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The sensitivity of the midlatitude storm track and eddy-driven wind to the sea surface temperature (SST) boundary forcing is studied over a wide range of perturbations using both simple and comprehensive general circulation models over aquaplanet lower boundary conditions. Under the single-jet circulation regime similar to the conditions of the present climate in the Northern Hemisphere winter or the Southern Hemisphere summer, the eddy-driven jet shifts monotonically poleward with both the global mean and the equator-to-pole gradient of the SST. The eddy-driven jet can have a reverse relationship to the gradient if it is well separated from the subtropical jet and Hadley cell boundary in a double-jet circulation regime. A simple scaling is put forward to interpret the simulated sensitivity of the storm-track/eddy-driven westerly wind position within the single-jet regime in both models. The rationale for the scaling is based on the notion that the wave activity flux can propagate horizontally away from the source region, resulting in a broader distribution of eddy potential vorticity (PV) flux in the upper troposphere than that of the flux in the opposite direction in the lower troposphere. As a consequence, the position of the maximum of the eddy-driven westerlies tends to be controlled by the profile of the relatively sharp-peaked low-level PV flux, which is dominated by the eddy heat flux component of the Eliassen-Palm (EP) flux. Thus, the position of the eddy-driven surface westerlies may be inferred from the vertical EP flux coming out of the lower troposphere. The vertical EP flux can be parameterized by a measure of baroclinicity, whose latitudinal variations show a linear relationship with the meridional displacement of the eddy-driven westerlies and the storm track. This relationship still holds well within the single-jet regime, even when only the variation of static stability is taken into consideration in estimating the baroclinicity (the temperature gradient component of which is fixed). To the extent that the static stability is deterministically constrained by and hence can be predicted from the given SST conditions through a moist scaling for the midlatitude stratification, one may, given SST perturbations, predict which way the storm track and eddy-driven wind should shift with respect to a chosen reference climate state. The resultant anomaly-wise scaling turns out to be valid for both the idealized and comprehensive models, regardless of the details in the model physics. By corollary, it can be argued that the poleward shift of storm track found in the global warming simulations by fully coupled climate models may be attributed, at least partially, to the increase in the subtropical and midlatitude static stability with global warming. DOI: 10.1175/2010JAS3477.1

Observational studies have shown that tropospheric zonal mean flow anomalies frequently undergo quasi-periodic poleward propagation, A set of idealized numerical model runs is examined to investigate the physical mechanism behind this poleward propagation. This study finds that the initiation of the poleward propagation is marked by the formation of negative zonal wind anomalies in the Tropics. These negative anomalies arise from meridional overturning/breaking of waves that originate in midlatitudes. This wave breaking homogenizes the potential vorticity (PV) within the region of negative zonal wind anomalies, and also leads to the formation of positive zonal wind anomalies in the subtropics. Subsequent equatorward radiation of midlatitude waves is halted, which results in wave breaking at the poleward end of the homogenized PV region. This in turn generates new positive and negative zonal wind anomalies, which enables a continuation of the poleward propagation. The shielding of the homogenized PV region from equatorward wave propagation allows the model's radiative relaxation to reestablish undisturbed westerlies in the Tropics, while extratropical westerly anomalies arise from eddy vorticity fluxes. The above process indicates that the poleward zonal mean anomaly propagation is caused by an orchestrated combination of linear Rossby wave propagation, nonlinear wave breaking, and radiative relaxation. The importance of the meridional wave propagation and breaking is consistent with the fact that the poleward propagation occurs only in the parameter space of the model where the PV gradient is of moderate strength. Implications for predictability are briefly discussed.

Winds at the Salt Lake City International Airport (SLC) during the April-October period from 1948 to 2003 have been observed to shift to the north (up-valley direction) between late morning and afternoon on over 70%, of the days without precipitation. Lake-breeze fronts that develop as a result of the differential heating between the air over the nearby Great Salt Lake and that over the lake's surroundings are observed at SLC only a few times each month. Fewer lake-breeze fronts are observed during late July-early September than before or after that period. Interannual fluctuations in the areal extent of the shallow Great Salt Lake contribute to year-to-year variations in the number of lake-breeze frontal passages at SLC. Data collected during the Vertical Transport and Mixing Experiment (VTMX) of October 2000 are used to examine the structure and evolution of a lake-breeze front that moved through the Salt Lake Valley on 17 October. The onset of upslope and up-valley winds occurred within the valley prior to the passage of the lake-breeze front. The lake-breeze front moved at roughly 3 m [s.sup.-1] up the valley and was characterized near the surface by an abrupt increase in wind speed and dewpoint temperature over a distance of 3-4, km. Rapid vertical mixing of aerosols at the top of the 600-800-m-deep boundary layer was evident as the front passed.

The dynamical equations of atmospheric flow are written using potential vorticity as the meridional coordinate and potential temperature as the vertical coordinate. Within this system, the atmosphere is divided into undulating tubes bounded by isentropic and constant potential vorticity surfaces, and, under adiabatic and frictionless conditions, the air moves through the tubes without penetrating through the walls. A model that uses this system of coordinates incorporates as built-in both the dry convective adjustment and barotropic adjustment processes, which prevent the folding of isentropic and potential vorticity surfaces but crudely represent the effects of interactions with the mean flow. The Eliassen--Palm flux vector in this frame of reference yields a new interpretation of the eddy momentum transport. Eddy momentum exchange is through the form drag created by pressure forces exerted on the potential vorticity-potential temperature tubes.

Unique facility of measuring vertical winds using Indian mesosphere--stratosphere--troposphere (MST) radar along with horizontal winds enables the study of the atmospheric circulation over Gadanki, India. Several important features are noted while analyzing the wind field. A tropical easterly jet stream of 35 m [s.sup.-1] strength is seen around 16 km during monsoon season. Relatively strong jetlike northward motion (southerlies) of 5-7 m [s.sup.-1] is seen around 14 km during winter months. These two maxima in zonal and meridional wind patterns, even though they differ in strength greatly, occur in two contrasting seasons. Downward motion combined with upper-level northward and lower-level southward motion observed during winter in normal years indicates the signature of tropical Hadley circulation over the study region. During the 1997/98 El Nino event, however, an anomalous pattern of winds is seen and Hadley circulation is observed to be weakened.

Coastally trapped wind reversals along the U.S. west coast, which are often accompanied by a northward surge of fog or stratus, are an important warm-season forecast problem due to their impact on coastal maritime activities and airport operations. Previous studies identified several possible dynamic mechanisms that could be responsible for producing these events, yet observational and modeling limitations at the time left these competing interpretations open for debate. In an effort to improve our physical understanding, and ultimately the prediction, of these events, the Office of Naval Research sponsored an Accelerated Research Initiative in Coastal Meteorology during the years 1993-98 to study these and other related coastal meteorological phenomena. This effort included two field programs to study coastally trapped disturbances as well as numerous modeling studies to explore key dynamic mechanisms. This paper describes the various efforts that occurred under this program to provide an advancement in our understanding of these disturbances. While not all issues have been solved, the synoptic and mesoscale aspects of these events are considerably better understood.

This study utilized conditional sampling to identify three frequent wind regimes in the lower Sacramento Valley. The major flow features of the mean diurnal wind patterns in the southern Sacramento Valley and surrounding areas were analyzed for each wind regime. Afternoon wind directions at a pivotal observing site (Davis, CA) in the south-central part of the valley were used to classify the regimes as south wind (marine air intrusion), north wind (no marine air intrusion), and transitional wind days. In the summer of 1991, these occurred 72%, 14%, and 14% of the days, respectively. Daily data from 21 surface observing stations were segregated by wind regime, then averaged for quarters of the day to produce wind roses grouped by regime and time of day. These data were then plotted on a base map. The most frequent direction in each of these wind roses was used to construct streamlines for the area by quarter of the day for each regime. These analyses provide a climatology of the diurnal variation of the average wind flow for each of these frequent flow regimes, providing a wind climatology with greater spatial and temporal resolution than those in extant publications. These analyses are especially useful for evaluating transport patterns of air pollutants or contaminants.

The usefulness of any simulation of atmospheric tracers using low-resolution winds relies on both the dominance of large spatial scales in the strain and time dependence that results in a cascade in tracer scales. Here, a quantitative study on the accuracy of such tracer studies is made using the contour advection technique. It is shown that, although contour stretching rates are very insensitive to the spatial truncation of the wind field, the displacement errors in filament position are sensitive. A knowledge of displacement characteristics is essential if Lagrangian simulations are to be used for the inference of airmass origin. A quantitative lower estimate is obtained for the tracer scale factor (TSF): the ratio of the smallest resolved scale in the advecting wind field to the smallest 'trustworthy' scale in the tracer field. For a baroclinic wave life cycle the TSF = 6.1 +/- 0.3 while for the Northern Hemisphere wintertime lower stratosphere the TSF = 5.5 +/- 0.5, when using the most stringent definition of the trustworthy scale. The similarity in the TSF for the two flows is striking and an explanation is discussed in terms of the activity of potential vorticity (PV) filaments. Uncertainty in contour initialization is investigated for the stratospheric case. The effect of smoothing initial contours is to introduce a spinup time, after which wind field truncation errors take over from initialization errors (2-3 days). It is also shown that false detail from the proliferation of finescale filaments limits the useful lifetime of such contour advection simulations to approximately 3(sigma)(super -1) days, where (sigma) is the filament thinning rate, unless filaments narrower than the trustworthy scale are removed by contour surgery. In addition, PV analysis error and diabatic effects are so strong that only PV filaments wider than 50 km are at all believable, even for very high-resolution winds. The minimum wind field resolution required to accurately simulate filaments down to the erosion scale in the stratosphere (given an initial contour) is estimated and the implications for the modeling of atmospheric chemistry are briefly discussed.

A study was conducted to analyze lithogenic particle flux and grain size distributions from sediment trap deployments to determine short-term differences in flux and transport of lithogenic matter off northwest Africa. Results indicated a dominant change of dust transport in the trade winds during winter/spring and transport in the Saharan Air Layer at the Cape Verde site. Off Cape Blanc, there was a fast and mostly undisturbed downward transport of particulate matter.

It is shown that a vertically sheared inviscid stratified flow over a sinusoid, starting from rest, results in one of two responses, both nonsteady: 1) resonance modes, in which the phase lines are vertical, and wave amplitude and drag grow linearly with time; or 2) oscillatory (in time) models, with one or more frequencies, phase lines sloping alternately upwind and downwind, momentum flux alternately negative and positive, and drag alternately positive and negative. Flow over an isolated ridge will excite all of the foregoing modes. The resonant modes are the familiar trapped waver; oscillatory modes will tend to be evident or dominant only when the ridge is broad and/or the Richardson number low.

Atmospheric transport and dispersion over complex terrain were investigated. Meteorological and sulfur hexafluoride ([SF.sub.6]) concentration data were collected and used to evaluate the performance of a transport and diffusion model coupled with a mass consistency wind field model. Meteorological data were collected throughout April 1995. Both meteorological and plume location and concentration data were measured in December 1995. The meteorological data included measurements taken at 11-15 surface stations, one to three upper-air stations, and one mobile profiler, A range of conditions was encountered, including inversion and postinversion breakup, light to strong winds, and a broad distribution of wind directions. The models used were the MINERVE mass consistency wind model and the SCIPUFF (Second-Order Closure Integrated Puff) transport and diffusion model. These models were expected to provide and use high-resolution three-dimensional wind fields. An objective of the experiment was to determine if these models could provide emergency personnel with high-resolution hazardous plume information for quick response operations. Evaluation of the models focused primarily on their effectiveness as a short-term (1-4 h) predictive tool. These studies showed how they could be used to help direct emergency response following a hazardous material release. For purposes of the experiments, the models were used to direct the deployment of mobile sensors intended to intercept and measure tracer clouds. The April test was conducted to evaluate the performance of the MINERVE wind field generation model. It was evaluated during the early morning radiation inversion, inversion dissipation, and afternoon mixed atmosphere. The average deviations in wind speed and wind direction as compared to observations were within 0.4 m [s.sup.-1] and less than 10 [degrees] for up to 2 h after data time. These deviations increased as time from data time increased. It was also found that deviations were greatest during inversion dissipation. The December test included the release and tracking of atmospheric tracers. The MINERVE-SCIPUFF modeling system was used to direct remote sensing equipment. Posttest analyses were performed to determine model reliability. It was found that plume centroid position as determined by the models was within 10% of the observed plume centroid.

Atmospheric dispersion code system SPEEDI (System for Prediction of Environmental Emergency Dose Information) has been applied to simulate the field experiments conducted over a complex terrain. A diagnostic mass-consistent wind field model of the code system simulates the flow over an isolated hill using the routinely measured data from sodars and a meteorological tower. An objective basis for the adjustment of the horizontal and vertical wind components has been incorporated in the model, and the results show a great improvement in modeling the flow past the hill. Calculated profiles of the vertical velocity component around the hill have been compared with those observed by the sodars. The model streamlines show close agreement with the tetroon trajectory and the ground-level concentration patterns. Dispersion calculations are carried out using a Lagrangian particle random walk model. The dispersion algorithm is modified in order to utilize the observed turbulence data instead of the conventional Pasquill-Gifford method, and the former scheme performs better in simulating the concentration distribution. Results suggest that the accuracy of the code system improves significantly when all these changes are introduced.

Recent satellite observations suggest that convection over the tropical continents is capable of exciting wave motions over a wide range of spatial and temporal scales. An equatorial beta-plane model was used to investigate the forcing by convective heating of equatorial waves with zonal wavenumbers from 1 to 15 and a wide range of periods, including diurnal oscillations. Also studied are the propagation of these waves in the equatorial middle atmosphere and their role in driving the tropical semiannual oscillation (SAO). Specification of the heating distribution used to force the model is guided by observations and analyses of tropical convection. It was found that intermediate-scale Kelvin and inertia-gravity waves provide between 25% and 50% of the forcing necessary to drive the westerly phase of the SAO near the stratopause, while the remainder is supplied by planetary-scale Kelvin waves. In the mesosphere, intermediate-scale waves account for an even larger fraction of the force required to drive the westerly phase and they are solely responsible for driving the easterly phase. The resulting SAO agrees well with ground-based and satellite observations in both the stratosphere and mesosphere. The dependence of the simulated SAO on various model parameters has also been explored. A simulation wherein only planetary-scale waves (k = 1-3) are included yields a weaker than observed stratopause oscillation and fails to produce a mesospheric oscillation. If the full range of zonal wavenumbers (k = 1-15) is included but the diurnal component of the forcing is omitted, the stratopause oscillation is again weaker than observed, while the amplitude of the mesospheric oscillation is greatly diminished. These results suggest that strong excitation of intermediate-scale equatorial waves depends on the diurnal cycle of convection and that the waves thus excited play an important role in the forcing of the tropical semiannual oscillation.

The third intensive observation period (IOP3) of PYREX was a case of strong lee waves generated by a southerly wind crossing the Pyrenees chain. Upstream radiosounds and measurements obtained by aircraft along the chain transect and by constant volume balloons launched near the crest provided spatial characteristics of the lee wave at different times and heights. Values ranging from 7 to 14 km for the horizontal wavelength, and from 3 to 5 m [s.sup.-1] for the maximum amplitude of the air vertical velocity, were observed. The lee wave horizontal extent, measured from the crest line, reached 30 to 55 km. In addition, two very high frequency stratosphere-troposphere (VHF ST) radars, one on the mountain mean axis and another downstream in the lee wave field, observed the temporal variations of the vertical profiles of the vertical velocity. The analysis of those observed variations and their vertical distribution allowed the stationarity of the wave to be studied. The lee wave was found to be far from stationary during its lifetime, but there were some periods, never longer than 1.5 h, during which the wave was quasi-stationary. Data obtained by the airborne instruments revealed that the wavelength, amplitude, and downstream wave extent underwent temporal variations. The time evolution of the vertical velocity profile observed by the radar below the wave field revealed that, most of the time, the lee wave was trapped, which was found to be consistent with radiosounding data. In addition, favorable comparison between airborne and radar data added further evidence that direct VHF ST radar measurement of the vertical air motion induced by lee waves is not seriously affected by beam tilting.

The surface wind pattern over the ice sheets of Antarctica is irregular with marked areas of airflow confluence near the coastal margins. Where cold air from a large interior area of the ice sheet converges (a confluence zone), an anomalously large supply of air is available to feed the coastal katabatic winds, which, as a result, are intensified and more persistent. The confluence zone inland of Siple Coast, West Antarctica, differs from its East Antarctic counterparts in that the terrain slopes become gentler rather than steeper as the coast is approached. In addition, synoptic processes exert substantially more impact on the behavior of the surface winds. A month-long field program to study the dynamics of the springtime katabatic wind confluence zone has been carried out near Siple Coast. Two sites, Upstream B (83.5 [degrees] S, 136.1 [degrees] W) and South Camp (84.5 [degrees] S, 134.3 [degrees] W), were established roughly perpendicular to the downslope direction. The field program involved the use of the ground-based remote sensing equipment (sodar and RASS) along with conventional surface and balloon observations. Previous analyses revealed the cross-sectional structure of the confluence zone as consisting of a more buoyant West Antarctic katabatic airflow overlying a less buoyant katabatic airflow originating from East Antarctica. The force balances inside the confluence zone are here investigated for three situations: mean (all available wind profiles from balloon launches), and two extreme cases (light and strong winds). A linear regression method is used to estimate the mean vertical wind shears and horizontal temperature gradients. The vertical wind shears are used to examine whether or not the airflows are in geostrophic balance. The results are 1) the airflow above the surface at both sites is in geostrophic balance for the three situations; 2) inside the West Antarctic katabatic wind zone, there are three forces in the north-south direction - the restoring pressure gradient force associated with blocking of the katabatic and synoptic winds, the downslope buoyancy force, and the synoptic pressure gradient force associated with the time-averaged low in the South Pacific Ocean; 3) above the West Antarctic katabatic wind layer, the observed easterly wind is due to the synoptic pressure gradient force associated with the low; 4) inside the East Antarctic katabatic wind zone, in addition to the above three forces, there is the downslope buoyancy force associated with the inversion; and 5) large-scale transient synoptic systems strongly influence the downslope wind speed and the boundary layer depth, resulting in the light and strong wind cases.

The evolution of summer katabatic wind events over the steep slopes of Adelie Land is examined, with emphasis on the sudden cessation of these events. Different idealized large-scale forcings are considered, including a situation that comes very close to one observed during the IAGO (Interaction Atmosphere Glace Ocean) campaign, held in the region in November-December 1985. The hydrostatic meso-[Gamma]-scale atmospheric model MAR (Modele Atmospherique Regional) is used to assess the sensitivity of the simulated cessation process to a prescribed large-scale forcing.

As a characterization of the variability of observed geopotential height fluctuations, their probability density function (PDF) and its skewness are studied in the global domain for winter and summer. The PDF of the geopotential, [Phi], is skewed toward low anomalies at midlatitudes and toward high anomalies in polar and tropical regions. When [Phi] is filtered spatially by discarding planetary wavenumbers less than [approximately]8, the skewness is small in the Tropics and in polar regions and large and negative in zonal bands approximately centered on the latitudes of the climatological jets. Where the skewness is large and negative, the zonally averaged PDF of [Phi] has an approximately exponential tail for negative anomalies. From a diagnostic study based on computing [Phi] from the observed winds through the balance equation, the negative skewness bands can clearly be attributed to the rectification of near-symmetric velocity fluctuations by the advective nonlinearity. This mechanism implies that where winds are highly variable, large synoptic-scale negative [Phi] anomalies are more likely than large positive [Phi] anomalies. The maximum of the (negative) zonally averaged skewness in the summer hemisphere tends to be larger than that in the winter hemisphere, and in both hemispheres these maxima lie [approximately]150 mb below the velocity variance maxima. The fact that skewness extrema do not precisely match maxima in the nonlinearities is attributed to asymmetries in the winds themselves. Interactions of the velocity fluctuations with the mean flow have a small but observable effect in modulating the skewness. The subtle dependence of the skewness on key flow parameters is illustrated through an analytic model for idealized fluctuations on a beta plane. General expressions for the PDF, its asymptotic form, criteria for the presence of exponential tails, and the generic dependence of the skewness on a generalized Rossby number are derived. For the case of a [Delta]-function velocity spectrum, closed-form expressions for the PDF and skewness are obtained and compared to observations.

The effect of the synoptic-scale wind on the predictability of landscape-induced atmospheric flow as a function of surface characteristics is studied by means of over 200 two-dimensional numerical simulations. The predictability measure is defined based on the domain-averaged signal to noise ratio. The decomposition of the signal into two different parts is also used to further understand the effect of the synoptic-scale wind on the mesoscale atmospheric predictability. It is emphasized that different predictability measures should be used for different purposes, and different predictability results can be obtained based on different measures. For the cases of surface heat patches with patch sizes from L = 0 to 150 km and constant large-scale mean winds of U = 0, 1, 3, 5, 7, and 9 m [s.sup.-1] , it is found that the potential temperature [Theta], the deviation of [Theta] from its horizontally domain-averaged value [Theta][prime], the horizontal velocity u, and u[prime] are all predictable at any time during the day when L = 10 to 150 km, regardless of the mean wind. In contrast to [Theta] and u, the predictability of the vertical velocity w (which is the same as w[prime]) at any time during the day is a very complicated function of both the mean wind U and the patch size L for all cases. However, based on the temporal- and domain-averaged signal to noise ratio during the day, w is predictable when L = 10 to 100 km. For the cases of sinusoidal hills with horizontal wavelengths of L = 2 to 100 km, hill heights of h = 0.2 to 2 km, and large-seale mean winds of U = 0, 3, and 7 m [s.sup.-1], it is found that [Theta], [Theta][prime], u, and u[prime] are all predictable at any time during the day for cases of L = 60 and 100 km, regardless of the mean wind. It is also found that w and w[prime] are predictable at any time during the day for cases of L = 60 and 100 km and h = 0.6 to 2 km.

The partial differential equation set for the horizontally homogeneous nocturnal boundary layer under first-order closure is discretized and truncated to a two-layer system. This system can be treated as a coupled four-layer ordinary differential equation set. Using techniques of nonlinear dynamics, including numerical continuation and nonlinear stability analysis, characteristics of the solutions are developed. The bifurcation diagrams show classic S-shaped behavior so that the equations support multivalued solutions for certain values of external parameters. Both stable and unstable solution regimes exist with multiple, stable limit points. The results have strong implications for the predictability of the stable boundary layer in that even slight changes in initial conditions (or perturbations) would lead to quite different solutions in terms of temperatare and wind speed for the regions of multivalued solutions. Practically, this means that predictions of frost or pollution dispersion may not be made with confidence for certain parameter regimes. If this type of behavior holds in the full partial differential equation set, it also means that additional physics or numerical sophistication in models will not improve the prediction of winds or temperature.

The theory of mountain waves is usually discussed for the case of a steady background wind. Here, the consequences of a superimposed diurnal (or other periodic) background wind variation are considered in outline. They are found to be sufficiently complicated as to warrant avoidance in detailed case studies. They include, however, the production of freely propagating waves and may be of interest on that account for other purposes.

The movement of a symmetric vortex embedded in a resting environment with a constant planetary vorticity gradient (the beta drift) is investigated with a shallow-water model. The authors demonstrate that, depending on initial vortex structure, the vortex may follow a variety of tracks ranging from a quasi-steady displacement to a wobbling or a cycloidal track due to the evolution of a secondary asymmetric circulation. The principal part of the asymmetric circulation is a pair of counterrotating gyres (referred to as beta gyres), which determine the steering flow at the vortex center. The evolution of the beta gyres is characterized by development/decay, gyration, and radial movement. The beta gyres develop by extracting kinetic energy from the symmetric circulation of the vortex. This energy conversion is associated with momentum advection and meridional advection of planetary vorticity. The latter (referred to as 'beta conversion') is a principal process for the generation of asymmetric circulation. A necessary condition for the development of the beta gyres is that the anticyclonic gyre must be located to the east of a cyclonic vortex center. The rate of asymmetric kinetic energy generation increases with increasing relative angular momentum of the symmetric circulation. The counterclockwise rotation of inner beta gyres (the gyres located near the radius of maximum wind) is caused by the advection of asymmetric vorticity by symmetric cyclonic flows. On the other hand, the clockwise rotation of outer beta gyres (the gyres near the periphery of the cyclonic azimuthal wind) is determined by concurrent intensification in mutual advection of the beta gyres and symmetric circulation and weakening in the meridional advection of planetary vorticity by symmetric circulation. The outward shift of the outer beta gyres is initiated by advection of symmetric vorticity by beta gyres relative to the drifting velocity of the vortex.

The evolution of zonal wind and zonal wavenumber one (wave 1) in the Southern Hemisphere subpolar middle atmosphere is described for the period December 1978-May 1979 using temperature and ozone measurements from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. In late December maximum zonal easterlies of approximately -70 m s to the -1 are observed at 0.1 mb, 60 degrees S. A zonal flow reversal occurs during late February and westerlies subsequently increase to 60-70 m s to the -1 in the upper stratosphere by April-May. LIMS zonal winds are compared with rocketsonde measurements and nadir sounder (derived) winds for summer and autumn. Although quantitative agreement is found at stratospheric levels, substantial discrepancies are evident in the mesosphere, most likely a reflection of sampling and resolution differences in the respective datasets. Stationary and traveling wave 1 temperature disturbances (amplitudes approximately 1-2 K at 60 degrees S) are observed by LIMS during summer. The stationary wave is confined to the lower stratosphere near the level of zero zonal-mean wind flow, whereas the traveling wave is prominent in the middle stratosphere, moves west at a rate similar to the zonal-mean wind, and exhibits a vertical-meridional structure similar to a P(super 1)(sub 4) normal mode Rossby wave. A substantial intensification of wave 1 activity occurs during autumn (amplitudes approximately 5-10 K), which is found to be associated with an upward-directed Eliassen-Palm flux near the subpolar tropopause level. Evidence relating wave 1 activity in the lower-middle stratosphere to the occurrence of zonal ozone perturbations of 10%-20% amplitude is presented for summer and autumn.

Observations from the German Front Experiment are presented here that show the existence - in conditions with a dominant flow component parallel to the main Alpine chain - of a mesoscale region to the north of the Alps where the absolute and potential vorticity (PV) are substantially negative. These structures exist before the front arrives to the Alps and appear to be affected little by the passage of the front. A dynamical explanation for these and other mesoscale structures is sought by considering a simple unsheared airflow impinging on the Alps from the west. A linear frictionless model for the steady-state response is used as well as a full nonlinear numerical model with and without friction. A vastly simplified Alpine orography is considered as well as one that adequately describes its mesoscale detail. The results show that the frictionless linear dynamics lead to a zone north of the Alps with anticyclonic vorticity but with uniform (positive) potential vorticity. With boundary-layer processes included in a nonlinear simulation substantial PV anomalies are produced. This leads to negative PV, and absolute vorticity, north of the Alps and positive PV south of the Alps. The region of PV anomalies in the model bears a suggestive similarity to that in the observations. The PV structures are attributed to frictional processes acting in a boundary layer that acquires a slope due to the sloping mountain sides. This mechanism only operates in this situation. Other mesoscale aspects of the flow are discussed in regions around the Alps for which we have as yet no detailed observational evidence; for example, there is strong flow retardation immediately downstream of the orography. An important conclusion is that the Alps, in conditions of parallel flow, are a significant source of potential vorticity anomalies in the lower troposphere. These are advected away from the orography and must be an important part of the tropospheric PV budget.

A series of idealized simulations using a nonhydrostatic cloud model is used to investigate the genesis of bow echoes (a bow-shaped system of convective cells that is especially noted for producing long swaths of damaging surface winds). It is hypothesized that severe, long-lived bow echoes represent a dynamically unique form of mesoconvective organization being produced for a restricted range of environmental conditions, including a convective available potential energy (CAPE) of at least 2000 m squared s to the -2 and vertical wind shears of at least 20 m s to the -1 over the lowest 2.5-5 km AGL. The key structural features include a 40-100-km-long bow-shaped segment of convective cells, with a strong rear-inflow jet extending to the leading edge of the bow at 2-3 km AGL, and cyclonic and anticyclonic eddies (referred to as 'bookend' vortices) on the northern and southern flanks of the bowed segment, respectively. This structure characteristically develops three to four hours into the lifetime of a convective system and may remain coherent for several hours. The evolution of this coherent structure occurs systematically as the convectively produced cold pool strengthens over time, eventually producing a circulation that overwhelms the ambient shear. This forces the convective cells to advect rearward above the cold air and weaken. The horizontal buoyancy gradients along the back edge of these rearward-advecting cells subsequently generate an elevated rear-inflow jet that extends to near the leading edge of the cold pool. The circulation of this jet helps negate the circulation of the cold pool, reestablishing deep, forced lifting at the leading edge of the system. This elevated rear-inflow jet is also enhanced through the development of bookend vortices. Such vortices are produced at the ends of a convective line segment as vortex lines inherent in the ambient vertically sheared environment are first tilted upward by the convective updrafts and then tilted downward and stretched by the convective downdrafts. The development of these features requires both large amounts of CAPE and strong vertical wind shear in the environment of these systems, as is consistent with the observed environments of many severe, long-lived bow echoes.

The formation and diurnal evolution of the dryline during fair weather have been investigated through the use of a two-dimensional mesoscale model that includes condensation/evaporation, budget equations of surface energy and moisture field, as well as turbulence and radiation parameterizations. A moderately strong, vertical wind shear was introduced on a sloping terrain, where the soil is very dry on the west side but moist on the east. Initially, a weak easterly geostrophic wind exists to the east but a weak westerly geostrophic wind to the west of the dryline. During daytime, deepening of the mixed layer due to vertical mixing, especially on the west side, forces the dryline to advance eastward. The westerly wind on the west side is maintained by downward transport of westerly momentum due to strong vertical mixing; the easterly wind on the east side is strengthened due to the inland sea-breeze circulation. Therefore, the resulting low-level convergence sustains a strong moisture gradient along the dryline. During the night, convection near the ground ceases, and a rapid surface cooling to the west of the dryline changes the pressure gradient force near the surface, which is able to maintain a weak westerly wind there. In contrast, a strong easterly wind persists on the east side and forces the dryline to retrograde until sunrise. The strong moisture gradient was also maintained by the low-level convergence. Sensitivity tests show that the most important factors in sustaining a strong moisture gradient along the dryline are the low-level wind shear, the sloping terrain, and the horizontal soil moisture gradient, in that order. Under favorable conditions, a very weak moisture gradient zone can develop into an intensive dryline within 36 hours.

Spatially resolved wind fields are derived by cross correlation of aerosol backscatter data from horizontal and vertical scans of the University of Wisconsin volume imaging lidar during the 1997/98 Lake-Induced Convection Experiment. Data from three cases are analyzed. The first two cases occurred on 10 and 13 January 1998 during cold-air outbreaks. Horizontal scans at 5 m above the lake reveal cellular structure of the steam fog. Vector winds are derived with 250-m spatial resolution over 60 and 36 [km.sup.2] areas. These wind fields show acceleration and veering of offshore flow in the convective internal boundary layer along the upwind edge of Lake Michigan. The wind fields are used to compute divergence and vorticity. Effects of shoreline shape and topography are evident in the data. Horizontal wind speeds derived from vertical scans show the effects of convection on the vertical distribution of momentum. In the third case, 21 December 1997, a well-defined, shallow density current flowing offshore at [approximately equals]1 m [s.sup.-1] is observed in the presence of larger-scale (3-4 m[s.sup.-1) onshore flow. Winds on both sides of the land-breeze boundary as well as the three-dimensional structure of the event were recorded and analyzed.

In this paper, the atmospheric circulations on an equatorial beta plane in response to steady tropical heating are investigated by analytically solving a set of linear equations. Special emphasis is placed on the horizontal structure of forced response under the different combinations of momentum damping and thermal damping, as well as the effect of the zonal domain on the forced responses. Two zonal domains are considered: a zonally cyclic domain and a zonally unbounded domain. The linear model is decomposed in terms of the vertical eigenfunctions in a vertically semi-infinite domain. A new feature of the solution is the existence of a continuous spectrum corresponding to energy propagation out the top of the troposphere. The resulting shallow-water equations are then solved using a method similar to that of Gill. Since the zonal decay scale is proportional to the inverse of the square root of the product of the Rayleigh friction rate and the Newtonian cooling rate, the solutions in a zonally unbounded domain can be good approximations for the solutions in a zonally cyclic domain only when both Rayleigh friction and Newtonian cooling are large enough. When either Rayleigh friction or Newtonian cooling is very weak, the solutions are essentially zonally uniform regardless of the longitudinal location of the heat source in a zonally cyclic domain except in a very narrow zone along the equator. The characteristic meridional scale of the shallow-water system is the equatorial radius of deformation of the shallow-water system multiplied by the fourth root of the ratio between the Rayleigh friction rate and the Newtonian cooling rate. Therefore, the characteristic meridional scale is very large for the Rayleigh friction--dominant case, and the forced response can extend far outside the heating latitude. In contrast, in the Newtonian cooling-dominant case the characteristic meridional scale is very small and the forced response is confined to the heating latitudes. The implications of these solutions for both the thermally driven surface winds and the zonally uniform low-frequency variation in pressure and temperature in the upper half of the tropical troposphere are also discussed.

Eastward propagating planetary waves of zonal wavenumber one in the zonal wind (u) with phase speeds in the range of 1-10 m [s.sup.-1], and also with frequencies in the 30-60-day range, are studied using 39 boreal winter (austral summer) seasons (each of length 180 days) from the reanalyses of the National Centers for Environmental Prediction. The purpose of the paper is to study the relationship between these low phase speed waves in the extratropics (which are candidates for instabilities) and the low-frequency tropical waves associated with the Madden-Julian oscillation (MJO). Planetary waves dominate the zonal wavenumber spectrum of all (eastward plus westward) transient fluctuations with phase speeds of 1-10 m [s.sup.-1] at upper levels. Using the theory of Y. Hayashi to separate out standing oscillations, it is found that eastward propagating waves for zonal wavenumber one have variance maxima at 63 [degrees] N, 32 [degrees] N, 13 [degrees] N, 32 [degrees] S, and 52 [degrees] S at upper levels. As a percentage of the total variance for phase speeds 1-10 m [s.sup.-1] and zonal wavenumber one, the eastward propagating waves have strong maxima at 200 hPa in the Tropics (13 [degrees] N, 13 [degrees] S) and at lower levels at 13 [degrees] S (indicative of the MJO). The standing wave variance is maximum in northern midlatitudes. The eastward propagating wave variance for zonal wavenumber two has similar properties. For zonal wavenumber one, eastward propagating waves at 52 [degrees] N and 300 hPa are highly coherent with mid-and upper-level waves at 32 [degrees] N, with a nearly perfectly out-of-phase relationship. For a base point at 32 [degrees] N, 200 hPa, we find a strong coherence maximum for phase speeds of 1-10 m [s.sup.-1] (coherence squared greater than 0.7) with upper levels in the Tropics (13 [degrees] N), accompanied by a 180 [degrees] relative phase shift. Coherence and phase plots with a base point at 13 [degrees] N and 200 hPa show not only strong coherence with waves at 32 [degrees] N, but also with waves at 13 [degrees] S at 700 hPa. While results for the MJO averaging (corresponding to periods of 30-60 days) are generally similar, there is in addition a strong cross-equatorial coherence between fluctuations at 13 [degrees] N and 13 [degrees] S at 200 hPa, and stronger coherence with the lower tropical troposphere. The strong coherence between waves in the subtropics (32 [degrees] N) and the Tropics (13 [degrees] N) indicates a potential role for dynamical instability in the organization of the MJO. Coherence and phase diagnostics for base points in the Southern Hemisphere have generally the same character, although the subtropical-tropical coherence is less dramatic (but still significant). Coherence and phase results for eastward propagating zonal wavenumber two are generally similar. These results represent, for midlatitudes, an extension of earlier work of C. R. Mechoso and D. L. Hartmann to phase speeds less than 10 [m s.sup.-1], for which an interpretation in terms of baroclinic instability becomes viable; a much larger dataset is also used here. The strong coherence found between the subtropics and Tropics lend support to the notion that planetary wave baroclinic instability and the MJO are connected with each other, as suggested by J. S. Frederiksen and C. S. Frederiksen. An origin of the MJO in which subtropical jet instability helps to organize tropical convection is suggested.

Although an El Nino-Southern Oscillation (ENSO) event is a coupled ocean-atmosphere phenomenon (1), some stages of ENSO events can be examined usefully if one imposes the behavior of one fluid [...]

This study examines the effect of temporal variations in either location or structure of both straight and curved jets on their associated divergence patterns. For each of these jets the time-dependent geopotential height field is prescribed analytically, from which the geostrophic and ageostrophic wind fields are extracted. The divergence fields are calculated separately for the steady state (which was commonly assumed in studies on this subject) and for the following time-dependent cases: progressing, retrograding, intensifying, and weakening jets. In this study, the 'intensification' of a straight jet implies an increase in the wind speed while in the case of a curved jet the intensification refers to an increase in the meandering aspect, that is, amplitude divided by wavelength. The dominant divergence pattern of a straight jet consists of a quadrupole structure of four divergence/convergence centers, and in the case of a curved jet the pattern consists of a periodic chain of centers (of alternating signs) located along the jet axis at the inflection points. In progressing and retrograding straight and curved jets, the divergence patterns remain unaltered compared with those of the steady state, but the amplitude decreases in the former and increases in the latter. For an intensifying straight jet, the divergence patterns increase at the entrance region and weaken at the exit, while at the same time being slightly shifted downstream. The reverse holds for the weakening straight jet. The changes in the meandering aspect of a curved jet are accompanied by an intensification of the divergence patterns in both the intensifying and weakening jets. These are accompanied by a shift in their location relative to the jet: downstream for the intensifying jet and upstream for the weakening one. These findings point to the effect of time dependence on the cyclogenesis associated with jet streams and on feedback mechanisms there. For each of the cases considered here, the authors demonstrate that the divergence patterns indicate a negative feedback, which tends to suppress the prescribed temporal change.

The technique of large-eddy simulations has been used to investigate thermally driven local circulations in deep valleys for a complete diurnal cycle. A soil model simulates the thermal forcing at the ground, which depends on the season, the soil characteristics, the valley orientation, and the atmospheric variables. The scales of interest are characteristic of an urban site located in a mountainous area, and the research focuses on low wind conditions without the influence of large-scale pressure gradients. This study highlights the influence of the season on the mechanisms responsible for the formation and the destruction of the thermal inversion layer. The spatial distribution of the convective boundary layer (CBL) within the valley is directly influenced by the season because of the variation of the solar warming. In summer, the altitude of the top of the CBL remains approximately constant across the valley, whereas in winter, this altitude varies with its location within the valley.