early radar echoes from small warm cumulus bragg and hydrometeor scattering abstract studies of small cumulus clouds in florida using x and s band radar 3 and 10 cm wavelengths reveal both hydrometeor and bragg scattering signals turbulent mixing between cloudy and drier environmental air can produce centimeter scale variations in refractive index that can lead to strong mantle echoes around the sides and tops of the clouds when the environmental air is exceptionally dry the s band bragg scattering signals are as strong as 10 dbz at cloud boundaries with weaker echoes in the cloud cores where hydrometeor scattering is also present the bragg signal at s band is typically about 19 db stronger than that at x band as expected from theory however there is in many cases an unexplained bragg like return from the clouds at s band that correlates with the x band echo but is only about 10 db stronger the x band echo is often dominated by backscattering from the cloud droplets and shows adiabatic ascent within the cloud cores fairly often up to at least 1 km above cloud base

relationship between a weakening cold front misocyclones and cloud development on 10 june 2002 during ihop abstract the finescale structure and evolution of a cold front in the presence of small scale circulations are examined using overdetermined dual doppler syntheses of mobile radar data along with thermodynamic data and cloud imagery collected on 10 june 2002 during the international h2o project ihop linear clear air reflectivity maxima and open cellular convection intersect the cold front causing spatial variations in convergence along the front small scale vertical vorticity maxima misocyclones often are coincident with these intersections and are associated with vertical velocity maxima throughout the deployment trajectory analyses indicate that parcels entering the frontal circulation travel predominately in the along front direction during the first hour of the deployment upward motion is nearly continuous along the front consequently parcels remain in regions of upward motion as they move along the front and many eventually ascend to cloud base in response a line of cumulus clouds dev

response of vietnam coastal upwelling to the 1997 1998 enso event observed by multisensor data abstract in this paper we examine the behavior of the vietnam coastal upwelling during the 1997 1998 el nino southern oscillation enso event the baseline is 4 years of national oceanic and atmospheric administration noaa satellite advanced very high resolution radiometer avhrr sea surface temperature sst data taken from 1997 to 2000 comparison of upwelling images to simultaneous ers 2 european remote sensing satellite wind fields indicates that the summer monsoon winds constitute a major generation forcing during the 1997 el nino the monsoon winds enhanced the upwelling and induced the upwelling center to move southward during the 1998 la nina the monsoon winds weakened the upwelling in contrast with the tropical pacific in the study area la nina implies a warm event and el nino a cold event we use empirical orthogonal function eof methods to analyze the spatial and temporal variance of the upwelling the three principal modes account for 37 15 and 8 of the total variance respectively the first eof modes reveal that the sst variance in the north and south subregions underwent a positive negative sign switch in summer 1997 the second eof modes represent the monthly evolution in normal years the third modes seem to be sensitive to the 1998 la nina event simultaneous topex poseidon and ers 2 altimeter data provide further evidence for our analysis comparison with california coastal upwelling and mid atlantic bight mab coastal upwelling indicates that the vietnam coastal upwelling is the most intensive one

a severe frontal rainband part ii tornado parent vortex circulation abstract by means of multiple doppler radar analysis part i established the stormwide hydrodynamic structure as an intense gravity current which advances on a prefrontal low level jet part ii examines the initiation and evolution of a tornado parent vortex circulation in the context of stormwide vorticity production tilting of the prefrontal inflow below the low level jet axis accounts for production of observed vorticity 10 2 s 1 at the surface front the two dimensional flow pattern is modified by the development of an inflectional instability with a wavelength of 13 km breaking or roll up of this instability occurs on a time scale of roughly 500 s and results in small scale vortices along the surface front one such vortex is examined in detail this vortex is the parent circulation of a small tornado the parent vortex structure is determined to be quite similar to those observed in summertime deep convection this suggests a generality to the findings concerning parent vortex initiation and

tidal and atmospheric forcing of the upper ocean in the gulf of california 1 sea surface temperature variability two years of satellite infrared imagery 1984 1986 are used to examine the sea surface temperature sst variability in the northern gulf of california empirical orthogonal functions eofs of the temporal and spatial sst variance for 20 monthly mean images show that the dominant sst patterns are generated by spatially varying tidal mixing in the presence of seasonal heating and cooling these patterns are modified in the fall and winter when shelf temperatures south of tiburon island cool in response to upwelling favorable winds these same winds bring cold dry air from off the continental united states causing local cooling of the shallow northern shelf during the rest of the year the broad shallow shelves are warmer than offshore the seasonally reversing temperature patterns are consistent with recent hydrographic observations which show a cyclonic surface circulation in the summer and a weaker anticyclonic circulation during the rest of the year atmospheric forcing of the northern gulf appears to occur over large spatial scales area averaged ssts for the guaymas basin island region and northern basin show significant fluctuations which are highly correlated these fluctuations in sst correspond to similar fluctuations in the air temperature which are related to synoptic weather events over the gulf during periods of particularly low wind speeds the air temperature over the gulf increases dramatically by afternoon intense heating of the sea surface results in the appearance of warm sst anomalies in the satellite data these ssts are approximately 2 c warmer than surrounding ssts and most likely occur as a result of a spatially varying wind field a regression analysis of the sst relative to the fortnightly tidal range shows that tidal mixing occurs over the sills in the island region as well as on the shallow norther shelf mixing over the sills however occurs as a result of large breaking internal waves or internal hydraulic jumps which mix water over the upper 300 500 m this mixing pumps heat away from the surface deep into the water column there by maintaining the cool ssts since mixing occurs over greater depths in the island region the temperatures there are much colder than those generated by tidal mixing on the shallow shelves resulting in the persistent pool of cool water evident in the satellite data this cooler water is mixed horizontally by the basin scale circulation lowering the ssts over much of the northern gulf these reduced ssts have a large impact on the surface heat flux by lowering the saturation vapor pressure of the air as a result the amount of heat lost to the atmosphere through the latent or evaporative heat flux is reduced this may explain why the gulf of california gains heat on an annual average rather than losing heat as occurs in the mediterranean and red seas where tidal mixing is not significant

summer sea surface temperature variability off vancouver island from satellite data satellite sensed advanced very high resolution radiometer avhrr sea surface temperature sst data over eight summers 1984 1991 were used to analyze the summer sst patterns of variability off the west coast of vancouver island empirical orthogonal function eof analysis of the spatial variance for 133 nearly cloud free summer images was performed the first eof mode which resembled the mean of all images showed a strong cool water band located at the northwest comer of vancouver island a cool tongue extending seaward from the strait of juan de fuca and a warm patch off barkley sound the second mode revealed topographically controlled upwelling cool water over the shelf region with its seaward boundary roughly following the 200 m depth contour plus a cold eddy located just north of the juan de fuca canyon the third mode displayed cool water extending southwestward off brooks peninsula while the fourth mode showed a cool water plume extending off cape scott at the northern tip of vancouver island these four modes accounted for 33 12 10 and 5 of the sst variance respectively the temporal amplitude of these eof modes revealed how the sst features changed as summer progressed from these images we also constructed an overall seasonal coolness index which revealed the summers of 1986 and 1991 to have the coolest coastal water with both summers immediately preceding an e1 nifio

an objective method for computing advective surface velocities from sequential infrared satellite images using cross correlations between sequential infrared satellite images an objective technique is developed to compute advective sea surface velocities cross correlations are computed in 32 x 32 pixel search second image and 22 x 22 template first image windows from gradients of sea surface temperature computed from the satellite images velocity vectors computed from sequential images of the british columbia coastal ocean generally appear coherent and consistent with the seasonal surface current in the region during periods of strong wind forcing as indicated by maps of sea level pressure the image advective velocities are stronger and more coherent spatially and appear to cross surface temperature gradients when winds are weaker the advective velocities correspond better with the infrared temperature patterns suggesting the increased contribution of the geostrophic current to the surface flow velocities determined from coincident near surface drogued 5 10 m buoys positioned every half hour by internal loran c units in mid june show excellent agreement with the image advective velocities in addition conductivity temperature and depth ctd measurements taken during the buoy tracking confirm the homogeneity of the upper 10 m and ctd derived geostrophic currents are consistent with

the role of boundary layer convergence zones and horizontal rolls in the initiation of thunderstorms a case study abstract the initiation of thunderstorms is examined through a combined observational and modeling case study the study is based on doppler radar aircraft mesonet balloon sounding and profiler and photographic data from the convection initiation and downburst experiment cinde conducted near denver colorado the study examines the initiation of a line of thunderstorms that developed along a preexisting quasi stationary boundary layer convergence line on 17 july 1987 the storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present the primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection the critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line the effect was to cause the updrafts in the convergence line

initiation of convective storms at radar observed boundary layer convergence lines abstract the origin of 653 convective storms occurring over a 5000 km2 area immediately east of the colorado rocky mountains from 18 may to 15 august 1984 was examined seventy nine percent of the 418 storms that initiated within the study area occurred in close proximity to radar observed boundary layer convergence lines this percentage increased to 95 when only the more intense storms 60 dbze were considered colliding convergence lines initiated new storms or intensified existing storms in 71 of the cases a new storm took a median time of 24 min to grow to 30 dbz following line collision the convergence lines ranged in length between ten and several hundred kilometers both radar and mesonet stations indicated that the primary convergence was concentrated in a zone 0 5 to 5 km in width these lines were characterized on doppler radar as thin lines of enhanced reflectivity between 0 and 20 dbze and as a line of strong radial or azimuthal gradient in doppler velocity these lines were observed ev

observation and modeling of satellite sensed meanders and eddies off vancouver island abstract a long time series of satellite infrared images spanning the period 21 july 8 october 1980 reveals the evolution of long 160 km and short 80 km wavelength meanders of the california current system off vancouver island midway in the series strong interactions occurred between the two meander scales although the shorter meanders were initially more energetic they were eventually dominated by the longer meanders near the end of the time series detached mesoscale eddies were formed from the longer meanders current meter data from the period before meander growth clearly indicates the presence of a strong vertical shear near 150 m which consists of a southeastward surface current and a northwestward undercurrent it is argued in this paper that the observed shear flow is the energy source for the meanders and eddies seen in the satellite images a four layer quasi geostrophic model is used to represent the california current system off vancouver island the two upper layers represent the

a modeling study of the dryline abstract results of a modeling study of the 24 may 1989 dryline are presented a nonhydrostatic two dimensional version of the colorado state university regional atmospheric modeling system csu rams is used to deduce the impact of east west variability of soil moisture and vegetation on convective boundary layer evolution and dryline formation the effects of the initial moisture and wind fields and the impact of the coriolis force on the model results are also examined model output is compared with special airborne and sounding observations of the 24 may dryline several findings of an earlier observational study of the 24 may dryline are supported in the present study the modeled drylines are broadly comparable to the observed dryline with respect to the following properties 1 width 10 km 2 strong horizontal moisture and virtual potential temperature gradients 4 g kg 1 10 km and 2 k 10 km 3 strong horizontal convergence updraft w e shear of n s wind component 8 10 4 s 1 1 m s 1 10 m

the influence of buoyancy flux from estuaries on continental shelf circulation abstract the release of freshwater from a midlatitude estuary to the continental shelf is modeled numerically as a rossby adjustment problem using a primitive equation model as the initial salinity front is relaxed a first baroclinic mode kelvin wave propagates into the estuary while along the continental shelf the disturbance travels in the direction of coastally trapped waves but with a relatively slow propagation speed when a submarine canyon extends offshore from the estuary the joint effect of baroclinicity and bottom relief provides forcing for barotropic flow the disturbance now propagates along the shelf at the first coastally trapped wave mode phase speed and the shelf circulation is significantly more energetic and barotropic than in the case without the canyon for both the experiments with and without a canyon an anticyclonic circulation generated by the surface outflow and deeper inflow over changing bottom topography is formed off the mouth of the estuary as the deeper inflow enco

the dryline on 22 may 2002 during ihop ground radar and in situ data analyses of the dryline and boundary layer evolution abstract on the afternoon and evening of 22 may 2002 high resolution observations of the boundary layer bl and a dryline were obtained in the eastern oklahoma and texas panhandles during the international h2o project using overdetermined multiple doppler radar syntheses in concert with a lagrangian analysis of water vapor and temperature fields the 3d kinematic and thermodynamic structure of the dryline and surrounding bl have been analyzed over a nearly 2 h period the dryline is resolved as a strong 2 4 g kg 1 km 1 gradient of water vapor mixing ratio that resides in a nearly north south oriented zone of convergence maintained through frontogenesis the dryline is also located within a gradient of virtual potential temperature which induces a persistent solenoidally forced secondary circulation initially quasi stationary the dryline retrogrades to the west during early evening and displays complicated substructures including small wavelike perturbations that travel from south to north at nea

analysis of a dryline during ihop implications for convection initiation abstract a detailed analysis of a dryline that formed on 22 may 2002 during the international h2o project ihop is presented the dryline was classified as a null case since air parcels lifted over the convergence boundary were unable to reach the level of free convection preventing thunderstorms from forming a secondary dryline associated with a distinct moisture discontinuity developed to the west of the primary dryline the primary dryline exhibited substantial along frontal variability owing to the presence of misocyclones this nonlinear pattern resembled the precipitation core gap structure associated with cold fronts during one of the analysis times although the misocyclones were positioned within the gap regions radar refractivity has been recently shown to accurately retrieve the low level moisture fields within the convective boundary layer however its use in forecasting the initiation of convection has been restricted to qualitative interpretations this study introduces the total derivati

satellite derived sea surface temperature variability off california during the upwelling season abstract an empirical orthogonal function analysis of an advanced very high resolution radiometer sea surface temperature data set encompassing the spring summer period during 1991 1993 was used to examine patterns of temperature variability off northern and central california this analysis decomposed the spatial variability of remotely sensed sea surface temperature in 124 images that covered an offshore distance of 200 km and an alongshore distance of 500 km the first mode empirical orthogonal function described 65 of the spatial variability in the data set the spatial amplitude pattern of the first mode which closely resembled the mean of all the images revealed a strong cross shore gradient with much cooler water nearshore and warmer water offshore these areas of cool waters were associated with known upwelling locales and the large scale temperature structure compared favorably to that derived from historical california cooperative oceanographic fisheries investigation calcofi climatology the mode 1 temporal amplitude function representing the temporal modulation of this spatial pattern was coupled with changes in the alongshore component of the coastal wind field interannual variations in the timing of the spring transition were reflected in this temporal amplitude time series the second mode empirical orthogonal function described 11 of the variability in the data set the spatial pattern revealed a north south structure that appeared to be related to regional spatial variations in the strength of upwelling favorable wind the third empirical orthogonal function described only 3 of the data variability and the spatial structure suggested a meandering of the coastal surface flow the results of this investigation indicate that the empirical orthogonal function method was effective at characterizing from the satellite data the dominant nearshore surface properties of the california current system related to upwelling and wind and by extension suggest that it is a suitable approach for remotely investigating other less well known eastern boundary current systems

the relationship between radar reflectivities and clouds associated with horizontal roll convection on 8 august 1982 abstract horizontal convective rolls hcrs and cloudstreets that occurred in northeastern colorado are studied using doppler radar data cloud photography and vertical soundings examination of the environment in which the hcrs form indicates a boundary layer wind speed that is much lower than those measured in previous studies evidence shows that the hcrs form in a well defined boundary layer yet the cloudstreets that develop seem to exist within a stable layer above a relationship between the radar reflectivity echoes and the cloudstreets is firmly established the echoes are aligned directly under the cloudstreets the echoes at lower levels are shown to be caused by convergence of particulate matter into updraft portions of the hcr circulation yet evidence indicates the cloud level echoes are due to backscatter from refractive index inhomogeneities at the surfaces of rising thermals examination of the echoes show that the strongest and tallest echoes are associated with cloud formation and tha

on the use of high resolution satellite data to describe the spatial and temporal variability of sea surface temperatures in the new zealand region a new high resolution 1 1 km regional data set of satellite derived sea surface temperatures sst is developed and validated against drifting and moored buoy data analysis of 5 years of monthly means and related statistics suggests that there is often a coherent relationship between the sst patterns and oceanographic features identifiable relationships are detailed then used to indicate the temporal and spatial characteristics of watermasses fronts and currents for example a gradient analysis suggests that the subtropical front east of new zealand and west of 177 w is bathymetrically locked to the southern flank of the chatham rise near 44 s beyond 177 w it loops southward to a mean position close to 47 s on the northern edge of the bounty plateau adjacent to the subantarctic front near 48 s harmonic analysis of the 60 months of data suggests that spatial variability in the amplitude of the sst annual cycle is related to circulation water mass features and varies from less than 1 c in subantarctic waters to more than 4 c in new zealand nearshore neritic waters the antarctic circumpolar east auckland and east cape currents also display coherent annual cycle amplitude signatures that differ from adjacent water masses however the amplitude of the annual cycle in subantarctic waters south of the chatham rise and north of the pukaki rise and bounty plateau is significantly different from that of this water mass at other similar latitudes an empirical orthogonal function eof analysis of seasonal anomalies shows that 90 of sst variance can be associated with the annual cycle however four eofs are required to explain 50 of the variance in sst monthly anomalies the first of these 25 4 of the variance indicates that anomalies north and east of new zealand are negatively correlated with those in the subantarctic the second eof represents a tasman sea mode and suggests that the tasman sea has been in a warming phase since december 1996 to at least april 1998 this might explain the exceptional size and sign of the sst anomaly in new zealand waters during the 1998 el nino where in february 1998 it reached 3 5 c in northern new zealand waters similar to the amplitude of the annual cycle

finescale vertical structure and evolution of a preconvective dryline on 19 june 2002 abstract high resolution airborne cloud radar data and other international h2o project datasets are used to describe the vertical structure of an unusual prefrontal dryline this dryline observed in northwestern kansas on 19 june 2002 first progressed eastward and tilted toward the west and later became more stationary and reversed its tilt toward the moist side the convective boundary layer cbl depth difference also reversed only in the later phase did the dry side cbl become deeper than on the moist side echo and single dual doppler velocity data in a vertical transect across the dryline suggest a solenoidal circulation dynamically consistent with the observed horizontal buoyancy gradient both this gradient and the solenoidal circulation reversed in the later phase simultaneously confluence toward the dryline increased resulting in an increasing moisture gradient as well as a deepening cbl in the dryline convergence zone it is speculated that the baroclinically generated horizontal vortici

numerical study of precipitation core gap structure along cold fronts abstract the mechanism responsible for the core gap structure of precipitation along narrow cold frontal rainbands ncfrs is investigated through analyses of idealized cloud resolving simulations of cold fronts the control simulation in which the prefrontal thermal stratification is characterized by a weak convective instability at low levels with convective available potential energy cape of 60 j kg 1 reproduces the typical alongfront variability of observed ncfrs the simulated ncfr is broken up into regularly spaced ellipsoidal cores oriented at a clockwise angle to the cold front while horizontal shear instability hsi has frequently been proposed as a mechanism leading to the alongfront variability of ncfrs no characteristic features of hsi are recognized in the simulated vertical vorticity field at the leading edge of the cold front the alongfront variability in precipitation is attributed to the formation of a wavelike disturbance just above the leading edge of the cold front the wave

a wind induced mesoscale eddy over the vancouver island continental slope we use nimbus 7 coastal zone color scanner imagery and direct oceanic measurements to investigate an extensive mesoscale circulation pattern that developed off the coast of vancouver island british columbia in july 1979 particular emphasis is on a well defined cyclonic eddy that formed in 1000 m of water over the central portion of the continental slope characteristic features of the eddy included a core diameter of 25 km a vertical scale of 500 m azimuthal current speeds of 0 20 0 05 m s 1 and an alongshore propagation speed of 1 7 0 6 10 2 m s 1 according to our interpretation the eddy and the attendant mesoscale circulation pattern evolved through mixed barotropic baroclinic instability of a poleward flowing coastal jet that had been generated during a 1 week period of strong 0 10 m s 1 southeast winds formation of the eddy appears to have taken place 5 7 days after onset of the jet and may have been triggered through perturbation of the mean flow by a 0 10 km bump in the continental slope results imply that eddylike circulation observed off the coast of vancouver island is frequently initiated by local alongshore winds rather than through instability of the prevailing california current regime as previously suggested in the literature the results also indicate that thermal imagery often fails to delineate major oceanic surface features off the coast