainband three different scenarios are found to be

variations in snowpack melt on the greenland ice sheet based on passive microwave measurements a simple microwave emission model is used to simulate 37 ghz brightness temperatures associated with snowpack melt conditions for locations across the greenland ice sheet the simulated values are utilized as threshold values and compared to daily gridded smmr and ssm i passive microwave data in order to reveal regions experiencing melt the spatial extent of the area classified as melting is examined on a daily monthly and seasonal may august basis for 1979 91 the typical seasonal cycle of melt coverage shows melt beginning in late april a rapid increase in the melting area from mid may to mid july a rapid decrease in melt extent from late july through mid august and cessation of melt in late september seasonal averages of the daily melt extents demonstrate an apparent increase in melt coverage over the 13 year period of approximately 3 8 annually significant at the 95 confidence interval this increase is dominated by statistically significant positive trends in melt coverage during july and august in the west and southwest of the ice sheet we find that a linear correlation between microwave derived melt extent and a surface measure of ablation rate is significant in june and july but not august so caution must be exercised in using the microwave derived melt extents in august nevertheless knowledge of the variability of snowpack melt on the greenland ice sheet as derived from microwave data should prove useful in detecting climate change in the arctic and examining the impact of climate change on the ice sheet

recent changes in the arctic melt season melt season duration melt onset and freeze up dates are derived from satellite passive microwave data and analyzed from 1979 to 2005 over arctic sea ice results indicate a shift towards a longer melt season particularly north of alaska and siberia corresponding to large retreats of sea ice observed in these regions although there is large interannual and regional variability in the length of the melt season the arctic is experiencing an overall lengthening of the melt season at a rate of about 2 weeks decade sup 1 in fact all regions in the arctic except for the central arctic have statistically significant at the 99 level or higher longer melt seasons by greater than 1 week decade sup 1 the central arctic shows a statistically significant trend at the 98 level of 5 4 days decade sup 1 in 2005 the arctic experienced its longest melt season corresponding with the least amount of sea ice since 1979 and the warmest temperatures since the 1880s overall the length of the melt season is inversely correlated with the lack of sea ice seen in september north of alaska and siberia with a mean correlation of 0 8

tropical cyclone evolution in a minimal axisymmetric model revisited an improved version of a minimal model for a tropical cyclone is described the model is used to revisit some fundamental aspects of vortex behaviour in the prototype problem for tropical cyclone intensification after rapidly intensifying to a mature phase in which the maximum tangential wind speed remains quasi steady for a few days the vortex ultimately decays in a 20 day simulation the vortex never becomes globally steady in particular the upper anticyclone continues to expand for the duration of the integration these results are consistent with those of recent studies using more sophisticated numerical models as in the latter models an important feature of the dynamics of spin up is the development of supergradient winds in the boundary layer and the vertical advection of the associated high tangential momentum air from the boundary layer to spin up the eyewall region this mechanism while consistent with some recently reported results is not part of the classical theory of spin up

axisymmetric balance dynamics of tropical cyclone intensification and its breakdown revisited abstractthis paper revisits the evolution of an idealized tropical cyclone like vortex forced by a prescribed distribution of diabatic heating in the context of both inviscid and frictional axisymm

development and nondevelopment of binary mesoscale vortices into tropical cyclones in idealized numerical experiments abstractthe evolution of two symmetric midlevel mesoscale vortices situated above a warm ocean is examined with a basic cloud resolving model idealized numerical experiments provide insight into how the evolution may vary with the initial vortex separation distance d and other parameters that influence the time scale for an isolated vortex to begin rapid intensification the latter parameters include the ambient middle tropospheric relative humidity rh and the initial midlevel wind speed of each vortex at relatively low rh there exists an interval of d where binary midlevel vortex interaction prevents tropical cyclone formation while tropical cyclones generally develop at high rh similar values of d can delay the process if the vortices are initially weak prevention or inhibition of tropical cyclone formation occurs in association with the outward expulsion of lower tropospheric potential vorticity anomalies as the two vortices merge in the middle troposphere it is proposed that the primary mecha

on the accuracy of hf radar surface current measurements intercomparisons with ship based sensors high frequency hf radar systems can provide periodic two dimensional vector current estimates over an area approaching 1000 km as the use of these hf systems has gained wider acceptance a number of attempts have been made to estimate the accuracy of such systems however comparisons of hf radar current estimates with in situ sensors are difficult to interpret since hf systems measure currents averaged over an area of 1 km 2 and to a depth of only 50 cm while in situ sensors measure currents at a point and somewhat greater depths 1 to 10 m previous studies of the accuracy of hf radar technology have thus attributed the differences observed between hf radar and in situ sensors to an unknown combination of vertical shear horizontal inhomogeneity in situ instrument errors and hf radar system errors this study examines the accuracy of hf radar current measurements using data from the 1993 high resolution remote sensing experiment conducted off cape hatteras north carolina data from four shipborne in situ current meters are compared with data from an ocean surface current radar oscr a commercial current measuring radar we attempt to discern the predominant sources of error in these data by using multiple simultaneous measurements from different sensors and by examining the variation of observed current differences as a function of location the results suggest an upper bound on the accuracy of the oscr derived radial currents of 7 to 8 cm s the measurement of near surface currents in the coastal environ ment is an important problem

a new view of near shore dynamics based on observations from hf radar abstract since 1984 the oscr hf radar system has been used in over 50 deployments to measure near shore surface currents for both scientific and engineering applications the enhanced scope resolution and accuracy of these measurements have yielded new insights into the tidal wind and density driven dynamics of the near shore zone tidal current ellipses obtained from these radar measurements have been shown to be in good aggrement with values calculated by numerical models both for the predominant constituents and also for higher harmonics coherent patterns of wind forced currents ahve been determined with strong evidence of a slab like surface response in one deployment with offshore winds blowing over relatively deep water this slab rotated clockwise at near inertial frequency strong up to 20cm s 1 persistent surface residual currents are commonly observed these are almost certainly generated by small horizontal density gradients these observed surface residuals provide ideal data for rigorous testing of 3 d numerical models with a threatened rise in sea level hf radar is well suited for observing the expected changes in the dynamics of near shore regions continuing development of these radar systems offers exciting prospects of remote sensing of both surface waves and currents future applications may extend beyond the near shore region to measurements along the shelf edge in oceanic gyres and for beach processes

clouds in tropical cyclones abstract clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts within the eyes of mature tropical cyclones stratus clouds top a boundary layer capped by subsidence an outward sloping eyewall cloud is controlled by adjustment of the vortex toward gradient wind balance which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state this balance is intermittently upset by buoyancy arising from high moist static energy air entering the base of the eyewall because of the radial influx of low level air from the far environment supergradient wind in the eyewall zone and or small scale intense subvortices the latter contain strong erect updrafts graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left

why is the tropical cyclone boundary layer not well mixed abstractplausible diagnostics for the top of the tropical cyclone boundary layer include i the top of the layer of strong frictional inflow and ii the top of the well mixed layer that is the layer over which potential temperature is approximately constant observations show that these two candidate definitions give markedly different results in practice with the inflow layer being roughly twice the depth of the layer of nearly constant here the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model the authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects the radial wind varies strongly with height and therefore so does radial advection of this process also stabilizes the boundary layer but to a lesser degree than diabatic effects the authors also show that this differential radial advection contributes to the observed superadiabatic layer ad

interannual variations of snowmelt and refreeze timing on southeast alaskan icefields u s a twice daily satellite observations from the special sensor microwave imager ssm i indicate melt onset and refreeze on southeast alaskan icefields melt and refreeze are based on 37 ghz vertically polarized brightness temperatures t b and diurnal amplitude variations dav two types of melt regime have different summer characteristics onset is characterized by increasing average daily t b and a switch from low to high amplitude dav melt timing calibrated using juneau icefield temperatures correlates well with nearby stream hydrographs some pixels maintain high t b throughout the melt season and return to low amplitude dav after melt onset refreeze on these pixels is identified by decrease in t b and accompanying high amplitude dav other pixels maintain high dav throughout the summer indicating nocturnal refrecze fall refreeze is determined by the end of high amplitude dav interannual variability in melt timing and ablation season length is high melt onset and refreeze timing show a regional tendency toward earlier glacier melt onset and longer ablation seasons from 1988 98

the fine structure of nearshore tidal and residual circulations revealed by h f radar surface current measurements abstract using the ocean surface current radar oscr developed by the rutherford appleton laboratory uk 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square tidal analyses of these data show that the currents associated with the predominant m2 constituent sweep smoothly and regularly through the area unaffected by the finer topographic features moreover contours of the amplitude of the m2 semi major axis are mutually consistent to a precision of better than 0 5 cm s 1 statistical analyses of these data indicate that the standard error of oscr current measurements is less than 4 cm s 1 by contrast the major higher harmonic constituent m4 shows pronounced but ordered spatial variability relating the observed distributions for m4 and m2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes standard relationships between residual surface currents and

a statistical perspective on wind profiles and vertical wind shear in tropical cyclone environments of the northern hemisphere abstracta statistical analysis of tropical cyclone tc environmental wind profiles is conducted in order to better understand how vertical wind shear influences tc intensity change the wind profiles are computed from global atmospheric reanalyses around the best track locations of 7554 tc cases in the northern hemisphere tropics mean wind profiles within each basin exhibit significant differences in the magnitude and direction of vertical wind shear comparisons between tc environments and randomly selected non tc environments highlight the synoptic regimes that support tcs in each basin which are often characterized by weaker deep layer shear because weaker deep layer shear may not be the only aspect of the environmental flow that makes a tc environment more favorable for tcs two new parameters are developed to describe the height and depth of vertical shear distributions of these parameters indicate that in both tc and non tc environments vertical shear most frequently occurs in shallow layer

cloud microphysics impact on hurricane track as revealed in idealized experiments analyses of tropical cyclones created in an idealized environment reveal how and why cloud microphysical assumptions can influencestorm motion including speed and direction it is well known that in the absence of a mean flow a leading factor in storm propagation is the establishment of beta gyres owing to planetary vorticity advection by the storm s circulation previous research demonstrated that tangential winds well beyond the core influence storm motion by helping to determine the gyres orientation and intensity microphysical assumptions especially involving average particle fall speeds can strongly influence the winds at outer radius more specifically microphysics modulates the radial distribution of column average virtual temperature which largely determines the radial surface pressure gradient and therefore the winds because they tend to be in gradient balance beyond the core microphysics schemes can differ markedly with respect to average fall speed depending on the complexity of the scheme and how interactions among condensation types are handled average fall speed controls the outward movement of particles produced in the eyewall into the anvil where they can influence the environment through cloud radiative interactions and phase changes with the assistance of some special sensitivity tests the influence of microphysics and fall speed on radial temperature gradients leading to different outer wind strengths and tracks is shown among other things this work demonstrates that the treatment of outer rainbands in operational models can potentially influence how simulated storms move thus affecting position forecasts

structural changes preceding rapid intensification in tropical cyclones as shown in a large ensemble of idealized simulations abstractstructural changes that precede rapid intensification ri of tropical cyclones tcs are examined in a full physics model by conducting a large ensemble 270 of idealized tc simulations the processes leading to ri in a representative case with moderate shear are consistent with previous studies for weakly sheared cases the most distinct changes are that the vortex tilt and the vortex size begin to decrease more rapidly 6 h before the onset of ri a vorticity budget analysis for the upper layer around the low level center reveals that the vertical vorticity is increased by vertical advection stretching and tilting terms before ri whereas the horizontal advection is small thus the upright vortex structure is not achieved through a vortex alignment process but rather is built upward by deep convection the ensemble simulations are generated by changing the intensity and size of the initial vortex the magnitude of vertical wind shear and the translation speed the ensemble members that show

observed tropical cyclone eye thermal anomaly profiles extending above 300 hpa abstractas recently pointed out by stern and nolan much of our knowledge of the warm core structure of the tropical cyclone eye has come from composites of in situ data taken from multiple aircraft studies of three storms in the late 1950s and 1960s further observational confirmation of eye thermal structure has been lacking since much of the dropsonde data analyzed to date have been limited to pressure levels of 500 hpa or lower however there exist a number of dropsonde eye profiles extending to near 250 hpa these profiles were acquired from nasa aircraft during various field campaigns here the author uses these data to calculate eye temperature anomaly profiles these data are supplemented by several surface based radiosonde releases in tropical cyclone eyes over the period 1944 2003 the author finds that the pressure altitude of the maximum anomaly varies between 760 and 250 hpa the author also finds positive correlations between the maximum anomaly level and storm intensity size upper leve

tropical and subtropical north atlantic vertical wind shear and seasonal tropical cyclone activity abstractgiven recent insights into the role of anticyclonic rossby wave breaking awb in driving subseasonal and seasonal north atlantic tropical cyclone tc activity this study further examines

effects of low frequency current variability on near inertial submesoscale vortices from september 30 to october 14 1993 and from may 18 to june 13 1994 ocean surface currents along the inshore boundary of the florida current were measured using a land based ocean surface current radar oscr deployed along the florida keys a dual station oscr mapped the current fields at 20 min intervals with a horizontal resolution of 1 2 km over a 30 44 km domain using the high frequency hf 25 4 mhz mode an upward looking narrowband acoustic doppler current profiler adcp was moored in 150 m of water seaward of the shelf break concurrently sampling the three dimensional current vector between 15 and 130 m at 30 min intervals regression analyses between surface and subsurface currents at 15 m depth from the adcp indicated biases of 3 12 cm s 1 and slopes of o 0 8 1 root mean square differences were about 18 cm s 1 because of a 15 m depth separation between the two measurements and bulk current shears were o 10 3 s 1 in the florida current where maximum velocities exceeded 150 cm s 1 between 40 and 70 m surface tidal currents were 2 10 cm s 1 in the diurnal and semidiurnal bands and revealed fine scale variability in the tidal ellipses due to topographical variations differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s 1 in these constituents and explained more of the current variance with depth during the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s 1 this feature was embedded in the near inertial wave band where the local inertial period was 29 hours that may have been forced by an abrupt change in the wind direction this transient feature was trapped and advected along the inshore edge of the florida current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f these energetic near inertial currents of 15 20 cm s 1 had horizontal and vertical wavelengths of 40 km and 50 100 m respectively

the onset of spring melt in first year ice regions of the arctic as determined from scanning multichannel microwave radiometer data for 1979 and 1980 sea ice ablation is an important physical process affecting the global climate system during the arctic melt season rapid changes occur in both sea ice surface conditions and the extent of ice these changes alter the albedo and vary the surface energy budget understanding variations in arctic sea ice is critical for global climate studies this paper investigates the spring onset of melt in the arctic seasonal sea ice zone through analysis of melt signatures derived from nimbus 7 scanning multichannel microwave radiometer data satellite derived melt signatures determined by 18 and 37 ghz vertical brightness temperatures are associated with the initial melt of the snow pack on the sea ice surface sea ice melt events vary spatially and temporally within the arctic basin the melt signature is observed first in the chukchi sea and the kara and barents seas as melting progresses the melt signature moves westward from the chukchi sea and eastward from the kara and barents seas to the laptev sea region the initial location of the melt signal also varies with year in 1979 the melt signature occurs first in the chukchi sea and in 1980 in the kara sea the date for the initial melt varies between 1979 and 1980 by an average of 7 10 days with a maximum of 25 days in the chukchi sea region monitoring the occurrence of melt signatures can be used as an indicator of climate variability in the arctic s seasonal sea ice zones

an assessment of local coastal dynamics observed with high frequency radar an assessment is made of the dominant processes governing the circulation in a 30 km by 40 km area off the coast of cape hatteras north carolina where surface current velocity vectors have been obtained using shore based high frequency radars in the high resolution remote sensing experiment in 1993 although the currents in this region are constantly under the influence of winds and tides frequent intrusion of energetic flows into the area apparently mostly of gulf stream origin is shown to have a strong influence on the current variability analysis of vorticity and divergence shows that the flow intruding into the area is strongly rotational with positive vorticity this rotational motion is not in geostrophic balance with the surface pressure gradient and an inertial oscillation is often excited as a result with vorticity and divergence oscillating 90 out of phase on average the geostrophic imbalance generates a net surface flow divergence which in turn induces negative vorticity that more than offsets the positive vorticity advected into the region however vertical viscous transfer of positive vorticity from subsurface to surface is shown to be required to complete the surface vorticity balance the same energetic cyclonic shear flows that intrude into the area are suggested as the source of the subsurface vorticity as these flows of gulf stream origin are known to be subducted under buoyant shelf water

dry intrusions lagrangian climatology and dynamical impact on the planetary boundary layer abstractdry air intrusions dis are dry deeply descending airstreams from the upper troposphere toward the planetary boundary layer pbl the significance of dis spans a variety of aspects including the interaction with convection extratropical cyclones and fronts the pbl and extreme surface weather here a lagrangian definition for di trajectories is used and applied to ecmwf interim reanalysis era interim data based on the criterion of a minimum descent of 400 hpa during 48 h a first global lagrangian climatology of di trajectories is compiled for the years 1979 2014 allowing quantitative understanding of the occurrence and variability of dis as well as the dynamical and thermodynamical interactions that determine their impact dis occur mainly in winter while traveling equatorward from 40 50 latitude dis typically reach the lower troposphere with maximum frequencies of 10 in winter in the storm track regions as well as over the mediterranean sea arabian sea and eastern north pa

vertical temperature structure associated with evaporation of stratiform precipitation in idealized wrf simulations abstracta recent study based on observations has shown that after precipitation over tropical oceans rather shallow temperature structures occur in the lower troposphere and that their magnitude de

a numerical model study of parameters influencing tidal currents in the irish sea the mathematical formulation of a mixed finite difference modal three dimensional tidal model of the irish sea using quadratic bottom friction is briefly presented by using an expansion of modes in the vertical a continuous current profile from sea surface to seabed can be computed and accurate comparisons made with tidal currents measured at a range of positions throughout the water column the sensitivity of current profiles to various eddy viscosity formulations and coefficients of bottom friction are examined in detail also the influence of open boundary conditions and water depths upon computed tidal elevations and currents is examined in common with other earlier published calculations initial results show that with previous formulations of eddy viscosity and friction an accurate distribution of sea surface elevations is accompanied with computed currents which exceed observations by the order of 20 cm s 1 however results from the sensitivity study presented here show that elevations and currents above the bottom boundary in good agreement with observations can be obtained with alternative formulations of viscosity and other friction coefficients together with current input to the radiation condition however computed near bed currents are still larger than those observed although a slight increase in water depth and bottom friction coefficients corrects this calculations clearly show that the computation of near bed currents subject to the constraints of obtaining accurate elevation and current distributions is the most sensitive test of the model excellent agreement between computed and observed currents and elevations was obtained in a final series of calculations using appropriate parameterizations of eddy viscosity and bottom friction

dramatic inner core tropopause variability during the rapid intensification of hurricane patricia 2015 abstractdropsondes with horizontal spacing as small as 4 km were released from the stratosphere in rapidly intensifying hurricane patricia 2015 during the office of naval research tropical cyclone intensity experiment these observations provide cross sections of unprecedented resolution through the inner core of a hurricane on 21 october patricia exhibited a strong tropopause inversion layer til across its entire circulation with a maximum magnitude of 5 1 k 100 m 1 this inversion weakened between 21 and 22 october as potential temperature increased by up to 16 k just below the tropopause and decreased by up to 14 k in the lower stratosphere between 22 and 23 october the til over the eye weakened further allowing the tropopause to rise by 1 km meanwhile over patricia s secondary eyewall the til restrengthened and bulged upward by about 700 m into what was previously the lower stratosphere these observations support many aspects of recent modeling studies including eyewall penetration i

revisiting the relationship between eyewall contraction and intensification abstractin the widely accepted convective ring model of tropical cyclone intensification the intensification of the maximum winds and the contraction of the radius of maximum winds rmw occur simultaneously this study shows that in idealized numerical simulations contraction and intensification commence at the same time but that contraction ceases long before peak intensity is achieved the rate of contraction decreases with increasing initial size while the rate of intensification does not vary systematically with initial size utilizing a diagnostic expression for the rate of contraction it is shown that contraction is halted in association with a rapid increase in the sharpness of the tangential wind profile near the rmw and is not due to changes in the radial gradient of the tangential wind tendency it is shown that a number of real storms exhibit a relationship between contraction and intensification that is similar to what is seen in the idealized simulations in particular the statistical

the midsummer drought over mexico and central america abstract the annual cycle of precipitation over the southern part of mexico and central america exhibits a bimodal distribution with maxima during june and september october and a relative minimum during july and august known as the midsummer drought msd the msd is not associated with the meridional migration of the intertropical convergence zone itcz and its double crossing over central america but rather with fluctuations in the intensity and location of the eastern pacific itcz during the transition from intense to weak weak to intense convective activity the trade winds over the caribbean strengthen weaken such acceleration in the trade winds is part of the dynamic response of the low level atmosphere to the magnitude of the convective forcing in the itcz the intensification of the trade winds during july and august and the orographic forcing of the mountains over most of central america result in maximum precipitation along the caribbean coast and minimum precipitation along the pacific