the evolution of oceanic and atmospheric anomalies in the northeast pacific during the el nino and la nina events of 1995 2001 abstract from late 1995 through early 2001 three major interannual climate events occurred in the tropical pacific the 1995 97 la nina ln 1997 98 el nino en and 1998 2001 ln we analyze atmospheric and upper oceanic anomalies in the northeast pacific nep during these events and compare them to anomalies both elsewhere in the north and tropical pacific and to typical en and ln anomaly patterns the atmospheric and oceanic anomalies varied strongly on intraseasonal and interannual scales during the 1995 97 ln and 1997 98 en the northeast pacific was dominated by negative slp and cyclonic wind anomalies and by upper ocean temperature and sea surface height ssh anomalies the latter were positive along the north american west coast and in the nep thermal anomaly pool between hawaii vancouver island and baja california and negative in the central north pacific this atmospheric oceanic anomaly pattern is typical of en an eastward shift in the atmospheric teleconnection from east asia created en like anomalies in the nep during the 1995 97 ln well before the 1997 98 en had begun the persistence of negative sea level pressure slp and cyclonic wind anomalies in the nep during the 1997 98 en intensified pre existing upper oceanic anomalies atmospheric anomalies were shifted eastward during late 1996 early 1998 leading to a similar onshore shift of oceanic anomalies this produced exceptionally strong positive upper ocean temperature and ssh anomalies along the west coast during the 1997 98 en and explains the unusual coastal occurrences of several species of large pelagic warm water fishes the growth and eastward shift of these pre existing anomalies does not appear to have been linked to tropical pacific en anomalies until late 1997 when a clear atmospheric teleconnection between the two regions developed prior to this remote atmospheric impacts on the nep were primarily from east asia as the 1998 2001 ln developed nep anomalies began reversing toward the typical ln pattern this led to predominantly negative slp and cyclonic wind anomalies in the nep and upper ocean temperature and ssh anomalies that were mainly negative along the west coast and positive in the central north pacific the persistence of these anomalies into mid 2001 and a number of concurrent biological changes in the nep suggest that a decadal climate shift may have occurred in late 1998 during 1995 2001 nep oceanic anomalies tracked the overlying atmospheric anomalies as indicated by the maintenance of a characteristic spatial relationship between these anomalies in particular wind stress curl and ssh anomalies in the nep maintained an inverse relationship that strengthened and shifted eastward toward the west coast during late 1996 early 1998 this consistent relationship indicates that anomalous ekman transport driven by regional atmospheric forcing was an important contributor to temperature and ssh anomalies in the nep and ccs during the 1997 98 en other studies have shown that coastal propagations originating from the tropical pacific also may have contributed to coastal nep anomalies during this en our results indicate that at least some of this coastal anomaly signal may have been generated by regional atmospheric forcing within the nep

the 1997 1998 oceanic el nino signal along the southeast and northeast pacific boundaries an altimetric view changes in the sea surface heights ssh and geostrophic currents along the eastern boundaries of the pacific north central and south america are examined during the 1997 1998 el nino using altimeter data and proxy winds these show that symmetric ssh signals left the equator and propagated into both hemispheres in two episodes with primary periods of high equatorial ssh during may july and october december 1997 these are the distant signals from the mid latitude perspective as the signals spread poleward in each hemisphere their loss of symmetry demonstrates the degree to which they were altered by topographic features local winds and or local currents the first four eofs are calculated for 2 d ssh fields in 10 wide strips along the eastern margins 60 n 60 s and extending out along the equator from the coast to 110 w these account for approximately 40 of the overall variability and represent the main features of the seasonal cycles and el nino interannual variability snapshots of the 2 d ssh fields depict the structure of the el nino signal at different phases of its evolution

mesoscale anticyclonic circulations in the lee of the central rocky mountains composite analyses of terrain forced mesoscale anticyclonic circulations over southern wyoming and northern colorado are constructed these suggest two different types of circulations based on upstream flow direction the air within type 1 circulations originates to the west of the continental divide and contains little moisture type 2 circulations form in more moist northerly flow and are sometimes associated with snowbands both types tend to form during the afternoon and dissipate after sunset although type 2 events may follow frontal passages and occur at night case studies of one event of each type suggest that anticyclonic vorticity generation occurs within the lowest kilometer above ground level when that layer is nearly vertically mixed in both potential temperature and velocity an analogy with vorticity generation in mixed layer models is considered and it is shown that the conditions for generating negative vorticity in those models are satisfied in each observed case the mixed layer mechanism is also favored as it naturally explains the diurnal tendency of the circulations and may therefore explain the observed late day snowfall maximum along the front range of colorado

sea level fluctuations in central california at subtidal to decadal and longer time scales with implications for san francisco bay california abstract sea level elevations from near the mouth of san francisco bay are used to describe the low frequency variability of forcing of the coastal ocean on the bay at a variety of temporal scales about 90 of subtidal fluctuations in sea level in san francisco bay are driven by the sea level variations in the coastal ocean that propagate into the bay at the estuary mouth we use the 100 year sea level record available at san francisco to document a 1 9 mm yr mean sea level rise and to determine fluctuations related to el nino southern oscillation enso and other climatic events at time scales greater than 1 year enso dominates the sea level signal and can result in fluctuations in sea level of 10 15 cm alongshore wind stress data from central california are also analyzed to determine the impact of changes in coastal elevation at the mouth of san francisco bay within the synoptic wind band of 2 30 days at least 40 of the subtidal fluctuations in sea level of the bay are tied to the large scale regional wind field affecting sea level variations in the coastal ocean with little local direct wind forcing of the bay itself the majority of the subtidal sea level fluctuations within the bay that are not related to the coastal ocean sea level signal are forced by an east west sea level gradient resulting from tidally induced variations in sea level at specific beat frequencies that are enhanced in the northern reach of the bay river discharge into the bay through the sacramento and san joaquin river delta also contributes to the east west gradient but to a lesser degree

observations and modeling of banded orographic convection radar images and numerical simulations of three shallow convective precipitation events over the coastal range in western oregon are presented in one of these events unusually well defined quasi stationary banded formations produced large precipitation enhancements in favored locations while varying degrees of band organization and lighter precipitation accumulations occurred in the other two cases the difference between the more banded and cellular cases appeared to depend on the vertical shear within the orographic cap cloud and the susceptibility of the flow to convection upstream of the mountain numerical simulations showed that the rainbands which appeared to be shear parallel convective roll circulations that formed within the unstable orographic cap cloud developed even over smooth mountains however these banded structures were better organized more stationary and produced greater precipitation enhancement over mountains with small scale topographic obstacles low amplitude random topographic roughness elements were found to be just as effective as more prominent subrange scale peaks at organizing and fixing the location of the orographic rainbands

a global climatology of atmospheric fronts 1 atmospheric fronts in the lower troposphere often mark regions of significant weather and serve as an important conceptual tool that is frequently used and well understood by the public this study uses an objective method to identify fronts in the era 40 reanalysis and to compile a quantitative global climatology of their occurrence the climatology confirms the mid latitude storm tracks as the regions of highest front frequency and identifies slow moving frontal boundaries in the subtropics corresponding to major climate features such as the mei yu front the climatology also shows fronts to be common features of the south pacific convergence zone in the northern hemisphere fronts shift poleward in summer in the southern hemisphere this and other aspects of seasonal behavior are rather different

a comparison of shear and buoyancy driven planetary boundary layer flows abstract planetary boundary layer pbl flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces these differences are not unexpected in that shear instabilities occur locally while buoyancy force sets up vigorous thermals which result in nonlocal transport of heat and momentum at the same time these two forces can act together to modify the flow field in this study four large eddy simulations less spanning the shear and buoyancy flow regimes were generated two correspond to the extreme cases of shear and buoyancy driven pbls while the other two represent intermediate pbls where both forces are important the extreme cases are used to highlight and quantify the basic differences between shear and convective pbls in 1 flow structures 2 overall statistics and 3 turbulent kinetic energy tke budget distributions results from the two intermediate les cases are used to develop and verify a velocity scaling and a tke budget mode

the contribution of shear to the evolution of a convective boundary layer the role of shear in the development and maintenance of a convective boundary layer is studied by means of observations and large eddy simulations less particular emphasis is given to the growth of the boundary layer and to the way in which this growth is affected by surface fluxes of heat and moisture and entrainment fluxes this paper analyzes the processes that drive the latter mechanism which accounts for approximately 30 of the growth of the mixing layer typically it is found that under pure convective conditions without shear the entrainment buoyancy flux at the inversion is about 220 of the surface buoyancy flux this value is widely used for entrainment rate closures in general circulation models the data collected during the atmospheric radiation measurement campaign allow one to introduce realistic vertical profiles and surface fluxes into the les runs and to compare the simulation results with the observed evolution of the boundary layer height during a convective situation with high entrainment rates and high geostrophic winds the analysis of the turbulent kinetic energy tke budget shows that the inclusion of geostrophic winds which produce shear at the surface and in the entrainment zone modifies the vertical profile of the various terms in the tke budget as a consequence the entrainment flux is enhanced resulting in increased growth of the boundary layer the numerical experiments and the observations enable one to validate the efficiency of earlier representations based on the tke equation which describe the evolution of the ratio between entrainment and surface buoyancy fluxes the proposed parameterization for the entrainment and surface buoyancy flux ratio b which includes the main buoyancy and shear contributions is in good agreement with the les results some aspects of the parameterization of b for instance the absence of entrainment flux and its behavior during the transition between convective to neutral conditions are discussed

the turbulent structure and diurnal growth of the saharan atmospheric boundary layer the turbulent structure and growth of the remote saharan atmospheric boundary layer abl is described with in situ radiosonde and aircraft measurements and a large eddy simulation model a month of radiosonde data from june 2011 provides a mean profile of the midday saharan abl which is characterized by a well mixed convective boundary layer capped by a small temperature inversion 1k and a deep near neutral residual layer the boundary layer depth varies by up to 100 over horizontal distances of a few kilometers due to turbulent processes alone the distinctive vertical structure also leads to unique boundary layer processes such as detrainment of the warmest plumes across the weak temperature inversion which slows down the warming and growth of the convective boundary layer as the boundary layer grows overshooting plumes can also entrain freetropospheric air into the residual layer forming a second entrainment zone that acts to maintain the inversion above the convective boundary layer thus slowing down boundary layer growth further a single column model is unable to accurately reproduce the evolution of the saharan boundary layer highlighting the difficulty of representing such processes in large scale models these boundary layer processes are special to the sahara and possiblyhot dry desertenvironmentsingeneral andhaveimplicationsforthelarge scalestructureofthesaharan heat low the growth of the boundary layer influences the vertical redistribution of moisture and dust and the spatial coverage and duration of clouds with large scale dynamical and radiative implications

large scale low frequency barotropic circulation on continental margins abstract analytic solutions are obtained for forced barotropic circulation at subinertial frequencies over a bilinear continental margin shelf and slope in situations where bottom friction is important three different alongshore forces are considered wind stress offshore oceanic pressure gradients and offshore currents forcing functions are assumed to vary sinusoidally in time and in space alongshore steady models are found to perform adequately provided that the forcing functions do not move in the same direction as the free modes continental shelf waves propagate near resonance when the alongshore velocity of the forcing approximates that of a free mode the response is dominated by the mode in the case of wind forcing signals are trapped nearshore if the shelf break occurs within this trapping length as occurs near resonance the shelf width becomes the elective trapping length in this instance there can be significant horizontal shear in the alongshore velocity on the shelf near the s

the spacing of orographic rainbands triggered by small scale topography abstract a combination of idealized numerical simulations and analytical theory is used to investigate the spacing between convective orographic rainbands over the coastal range of western oregon the simulations which are idealized from an observed banded precipitation event over the coastal range indicate that the atmospheric response to conditionally unstable flow over the mountain ridge depends strongly on the subridge scale topographic forcing on the windward side of the ridge when this small scale terrain contains only a single scale of terrain variability the band spacing is identical to but when a spectrum of terrain scales are simultaneously present the band spacing ranges between 5 and 10 km a value that is consistent with observations based on the simulations an inviscid linear model is developed to provide a physical basis for understanding the scale selection of the rainbands this analytical model which captures the transition from lee waves upstream of the orographic cloud t

wind forced motion on the northern great barrier reef abstract observations of current bottom pressure and wind from the shelf of the northern great barrier reefare interpreted using a two layer frictional step shelf model forced by propagating wind stress distinguishing features of the region are the many reefs on the shallow shelf the well mixed nature of the shelf waters and the steep continental slope the instrument army was deployed from april to october 1982 and comprised 14 current meters nine pressure gauges and two weather stations low pass 0 6 cycles per day cpd filtered velocity and pressure data are analyzed separately using a time domain empirical orthogonal function eof analysis the first eofs of both velocity and pressure data are found to account for most of the observed variance on the shelf time series of these two eofs are both highly coherent with the local longshore wind stress at weatherband frequencies 0 3cpd however the frequency transfer functions of the wind to the current and of the wind to the pressure ar

thermodynamic variability within the convective boundary layer due to horizontal convective rolls abstract data from the convection and precipitation electrification cape experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer cbl potential temperature variations were only about 0 5 k while variations in water vapor mixing ratio values of 1 5 2 5 g kg 1 were observed throughout the cbl using radar aircraft and sounding data it is shown that horizontal convective rolls are the likely cause of these variabilities the enhanced moisture occurred within the roll updraft regions thus rolls were transporting moist air from the surface upward the observed cloud base heights obtained from cloud photogrammetry were produced from the highest moisture values within the roll updraft regions since the roll ascending branches contained moisture values that were most representative of the observed cloud base heights it is likely that measurements from within the roll updrafts would prov

the large scale coastal wind field along the west coast of north america 1981 1982 we study the statistical properties of the coastal wind field along the west coast of north america for two summers 1981 and 1982 coastal ocean dynamics experiments 1 and 2 respectively and the intervening winter using measured winds and geostrophic winds calculated from fleet numerical oceanography center atmospheric pressure analyses summer wind fluctuations are driven primarily by the interaction between two relatively stationary pressure systems the north pacific subtropical high and southwest u s thermal low and by their interactions with propagating atmospheric systems to the north in particular propagating cyclones and associated fronts are often followed by a northeastward intensification of the high producing strong upwelling events along the california coast this summer event sequence occurs more frequently and is displaced farther to the south on average during summer 1981 winter wind fluctuations are primarily driven by propagating cyclones and anticyclones and they tend to have larger variance and space scales than in summer a preference for poleward equatorward propagation exists in summer winter and the largest time scales are observed in summer 1982 coastal atmospheric boundary layer processes substantially modify winds within 100 200 km of the coast consequently measured wind fluctuations are strongly polarized in the alongshore direction and have means and rms amplitudes that can vary considerably between nearby stations along the coast calculated wind fluctuations are less polarized in the alongshore direction and have alongshore correlation scales about 60 larger than those for measured winds they represent fluctuations with alongshore wavelengths of 900 km rather well but represent poorly those with smaller wavelengths and those due to coastal atmospheric boundary layer effects

a new measure of entrainment zone structure 1 a wavelet covariance algorithm designed to objectively identify the upper and lower limits of the local transition zone between boundary layer and free troposphere is applied to the output fields from a series of large eddy simulations of the convective atmospheric boundary layer a new measure of entrainment zone structure is defined the normalized difference between the standard deviations of the local transition zone limits this is found to be a function of the convective richardson number and a predictor of normalized entrainment rate this provides a robust and objective means for determining the richardson number from remote sensing observations and the basis for determining the entrainment rate

characteristics of mid level clouds over west africa mid level clouds located between 2 and 9 km height are ubiquitous in the tropical belt however few studies have documented their characteristics and tried to identify the associated thermodynamic properties particularly in west africa this region is characterized by a strong seasonality with precipitation occurring in the sahel from june to september monsoon season this period also coincides with the annual maximum of the cloud cover here we document the macro and microphysical properties of mid level clouds the environment in which such clouds occur as well as their radiative properties across west africa to do so we combined high resolution observations from two ground based sites including lidar and cloud radar in contrasted environments one in the sahel niamey amma campaign 2006 and the other in the sahara bordj badji mokhtar fennec campaign june 2011 along with the merged cloudsat calipso satellite products the results show that mid level clouds are found throughout the year with a predominance around the monsoon season early in the morning they also are preferentially observed in the southern and western parts of west africa they are usually thin most of them are less than 1000 m deep and as observed in niamey mainly composed of liquid water a clustering method applied to niamey data allows us to distinguish three different types of cloud one with low bases one with high bases and another with large thicknesses the two first cloud families are capped by an inversion the last family is associated with a large vertical moisture transport and likely has the highest radiative effect at the earth s surface among the three cloud types

the role of moist processes in the formation and evolution of mesoscale snowbands within the comma head of northeast u s cyclones abstract the role of moist processes in regulating mesoscale snowband life cycle within the comma head portion of three northeast u s cyclones is investigated using piecewise potential vorticity pv inversion modeling experiments and potential temperature tendency budgets snowband formation in each case occurred along a mesoscale trough that extended poleward of a 700 hpa low this 700 hpa trough was associated with intense frontogenetical forcing for ascent a variety of pv evolutions among the cases contributed to midlevel trough formation and associated frontogenesis however in each case the induced flow from diabatic pv anomalies accounted for a majority of the midlevel frontogenesis during the band s life cycle highlighting the important role that latent heat release plays in band evolution simulations with varying degrees of latent heating show that diabatic processes associated with the band itself were critical to the development and maintenance of the band however changes in the meso

synoptic versus orographic control on stationary convective banding quasi stationary convective bands can cause large localised rainfall accumulations and are often anchored by topographic features here the predictability of and mechanisms causing one such band are determined using ensembles of the met office unified model at convection permitting resolution 1 5 km grid length the band was stationary over the uk for 3 h and produced rainfall accumulations of up to 34 mm the amount and location of the predicted rainfall was highly variable despite only small differences between the large scale conditions of the ensemble members only three of 21 members of the control ensemble produced a stationary rain band these three had the weakest upstream winds and hence lowest froude number band formation was due to the superposition of two processes lee side convergence resulting from flow around an upstream obstacle and thermally forced convergence resulting from elevated heating over the upstream terrain both mechanisms were enhanced when the froude number was lower by increasing the terrain height thus reducing the froude number the band became more predictable an ensemble approach is required to successfully predict the possible occurrence of such quasi stationary convective events because the rainfall variability is largely modulated by small variations of the large scale flow however high resolution models are required to accurately resolve the small scale interactions of the flow with the topography upon which the band formation depends thus although topography provides some predictability the quasi stationary convective bands anchored by it are likely to remain a forecasting challenge for many years to come

the effect of orography and surface albedo on stratification in the summertime saharan boundary layer dynamics and implications for dust transport 1 observations have shown that there are significant day to day variations in the stratification of the saharan boundary layer which is a control on regional dust transport and mixing it has been suggested that the hoggar mountains in the east part of the sahara desert act as an elevated heat source that injects air out of the convective boundary layer induces subsidence and inhibits the growth of the daytime convective boundary layer cbl over the western remote part of the sahara here model experiments are performed for three case study periods control cases and runs with the surface albedo and orographic variations in the hoggar region removed it is shown that plumes of hot air over the mountains can inject hot air which may be dust laden through the saharan residual layer to altitudes of 8 km the mountains and to a lesser extent the patches of low surface albedo produce a deeper and warmer cbl up to 400 km west of the main peaks the model simulations also provide evidence that the mountains inhibit the daytime growth of the remote convective boundary layer as postulated in earlier observational papers however the mechanism for this remote influence on the boundary layer is not subsidence induced by the mountains instead the mountains act as an obstacle to the larger scale northeasterly flow and divert cooler air to the west and east of the mountain range this cools the remote boundary layer and therefore reduces the depth of the daytime cbl

atmospheric factors governing banded orographic convection abstract the three dimensional structure of shallow orographic convection is investigated through simulations performed with a cloud resolving numerical model in moist flows that overcome a given topographic barrier to form statically unstable cap clouds the organization of the convection depends on both the atmospheric structure and the mechanism by which the convection is initiated convection initiated by background thermal fluctuations embedded in the flow over a smooth mountain without any small scale topographic features tends to be cellular and disorganized except that shear parallel bands may form in flows with strong unidirectional vertical shear the development of well organized bands is favored when there is weak static instability inside the cloud and when the dry air surrounding the cloud is strongly stable these bands move with the flow and distribute their cumulative precipitation evenly over the mountain upslope similar shear parallel bands also develop in flows where convection is

snowbands over the english channel and irish sea during cold air outbreaks persistent northerly to easterly cold air outbreaks affected the uk during the winters of 2009 10 and 2010 11 with the resulting convection frequently organizing into snowbands over the english channel and irish sea sounding data and composite radar reflectivity images from the met office nimrod precipitation radar network reveal that these bands formed along the major axis of each body of water or sea when the boundary layer flow was roughly parallel to each of those axes along channel for both seas a band was present the majority of times that the 850 hpa flow was along channel of these times of along channel flow the 850 hpa wind speed and surface to 850 hpa temperature difference were significantly greater when bands were present than when they were not for the english channel only the land sea temperature difference was also significantly greater when bands were present than when they were not in a real data weather research and forecasting model wrf control simulation of a typical band over the english channel a trough develops over the water and offshore air streams from either side converge along it in the absence of surface fluxes the trough convergence and organized precipitation fail to develop altogether orography and roughness length variations are less important in band development affecting only the location and morphology

a new boundary layer mixing scheme part i scheme description and single column model tests a new boundary layer turbulent mixing scheme has been developed for use in the ukmo weather forecasting and climate prediction models this includes a representation of nonlocal mixing driven by both surface fluxes and cloud top processes in unstable layers either coupled to or decoupled from the surface and an explicit entrainment parameterization the scheme is formulated in moist conserved variables so that it can treat both dry and cloudy layers details of the scheme and examples of its performance in single column model tests are presented

mean radar echo characteristics during project gale abstract the mean radar echo characteristics during the genesis of atlantic lows experiment gale are presented for the southeastern united states during the 15 january 15 march 1986 field phase of the program the echo characteristics were derived from the operational national weather service nws wsr 57 10 cm network radars radar echo frequency and intensity were subjectively tabulated on a 0 5 latitude longitude grid from objectively contoured three hourly nws maps routinely transmitted over facsimile the objectively contoured maps were derived from manually digitized radar observations at each nws radar location the principal findings include 1 a greater occurrence of vip intensity level 1 and greater echoes in the coastal waters from georgia to north carolina 2 a concentrated region of vip intensity level 1 3 and 5 radar echoes 100 km cast of cape hatteras north carolina along the edge of the gulf stream 3 a considerably greater likelihood of offshore versus onshore radar echoes along t

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones xi comparisons between observational and theoretical aspects of rainbands abstract comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale the warm sector flow ahead of the cold front and above the cold frontal zone aloft was unstable to conditional symmetric instability and theoretical predictions for this mechanism am consistent with several aspects of the warm sector and wide cold frontal rainbands in the case of the warm sector rainbands other mechanisms e g wave cisk and mixed dynamic convective instabilities may have also played a role the core structure of the narrow cold frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front also many aspects of the narrow cold frontal rainband were similar to a density current shear induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold frontal zone aloft the orientation of the

potential vorticity generation by west african squall lines abstractthe west african summer monsoon features multiple complex interactions between african easterly waves aews moist convection variable land surface properties dust aerosols and the diu

the role of moist convection in the west african monsoon system insights from continental scale convection permitting simulations 1 predicting the west african monsoon wam remains a major challenge for weather and climate models we compare multiday continental scale simulations of the wam that explicitly resolve moist convection with simulations which parameterize convection simulations with the same grid spacing but differing representations of convection isolate the impact of the representation of convection the more realistic explicit convection gives greater latent and radiative heating farther north with latent heating later in the day this weakens the sahel sahara pressure gradient and the monsoon flow delaying its diurnal cycle and changing interactions between the monsoon and boundary layer convection in explicit runs cold storm outflows provide a significant component of the monsoon flux in an operational global model biases resemble those in our parameterized case improved parameterizations of convection that better capture storm structures their diurnal cycle and rainfall intensities will therefore substantially improve predictions of the wam and coupled aspects of the earth system

global sea level change determination and interpretation the notion of sea level rise brings to the popular mind the specter of deep inundation of coastal regions one pictures skyscrapers emerging from the waters like so many sleeping flamingos standing in the shallows of a lake of course if all of the world s ice sheets suddenly melted or collapsed this vision would apply to new york city and its coastal counterparts but there is a general consensus that such a calamity is not an immediate threat houghton et al 1990 the actual situation for the recent historical past and near future appears to be more benign but with nonetheless extremely significant even devastating impacts due to erosion and flooding of coastal areas

evolution properties and spatial variability of mjo convection near and off the equator during dynamo abstractthis study investigates the evolution structure and spatial variability of madden julian oscillation mjo convection observed during the 2011 12 dynamics of the mjo dynamo field campaign generally the c band radars located in the near equatorial indian ocean shared mobile atmospheric research and teaching radar smart r on addu atoll gan and nasa toga on the r v roger revelle revelle observed similar trends in echo top heights stratiform rain fraction and precipitation feature size across the mjo life cycle these trends are closely related to changes in mid to upper tropospheric moisture sea surface temperature sst zonal wind and diagnosed vertical air motions however the evolution of convection moisture and vertical air motion at the r v mirai mirai located in the intertropical convergence zone itcz at 8 s exhibited a pattern nearly opposite to gan and revelle when the mjo was active over the equator convection was suppressed around mirai owing to induced subsiden

evolution of mesoscale precipitation band environments within the comma head of northeast u s cyclones abstract this paper explores the mesoscale forcing and stability evolution of intense precipitation bands in the comma head sector of extratropical cyclones using the 32 km north american regional reanalysis hourly 20 km rapid update cycle analyses and 2 km composite radar reflectivity data a statistical and composite analysis of 36 banded events occurring during the 2002 08 cool seasons reveals a common cyclone evolution and associated band life cycle a majority 61 of banded events develop along the northern portion of a hook shaped upper level potential vorticity pv anomaly during the 6 h leading up to band formation lower tropospheric frontogenesis nearly doubles and the conditional stability above the frontal zone is reduced the frontogenesis increase is primarily due to changes in the kinematic flow associated with the development of a mesoscale geopotential height trough this trough extends poleward of the 700 hpa low and is the vertical extension of the surface warm front and surface

topographically forced convergence in western washington state abstract several times a year when the low level winds from off the pacific ocean are within a narrow range of speed and direction air passes both north and south of the olympic mountains of washington state and is forced to converge in puget sound by the north south oriented cascade range this phenomenon termed the puget sound convergence zone often results in a band of cloudiness and precipitation in northern and central puget sound with clear subsiding air to the north and south this paper presents the results of a series of case studies in which the structure of the zone its meteorological manifestations and the environmental conditions necessary for its formation are explored it is shown that the convergence zone is skillfully forecast by surface coastal winds and undergoes a strong annual and diurnal cycle being most frequent during the late spring and early summer months and during the afternoon and early evening it is also found that the zone is structurally similar to a shallow cold fr

stability and charging characteristics of the comma head region of continental winter cyclones abstractthis paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16 storm climatology analyzing the distribution of lightning within the comma head a case study of a deep cyclone is presented illustrating how upper tropospheric dry air associated with the dry slot can intrude over moist gulf air creating two zones of precipitation within the comma head a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection lightning when it occurred originated from the elevated convection a second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection updrafts within convective cells in both storms approached 6 8 m s 1 and convective available potential energy in the cell environment reached approximately 50 250 j kg 1 radar measurements obtained in convective updraft regions showed enhanced spectral

structure and dynamics of the saharan atmospheric boundary layer during the west african monsoon onset observations and analyses from the research flights of 14 and 17 july 2006 this paper presents the results of co ordinated research flights over the saharan heat low conducted during july 2006 as part of the african monsoon multidisciplinary analysis amma special observing periods the flights consisted of a morning transect on 14 july 2006 with the falcon f20 aircraft of the deutsches zentrum fur luft und raumfahrt dlr using the doppler lidar wind to observe the tropospheric winds followed by an afternoon flight on 17 july with the bae146 aircraft of the facility for airborne atmospheric measurements faam releasing a curtain of 16 dropsondes measurements from radiosondes at the algerian stations of tamanrasset and in salah as well as the calipso spaceborne lidar are analysed also it is shown that the daytime saharan atmospheric boundary layer exhibits a remarkable split structure with a well mixed convective layer lying beneath a residual layer whose dynamics appear to be more nearly laminar observations from saharan radiosonde stations confirm that the saharan residual boundary layer is on some days a persistent rather than a transient feature and that on occasion the residual layer can last right through the day the broad features of the saharan heat low thermodynamics and winds are successfully captured by the ecmwf operational analyses and the discrepancies are quantified here the lidar winds of 14 july confirm that the analysis represents the main airflows accurately apart from the zone to the southeast of a mid level trough for which the southerly component of winds was underestimated by some 8 m s 1 on 17 july the strengths of both the southwesterly monsoon and the northeasterly harmattan winds were slightly underestimated also the region of maximum boundary layer temperature over the desert was around 1 degree too far north and too broad in the analysis while the equivalent potential temperature in the saharan convective boundary layer was around 2 k lower than observed the near saturated layer observed at the top of the saharan atmospheric boundary layer was underestimated also in the analysis in horizontal extent and altitude the intertropical front of 17 july was coincident with strong surface contrasts in vegetation and fluxes on each study day the intertropical front in the analyses was around 1 degree further north than observed copyright 2009 royal meteorological society

dynamics of sheared convective boundary layer entrainment part i methodological background and large eddy simulations abstract the reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer cbl when wind shear is present the sheared cbl can be studied by means of direct measurements in the atmosphere laboratory studies and numerical techniques the advantages and disadvantages of each technique are discussed in the present paper which also describes the methodological background for studying the dynamics of entrainment in sheared cbls for the reported study large eddy simulation les was chosen as the primary method of convective entrainment investigation twenty four les runs were conducted for cbls growing under varying conditions of surface buoyancy flux free atmospheric stratification and wind shear the simulations were divided into three categories cbl with no mean wind ns cbl with a height constant geostrophic wind of 20 m s 1 gc and cbl with geostrophic wind shear gs in the simulated cases the sheared cbls grew fastes

influence of terrain resolution on banded convection in the lee of the rocky mountains abstracton 16 17 february 2007 snowbands formed in the lee of the rocky mountains in wyoming colorado and nebraska on the anticyclonic shear side of a midlevel jet streak two types of bands were prevalent a longer wider band associated with frontogenesis along an equatorward moving cold front major band and multiple shorter narrower bands farther poleward minor bands to understand how the upstream terrain affected the occurrence and intensity of the bands multiple mesoscale model simulations were performed in which the terrain was incrementally smoothed the evolutions of the synoptic patterns were similar in all simulations that included topography but the synoptic pattern differed and no bands developed in a simulation with a flat land surface these results allowed a focus on the changes to the banded precipitation due to the terrain resolution remarkably although the exact location of the bands differed from run to run the bands in all simulations with topography were in roughly the s

numerical simulation of orographic rainbands 1 this study based on a statistical analysis of simulated warm rain event and radar data aims at highlighting the main physical mechanisms that lead to organize shallow convection on the relief the region of investigation the cevennes vivarais is located in the southeast part of france radar images from the cevennes experiment fall 1986 1988 reveal a characteristic and repetitive structure of the rain distribution organized in narrow bands or plumes oriented south north in the case of stationary southerly mediterranean flow the event of 14 november 1986 has been selected and constitutes the data set of this numerical study this work is closely associated with the previous work by miniscloux et al 2001 which presents in detail the results of a geostatistical analysis of the radar data set extracted from the cevennes experiment data base the main results highlight the physical characteristics and the dynamics of the rain patterns following the recent work of cosma et al 2002 high resolution i km simulations have been continued with the nonhydrostatic three dimensional 3d atmospheric model mesonh in order to reproduce the observed rainbands over the ceveunes region the numerical model correctly reproduces the structure and the dynamics of the rainbands the geostatistical analysis has been applied for the simulated rain fields the model slightly overestimates the northward advection velocity of the rain cells within the bands 75 km h 1 against 60 km h 1 for the observation and the simulated rainbands are narrower and more organized around the n 180 direction than the observed rain field the comparison allows the qualification and validation of the choice of the numerical methodology and realism of the physical parameterizations the analysis of the 3d simulated fields confirms the physical mechanisms responsible for the rain organization demonstrated by cosma et al 2002 through idealized simulations the statistical analysis highlights the presence of mean topographic features under low level convergence zones composed of a succession of ridges and penetrating valleys orientated east west the rainbands are generated upstream of these topographic features and enhanced on the leeside due to the convergence created by the flow deflection around the obstacle and its penetration into the valleys the simulated triggering takes place further to the south than the observed one and the triggering is active as soon as the relief is suitably described in the model

the vertical cloud structure of the west african monsoon a 4 year climatology using cloudsat and calipso the west african summer monsoon wam is an important driver of the global climate and locally provides most of the annual rainfall a solid climatological knowledge of the complex vertical cloud structure is invaluable to forecasters and modelers to improve the understanding of the wam in this paper 4 years of data from the cloudsat profiling radar and calipso are used to create a composite zonal mean vertical cloud and precipitation structure for the wam for the first time the near coincident vertical radar and lidar profiles allow for the identification of individual cloud types from optically thin cirrus and shallow cumulus to congestus and deep convection a clear diurnal signal in zonal mean cloud structure is observed for the wam with deep convective activity enhanced at night producing extensive anvil and cirrus while daytime observations show more shallow cloud and congestus a layer of altocumulus is frequently observed over the sahara at night and day extending southward to the coastline and the majority of this cloud is shown to contain supercooled liquid in the top the occurrence of deep convective systems and congestus in relation to the position of the african easterly jet is studied but only the daytime cumulonimbus distribution indicates some influence of the jet position

alongshore wind forcing of coastal sea level as a function of frequency abstract the amplitude of the frequency response function between coastal alongshore wind stress and adjusted sea level anomalies along the west coast of the united states increases linearly as a function of the logarithm log10 of the period for time scales up to at least 60 and possibly 100 days the amplitude of the frequency response function increases even more rapidly at longer periods out to at least 5 yr at the shortest periods the amplitude of the frequency response function is small because sea level is forced only by the local component of the wind field the regional wind field which controls the wind forced response in sea level for periods between 20 and 100 days not only has much broader spatial scales than the local wind but also propagates along the coast in the same direction as continental shelf waves hence it has a stronger coupling to and an increased frequency response for sea level at periods of a year or more observed coastal sea level fluctuations are not only forced b

airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter tropical discontinuity region of the west african monsoon the impact of a convective system downdraught and associated gravity current on the monsoon and harmattan flow structure as well as on aerosol vertical distribution over the sahel and the sahara is investigated using dropsondes and an airborne lidar complementary ground based and satellite observations as well as european centre for medium range weather forecasts analyses are also used the mission was conducted on 5 june 2006 in the framework of the african monsoon multidisciplinary analysis special observing period the targeted area was the saharan heat low region and mali in particular over which the inter tropical discontinuity was strongly perturbed by the convective system to the north away from the influence of the gravity current the atmosphere exhibited a two layer structure with a well mixed 3 km deep internal boundary layer capped by a stable layer 2 2 5 km deep layer referred to as the saharan aerosol layer sal the aerosol loading in the internal boundary layer as determined by lidar was observed to be much less than in the sal above plumes of dust generated by strong low level winds west of the hoggar as observed in spinning enhanced visible and infra red imager seviri images and possibly injected in the upper sal through orographic lifting contributed to the dust loading of the elevated sal over much of the domain observed by the lidar in the northernmost part of the domain investigated the depth of the sal top was decreased as the result of subsidence enhanced by the presence of an elevated jet associated with the monsoon trough and or related to the return circulation of plain mountain wind system involving the atlas range associated with the gravity current passing over dust sources a large aerosol plume was observed by means of lidar measurements and seviri imagery the plume was seen to reach heights of approximately 3 km above ground level a few tens of kilometres behind the leading edge lidar measurements suggest that a fraction of the dust lifted by the gravity current is mixed into the sal and in some instances injected above the sal

convective snowbands downstream of the rocky mountains in an environment with conditional dry symmetric and inertial instabilities abstract convective snowbands moved slowly over wyoming and northern colorado on 16 17 february 2007 and produced up to 71 mm 2 8 in of snow that was unpredicted by operational numerical weather prediction models and human forecasters the northwest southeast oriented bands lasted for over 6 h comprising both a single major band more than 30 km wide and multiple minor bands about 10 km wide the convective bands initiated within the ascending branch of a secondary circulation associated with both near surface and elevated frontogenesis but the bands remained nearly stationary while the near surface frontogenesis moved quickly equatorward the bands occurred downstream of complex terrain on the anticyclonic shear side of a midlevel jet streak where conditional dry symmetric negative potential vorticity and inertial negative absolute vorticity instabilities were present to determine the mechanisms responsible for the development and organization of these bands simulations using a convectio

decomposing observations of high frequency radar derived surface currents by their forcing mechanisms locally wind driven surface currents 1 the wind impulse response function and transfer function for high frequency radar derived surface currents off southern san diego are calculated using several local wind observations the spatial map of the transfer function reflects the influence of the coast on wind current dynamics near the coast within 20 km from the shoreline the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry meanwhile the amplitude of low frequency currents increases near the coast which is attributed to the local geostrophic balance between cross shore pressure gradients against the coast and currents locally wind driven surface currents are estimated from the data derived response function and their power spectrum shows a strong diurnal peak superposed on a red spectrum similar to the spectra of observed winds current magnitudes and veering angles to a quasi steady wind are typically 2 5 of the wind speed and vary 50 90 to the right of the wind respectively a wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance moreover the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change

sea level response to enso along the central california coast how the 1997 1998 event compares with the historic record abstract long term monthly sea level and sea surface temperature sst anomalies from central california show that during winter months positive anomalies are associated with el nino events and the negative ones with la nina events there is no significant impact on monthly mean anomalies associated with pacific decadal oscillations although there is a tendency for more extreme events and greater variance during positive decadal oscillations the very strong 1997 1998 el nino was analyzed with respect to the long term historic record to assess the forcing mechanisms for sea level and sst beginning in the spring of 1997 we observed several long period 30 days fluctuations in daily sea level with amplitudes of over 10 cm at san francisco california fluctuations of poleward long period alongshore wind stress anomalies awsa are coherent with the sea level anomalies however the wind stress cannot entirely account for the observed sea level signals the sea level fluctuations are also correlated with sea level fluctuations observed further south at los angeles and tumaco columbia which showed a poleward phase propagation of the sea level signal we suggest that the sea level fluctuations were to a greater degree forced by the passage of remotely generated and coastally trapped waves that were generated along the equator and propagated to the north along the west coast of north america however both local and remote awsa can significantly modulate the sea level signals the arrival of coastally trapped waves began in the spring of 1997 which is earlier than previous strong el nino events such as the 1982 1983 event

subtidal currents over the central california slope evidence for offshore veering of the undercurrent and for direct wind driven slope currents abstract in february 1991 an array of six current meter moorings was deployed for one year across the central california outer shelf and slope the main line of the array extended 30 km offshore of the shelf break out to water depths of 1400 m a more sparsely instrumented line displaced 30 km to the northwest extended 14 km offshore though shorter the northern line spanned similar water depths because the gradient of the topography steepened in the northern region a poleward flow pattern typical of the california undercurrent was seen across both lines in the array over most of the year the poleward flow was surface intensified in general the portion of the undercurrent that crossed the southern line had larger amplitudes and penetrated more deeply into the water column than the portion that crossed the northern line transport over the year ranged from 0 to 2 5 sverdrups sv poleward across the southern line 0 to 1 sv poleward across the northern line we suggest the difference in transport was caused by topographic constraints which tended to force the poleward flow offshore of the northern measurement sites the slope of the topography steepened too abruptly to allow the poleward flow to follow isobaths when currents were strong when current velocities lessened a more coherent flow pattern was seen across both lines in the array in general the poleward flow patterns in the undercurrent were not affected by local winds or by the local alongshore pressure gradient nor was a strong seasonal pattern evident rather unexpectedly a small but statistically significant fraction of the current variance over the mid and outer slope was driven by the surface wind stress an alongshelf wind stress caused currents to flow along the slope parallel to the wind field down to depths of 400 m below the surface and out to distances of 2 rossby radii past the shelf break the transfer functions were weak 3 4 cm s per dyn cm 2 but comparable to wind driven current amplitudes of 4 6 cm s per unit wind stress over the middle shelf equatorward alongshelf winds also caused water from 200 300 m over the slope to upwell onto the shelf as the surface water moved offshore

structure of the entrainment zone capping the convective atmospheric boundary layer the authors use large eddy simulation les to investigate entrainment and structure of the inversion layer of a clear convectively driven planetary boundary layer pbl over a range of bulk richardson numbers ri the les code uses a nested grid technique to achieve fine resolution in all three directions in the inversion layer extensive flow visualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion it is found that coherent structures in the convective pbl that is thermal plumes are primary instigators of entrainment in the ri range 13 6 ri 43 8 at ri 5 13 6 strong horizontal and downward velocities are generated near the inversion layer because of the plume interface interaction this leads to folding of the interface and hence entrainment of warm inversion air at the plume s edge at ri 5 34 5 the inversion s strong stability prevents folding of the interface but strong horizontal and downward motions near the plume s edge pull down pockets of warm air below the nominal inversion height these pockets of warm air are then scoured off by turbulent motions and entrained into the pbl the structure of the inversion interface from les is in good visual agreement with lidar measurements in the pbl obtained during the lidars in flat terrain field experiment a quadrant analysis of the buoyancy flux shows that net entrainment flux or average minimum buoyancy flux wu min is identified with quadrant iv w2u1 0 motions that is warm air moving downward plumes generate both large negative quadrant ii w1u2 0 and positive quadrant iii w2u2 0 buoyancy fluxes that tend to cancel the maximum vertical gradient in potential temperature at every x y grid point is used to define a local pbl height zi x y a statistical analysis of zi shows that skewness of zi depends on the inversion strength spectra of zi exhibit a sensitivity to grid resolution the normalized entrainment rate we w where we and w are entrainment and convective velocities varies as ari21 with a 0 2 in the range 13 6 ri 43 8 and is in good agreement with convection tank measurements for a clear convective pbl the authors found that the finite thickness of the inversion layer needs to be considered in an entrainment rate parameterization derived from a jump condition

precipitation banding in idealized baroclinic waves abstractmoist idealized baroclinic wave simulations show the development of precipitation bands from a zonally uniform initial midlatitude jet for a frictionless lower boundary and with no latent heat release or surface heat and moisture fluxes warm advection is strong and a bent back warm front forms although a narrow vertical velocity maximum forms within the area of synoptic scale ascent near the triple point only a wide warm frontal band forms as surface roughness length increases between simulations to that of an ocean then a land surface warm advection is reduced and the cold front becomes stronger relative to the warm front a separate narrow rainband forms along the cold front which is more intense and farther removed from the wide warm frontal band when roughness length is greater in the simulation with roughness length appropriate to the ocean after the narrow band decays the precipitation becomes oriented along the warm conveyor belt in the warm sector when latent heat release is in

climatology of banded precipitation over the contiguous united states abstracta climatology of banded precipitation features over the contiguous united states from 2003 to 2014 is constructed a band is defined as a precipitation feature with a major axis of 100 km or greater and a ratio of major axis length to minor axis length hereafter aspect ratio of 3 1 or greater by applying an automated feature based detection algorithm to composite radar imagery a database of 48 916 844 precipitation features is created of which 7 213 505 14 8 are bands this algorithm produces the first climatology of precipitation bands over the contiguous united states banded precipitation occurrence is broadly similar to total precipitation occurrence with a maximum of 175 h of banded precipitation annually over the ohio river valley in the warm season there is a strong diurnal signature associated with convective storm development for both the precipitation feature area and total area covered by precipitation but little diurnal signature in aspect ratio strong west east gradients

large scale sea level response to atmospheric forcing along the west coast of north america summer 1973 abstract along the west coast of north america the response of sea level to fluctuations in alongshore wind stress at large alongshore scales 1000 km accounted for a substantial faction of the total sea level variance during summer 1973 space time contour plots of sea level and alongshore stress show that the response of sea level to poleward propagating wind stress events was generally stronger than the response to equatorward propagating events atmospheric forcing was most effective in two regions along the coast with relatively strong forcing and response along northern california and oregon and somewhat weaker forcing and response along northern baja california the forced fluctuations in sea level propagated poleward away from these forcing regions causing local sea level to be most correlated with alongshore wind stress earlier in time and at a distant equatorward 1ocation along the southern and central california coast fluctuation in sea level were partly forced along northern baja cali

banded convection caused by frontogenesis in a conditionally symmetrically and inertially unstable environment abstract several east west oriented bands of clouds and light rain formed on 20 july 2005 over eastern montana and the dakotas the cloud bands were spaced about 150 km apart and the most intense band was about 20 km wide and 300 km long featuring areas of maximum radar reflectivity factor of about 50 dbz the cloud bands formed poleward of an area of lower tropospheric frontogenesis where air of modest convective available potential energy was being lifted during initiation and maintenance of the bands mesoscale regions of dry symmetric and inertial instability were present in the region of the bands suggesting a possible mechanism for the banding interpretation of the extant instabilities in the region of the bands was sensitive to the methodology to assess the instability the release of these instabilities produced circulations with enough vertical motion to lift parcels to their lifting condensation level resulting in the observed cloud bands a high resolution numerical weather prediction m

the role of small scale orographic features in the spatial distribution of precipitation radar images from the cevennes experiment which took place during the autumns of 1986 88 on the southeastern flank of the massif central in france reveal a characteristic and repetitive structure of the precipitation distribution in southerly mediterranean flows which are frequent there in autumn precipitation is often organized into narrow bands or plumes oriented south north the period 14 and 15 november 1986 has been selected as an example on which to perform numerical simulations and analyse the mechanism yielding this specific precipitation organization the numerical model correctly reproduces the bands especially at high horizontal resolution 1 km these rainfall bands are focused on the small scale topography oriented east west composed of a succession of ridges and penetrating valleys the bands are generated upstream of these ridges and enhanced on the lee side by convergence created by deflection around the obstacle and penetration of the flow into the valleys idealized simulations and sensitivity experiments have been carried out to verify these hypotheses these simulations suggest that small scale orography plays a decisive role in forcing the precipitation into bands while upstream moisture conditions mainly affect the precipitation intensity and its south north extent copyright 2002 royal meteorological society