Zonal Wind Profile Research Articles

Principal Component Analysis of Wind Profiler Observations

Abstract Principal component analysis (PCA) is applied to wind profiler observations to study the vertical profile of the wind field and its temporal evolution. The rationale for decomposing time–height wind profiler data using PCA is twofold. The orthogonal vertical profile vectors are determined empirically from the variance of the observations, and the time evolutions of these vectors are not simple sinusoids, but are temporal varying signals that can be directly related to other measurements. As an example of its utility, PCA is used to compare the annual and interannual variation of zonal wind measured with a 50-MHz VHF wind profiler above Christmas Island, Kiribati, with the difference between surface pressures measured at Tahiti, French Polynesia, and Darwin, Australia. The high correlation coefficients relate the vertical profile of zonal wind observed in the central Pacific with the variation of convection in the western Pacific. Complex PCA (C-PCA) allows the analysis of data consisting of ampli...

Empirical wind model for the upper, middle and lower atmosphere

The HWM90 thermospheric wind model has been revised in the lower thermosphere and extended into the mesosphere, stratosphere and lower atmosphere to provide a single analytic model for calculating zonal and meridional wind profiles representative of the climatological average for various geophysical conditions. Gradient winds from CIRA-86 plus rocket soundings, incoherent scatter radar, MF radar, and meteor radar provide the data base and are supplemented by previous data driven model summaries. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, local time (tides), and longitude (stationary wave 1), with a cubic spline interpolation in altitude. The model represents a smoothed compromise between the original data sources. Although agreement between various data sources is generally good, some systematic differences are noted, particularly near the mesopause. Overall root mean square differences between dar.a and model values are on the order of 15 m/s in the mesosphere and 10 m/s in the stratosphere for zonal winds, and 10 m/s and 5 m/s respectively for meridional winds.

Jovian Tropospheric Features—Wind Field, Morphology, and Motion of Long-Lived Systems

A map spanning 360° of longitude and +5° to −65° latitude has been constructed from Hubble Space Telescope SL-9 Campaign F410M images obtained on July 23, 1994 with the WFPC2 in PC and WF mode. For comparison, a map spanning +65° to −65° has been generated from WFPC2 PC observations on February 17, 1995. Because the WFPC2 data has the highest spatial resolution of any data obtained during the SL-9 campaign, these reference maps are intended for general digitized distribution for comparison with other SL-9 data sets. Multicolor images have been utilized to provide fundamental data for interpreting dispersal and evolution of impact sites. Although temporal coverage was less than ideal, a zonal wind profile has been generated from 953-nm images and is comparable to that seen with Voyager. The temporal behavior associated with individual impact sites varies. Impacts that occurred near cyclonic and anticyclonic storms show evidence of interaction with these systems, indicating that winds associated with individual cyclonic and anticyclonic storm centers do not decrease with height as rapidly as the zonal winds. Thus, the translation rates for well-defined anticyclonic cloud systems are tabulated and cyclonic circulation of a storm system which modifies the L-site evolution is characterized.

Radio Scintillations in Venus's Atmosphere: Application of a Theory of Gravity Wave Generation

Radio scintillations in Pioneer Venus radio Occultation data are simulated assuming that the index of refraction fluctuations in Venus's atmosphere responsible for the scintillations are directly caused by gravity wave fluctuations. The gravity waves are created by a global convection layer between 50- and 55-km attitude in Venus's atmosphere and propagate vertically. The authors compare the simulated scintillations with data from Pioneer Venus. These gravity waves can explain the spectral shape and amplitude of the radio scintilations. The shape at high frequencies is controlled by wave breaking, which yields a saturated spectrum. The amplitude is subject to parameters such as the intensity of the convection, the angle between the zonal winds and the beam path, and the zonal wind profile at polar latitudes. To match the observed amplitude of the scintillations, the velocity variations of the energy-bearing eddies in the convection must be at least 2 m s−1. This value is consistent with the Venus balloon results of Sagdeev et al. and is in the middle of the range considered by Leroy and Ingersoll in their study of convectively generated gravity waves. The later study, combined with the lower bound on velocity from the present study, then yields lower bounds on the vertical fluxes of momentum and energy in the Venus atmosphere.

Mean winds at 60–90 km observed with the MU radar (35°N)

Mean winds at 60–90 km altitudes observed with the MU radar (35°N, 136°E) in 1985–1989 are presented in this paper. The zonal wind at 70 km became westward and eastward in summer and winter, respectively, with a maximum amplitude of 45 m s −1 westward in early July and 80 m s −1 eastward at the end of November. The meridional wind below 85 km was generally northward with the amplitudes less than 10 m s −1. In September to November, the meridional wind at 75–80 km becomes as large as 20–30 m s −1. Those zonal wind profiles below 90 km show good coincidence with the CIRA 1986 model, except for the latter half of winter, from January to March, when the observational result showed a much weaker eastward wind than the CIRA model. The height of the reversal of the summer wind from westward to eastward was determined as being 83–84 km, which is close to the CIRA 1986 model of 85 km. The difference between the previous meteor radar results at 35–40°N, which showed the reversal height below 80 km, could be due to interannual variations or the difference in wind measurement technique. In order to clarify that point, careful comparative observations would be necessary. These mean winds were compared with Adelaide MF radar observations, and showed good symmetry between the hemispheres, including the summer reversal height, except for the short period of eastward winds above Kyoto and the long period over Adelaide.

The Role of Moist Baroclinic Instability in the Growth and Structure of Monsoon Depressions

Abstract The effect of latent heat of condensation on baroclinic instability process is studied using the model developed by Bonatti and Rao. This model includes the effect of latent heat of condensation as parameterized by Mak, a zonal wind profile that varies linearly with pressure, and a constant lapse rate of −6°C km−1. The effect of dissipative terms and vertical variation of heating on baroclinic instability with positive and negative zonal wind shears is studied. Dissipative terms and the type and intensity of heating change the selection of the most unstable wave. Viscosity and Newtonian cooling stabilize short waves. The characteristics of the most unstable wave obtained using a double maximum in the vertical distribution of heating and observed vertical profiles of zonal wind and temperature representative of the summer monsoon of India resemble those of observed monsoon depressions. The inclusion of dissipative effects makes the Green mode the most unstable wave.

Observations of strong wind shears and temperature enhancements during several sporadic Na layer events above Haleakala

We describe two sporadic Na (Nas) layer events observed on 26 September and 6 October 1993 by the Na Wind/Temperature (W/T) lidar above Haleakala, Maui (20.8°N, 156.2°W) during ALOHA‐93. Both events occurred between 90 and 95 km altitude and appeared to be associated with large amplitude gravity waves. Both were accompanied by temperature enhancements of more than 40 K. The temperature and Na density variations at the peak of the Nas, as well as the vertical motions of the Nas layers and regions of elevated temperature were highly correlated. The vertical wind structure exhibited no unusual characteristics in the vicinities of the Nas. The zonal wind profile for the 6 October event exhibited a very strong vertical shear approximately 1 km below the Nas layer that approached the dynamic instability limit of 42 m/s/km. However the polarity of the shear was opposite that predicted by the classical wind shear theory of sporadic E layers (Es) formation.

Estimate of Jupiter's Deep Zonal-Wind Profile from Shoemaker-Levy 9 Data and Arnol'd's Second Stability Criterion

A number of one- and two-layer atmospheric models have been applied to the study of Jupiter's tropospheric circulations over the past two decades in an effort to characterize the basic-state properties of the zonal winds and to study the influence these winds have on the dynamics of long-lived vortices like the Great Red Spot. By singling out the basic state that corresponds to neutral stability with respect to Arnol'd's second stability criterion, as motivated by the results of a vortex-tube stretching analysis of the Voyager wind data and by fixing the stratification parameter using the c = 454 ± 20 msec -1 gravity wave speed determined from the Shoemaker—Levy 9 impacts, we arrive at a unique model with no free parameters. The speed and direction of the zonal wind as a function of latitude for the model's lower layer is thus determined, which roughly indicates the circulation in Jupiter's interior below the water clouds. Predictions are that the westward jets change little with depth but that the eastward jets become stronger by 50-100%.

The Thermal Structure and Dynamics of the Atmosphere of Venus between 70 and 90 km from the Galileo-NIMS Spectra

An analysis of thermal profiles and dynamics over a wide range of latitudes for the venusian atmosphere between 70 and 90 km is presented based on high spatial resolution infrared spectra of the night side obtained by the near infrared mapping spectrometer (NIMS) experiment during the Galileo-Venus encounter in February 1990. Using the 4.3-μm CO 2 absorption band, the temperature profile is retrieved in the 75- to 91-km altitude region over a latitudinal range of -59° to + 64°. Compared to earlier observations from the Pioneer Venus mission, the temperature at 91 km is about 10 K higher and between 74 and 83 km about 3.6 K colder. An equator to pole warming at constant pressure levels is found and implications for the zonal wind profiles are drawn under the assumption that the atmosphere is in cyclostrophic balance in the region of 70 to 90 km. The results are in correspondence with direct wind measurements from ground-based observations at 95 km and 105 km altitude.

Galileo entry probe recovery of fine structure in the zonal wind profile at Jupiter

The algorithm developed to recover the zonal (east‐west) wind profile at Jupiter by Doppler tracking of the Galileo probe is well established in theory and verified by computer simulation. We extend the Doppler wind recovery algorithm to include detection and extraction of microscale wind structure. When the probe encounters regions of high wind shear, turbulence, atmospheric waves, or nonsteady aerodynamics (buffeting) a rapidly varying component is introduced into the probe telemetry Doppler data. These effects can be isolated by their respective Doppler signatures. Analysis of residuals allows fine structure in the wind profile to be located and retrieved. The overall wind recovery is a three‐step process: (1) The large‐scale zonal wind profile is recovered by converting the probe telemetry frequency residuals (as caused by the Doppler effect) to a velocity and back‐projecting it into the local horizontal east‐west direction at the probe location. The zonal winds are derived from the velocity residuals by a least squares algorithm. (2) Regions of high zonal wind shear are recovered numerically relative to the large‐scale wind structure and serve as a correction to the previously derived large‐scale profile. (3) The probe trajectory is updated to reflect both the large‐ and small‐scale wind structure, and the large‐scale wind profile is recalculated. The Galileo Doppler wind experiment described previously and the wind microstructure retrieval algorithm outlined here are the only reasonable means by which the vertical profile of the zonal winds at Jupiter can be directly sampled.

Saturn's Central Flash from the 3 July 1989 Occultation of 28 Sgr

We present observations of Saturn's central flash obtained from Palomar and McDonald Observatories during the 3 July 1989 occultation of 28 Sgr. As the star passed close to the geometric center of Saturn's shadow, the focusing of the incident starlight by the planet's atmosphere formed multiple stellar images along the limb which were detected in infrared images obtained at wavelengths of 3.9 (Palomar) and 2.1 μm (McDonald). These are the first reported observations of a central flash due to Saturn, and the first for any planet in which the signal from each stellar image could be determined separately, permitting a comparison of both intensity and position for each image with model predictions. The starlight forming the central flash was strongly modulated by passage through Saturn's rings. Four separate flashes were observed from each observatory, corresponding to the points on the limb where the starlight passed through the Cassini Division and the relatively transparent C Ring, with maximum brightnesses reaching 1-2% of the unocculted stellar intensity. The more usual brightening seen during previous central flashes as the observer crossed the caustic formed by the planet's oblate limb was not detectable in this event due to strong attenuation by the B Ring. The timing of the flashes is quite sensitive to the shape of Saturn's limb, which depends in turn on the planet's zonal gravity harmonics and on the zonal wind profile in the lower stratosphere, near the 2.5-mbar pressure level. The locations of the images along the limb, as well as the timing, shapes, and amplitudes of the individual flash light curves, are well matched by a smoothed model based on a tropospheric zonal wind profile obtained from tracking cloud features in Voyager images and the Saturn ring optical depth profile obtained from the Voyager Photopolarimeter experiment. The smoothing required to give the best match to the data exceeds that attributable to the finite angular extent of the occulted star and may be due to refractive scattering by turbulence or wave structure in Saturn's atmosphere. The detailed agreement of the models and observations is improved by assuming a uniform zonal wind speed of 40 msec -1 at 25-70° north latitude in the stratosphere, suggesting that midlatitude zonal winds on Saturn decay with height above the troposphere to the nonzero mean of local tropospheric winds. There is no evidence of significant atmospheric absorption at the observed wavelengths, which correspond to spectral regions of weak methane absorption.

Molluscan evidence for enhanced deglacial advection of Agulhas water in the Benguela current, off southwestern Africa

Shells in surficial, unconsolidated sediments at −120 to −140 m depth on the Orange Shelf, Benguela region, off southwestern Africa, have accumulated during the post-glacial transgression. A large sample of the shells, from 53 individual sites, revealed the presence of a significant percentage of bivalves (∼14%) that do not live along the western shelf at present, but inhabit the warm-temperate Algoa biogeographic province of the Agulhas Bank region. Altogether 17 Algoa species have been found on the Orange Shelf, which today is dominated by a cool-temperate, upwelling regime. Their presence suggests warmer conditions in the Benguela during the deglacial transgression, as a result of enhanced advection close to the coast of water derived from the Agulhas Current, and concomitant suppression of upwelling. Bathymetric preferences, comparative taphonomy and radiocarbon-dating of the Algoa taxa from the Orange Shelf suggest that two periods of enhanced advection of Agulhas water were involved, 13.5-12.5 kyr B.P. and ∼10 kyr B.P. This timing corresponds with major steps away from the Ice Age World, such as warming in the Southern Ocean, North Atlantic warming and increasing production of North Atlantic Deep Water. Previous suggestions for enhanced deglacial advection of Agulhas water in the Benguela are now supported by molluscan evidence. Rounding of Africa by the Agulhas Current, at times of warming in the Southern Ocean and reduced austral summer insolation due to orbital parameters, is consistent with modelling and observations that indicate such behaviour under conditions of reduced volume transport associated with southward shifting of the zonal wind profile south of the continent and reduced trade winds over the southwest Indian Ocean.

Quasi-Biennial Oscillations of Ozone and Diabatic Circulation in the Equatorial Stratosphere

The quasi-biennial oscillation (QBO) in ozone in the equatorial stratosphere is obtained by analyzing the Stratospheric Aerosol and Gas Experiment (SAGE) data from 1984 to 1989. The phase of the ozone QBO in the lower stratosphere is found to precede the zonal wind QBO by several months as opposed to the theoretically expected in-phase relationship between the two. A mechanistic model is developed to explore possible reasons for this disagreement. The model is capable of simulating the actual time evolution of the ozone QBO by introducing the observed zonal wind profile as input. The modeled results confirm the conventional view that the ozone QBO is generated by the vertical ozone advection that is driven to maintain the temperature structure against radiative damping. However, a series of experiments emphasizes the importance of the feedback of the ozone QBO to the diabatic heating through the absorption of solar radiation. Due to this effect, the phase of the ozone QBO shifts up to a quarter cycle ahead and approaches that of the temperature QBO. Because of this inphase relationship, the feedback of the ozone QBO to the diabatic heating acts to compensate for the radiative damping of the temperature structure, thus reducing the magnitude of the induced diabatic circulation. Because the reduction of the magnitude of the vertical motion facilitates downward transport of easterly momentum by the mean flow, this feedback process can help to resolve the insufficiency of the easterly momentum in driving the dynamical QBO in general circulation models (GCMs). It should be emphasized that more sophisticated models that allow for full interaction between the chemical species and radiative and dynamical processes should be developed to improve our understanding of both dynamical and ozone QBOs.

Jupiter's winds and Arnol'd's second stability theorem: Slowly moving waves and neutral stability

Since the Voyager encounters in 1979, it has been known that Jupiter's cloud‐top zonal winds violate the barotropic stability criterion. A vortex‐tube stretching analysis of the Voyager wind data indicates that the more general Charney‐Stern stability criterion is also violated. On the other hand, the zonal winds determined by tracking cloud features in Hubble Space Telescope images taken in 1991 precisely match the zonal winds determined by tracking cloud features in Voyager images, and it is hard to understand how a complicated zonal wind profile like Jupiter's could be unstable and yet not change at all in 12 years. In fact, there are at least two known ways to violate the Charney‐Stern stability criterion and still have a stable flow. The better known of these is called Fjørtoft's theorem, or Arnol'd's 1st theorem for the case of large‐amplitude perturbations. Although the Fjørtoft‐Arnol'd theorem has been extended from the quasi‐geostrophic equations to the primitive equations, the basic requirement that the potential vorticity be an increasing function of streamfunction is opposite to the case found on Jupiter, where the Voyager data indicate that the potential vorticity is a decreasing function of streamfunction. But this second case is precisely that which is covered by Arnol'd's 2nd stability theorem. In fact, the Voyager data suggest that Jupiter's zonal winds are neutrally stable with respect to Arnol'd's 2nd stability theorem. Here, we analyze the linear stability problem of a one‐parameter family of sinusoidal zonal wind profiles that are close to neutral stability with respect to Arnol'd's 2nd stability theorem. We find numerically that the most unstable mode is always stationary, which may help to explain the slowly moving mode 10 waves observed on Jupiter. We find that violation of Arnol'd's 2nd stability theorem is both necessary and sufficient for instability of sinusoidal profiles. However, there appears to be no simple extension of Arnol'd's 2nd stability theorem to the primitive equations. Nevertheless, the primitive growth rates are small, and the primitive system is still governed by the quasi‐geostrophic neutral‐stability configuration.

2. Morphology and dynamics of the Pinatubo aerosol layer in the northern hemisphere as detected from a ship‐borne lidar

Scattering ratios of the Pinatubo aerosol layer obtained by the ship‐borne lidar of the Roscomhydromet of Russia on‐board PROFESSOR ZUBOV are presented. During a campaign primarily in the low‐latitude (8°N–43°N) Atlantic Ocean, 48 days of data were obtained within the 73 day period spanning 11‐July and 21‐September of 1991 [Avdyushin et al., 1992]. These data show the aerosol cloud to be primarily in the 16–27 km altitude region, with a high degree of variability in the vertical structure and in the longitudinal structure, and to a lesser degree, in the latitudinal structure. Scattering ratios of up to 50 were recorded in the multi‐layered structure which was often observed. These are the first lidar observations showing a high degree of zonal variability of the vertical structure of the aerosol layer. We show the latitudinal range of the Pinatubo aerosol cloud in the northern hemisphere over a 2 month period and estimate the latitudinal dispersion rate for the months of the measurements to be approximately 5° latitude per month. A clear periodic variability was observed in the altitude structure during the entire data‐set primarily within 8°N–22°N latitude and within the altitude region of the layer, from which a mean zonal wind profile was produced by a spectral analysis of the aerosol profiles.

Dynamics of Neptune's Major Cloud Features

We used Voyager 2 images of Neptune to investigate the dynamics of the Great Dark Spot (GDS), the Scooter, the Second Dark Spot (DS2), and the South Polar Features (SPF). Relative to previous studies, we covered a longer time base, achieved more complete sampling of the available data, and improved accuracy by using stars to correct navigation errors. We tracked the GDS 8-day shape oscillation for more than 2000 hr, measuring amplitudes of 7.4° in longitude and 1.5° in latitude. We found a surprisingly large increase in the mean latitudinal size of the GDS from 8° in January 1989 to 17° in August. During the same period, we measured steady GDS northward motion from 26°S to 17°S and found no evidence of the latitudinal limits. We confirmed that the varying longitudinal drift rates of the GDS and DS2 were simple functions of latitude that approximately match the zonal wind profile. Measurements of at least three cycles of the periodic motions of the DS2 defined a 36-day period and amplitudes of 2.4° in latitude and 47.5° in longitude relative to its mean drift longitude. We found that the bright core of the DS2 varied in step with the DS2 latitudinal oscillation, reaching a maximum area when the DS2 was furthest north. A large increase in relative core brightness at high phase angles is consistent with forward scattering cloud particles. We measured one cycle of a Scooter longitudinal oscillation of 20° about its mean drift position, with a period of 120 days; but the Scooter's latitudinal motions are too small and of the wrong frequency to be consistent with an advective model of the zonal motions. We found that the highly variable SPF formation region has the same mean longitudinal drift rate as the DS2, but an offset of 180° in longitude and a 20° difference in latitude. The apparent synchronization of these widely separated and dramatically different features is unexplained. Coupling to the deep interior is ruled out by the significant difference between the radio and cloud periods. Observed motions of all features are described by empirical models valid over the Voyager time period. Comparisons of Voyager results with groundbased observations did not lead to conclusive constraints on the long-term dynamics, although much of the data seem to be consistent with a 5.8-year oscillation in GDS latitude.

Equatorial Superrotation in a Slowly Rotating GCM: Implications for Titan and Venus

The maintenance of strong superrotating zonal winds in the equatorial Venus atmosphere and the possibility of a similar regime on Titan are examined with a modified version of an efficient terrestrial general circulation model (GCM). The model is simplified by removal of the hydrologic cycle, the diurnal cycle, and all seasonal and geographic variations. We describe a suite of equilibrium simulations in which rotation rate, radiative heating profile, surface drag, and stratospheric drag are varied. The key to superrotation in these experiments is the presence of an upper troposphere cloud which intercepts much of the incoming solar flux and produces a statically stable radiative equilibrium state in the lower and middle troposphere. The radiative heating profile limits the depth of boundary layer convection and detaches the upper level flow from the surface. At Titan's presumed rotation period, the cloud-covered GCM produces equatorial winds in excess of 50/100 m sec-1 with/without stratospheric drag. Superrotation extends throughout most of the atmosphere. Hadley cell interaction with quasi-barotropic eddies which transport momentum equatorward is responsible for the excess angular momentum, confirming certain aspects of proposals by Gierasch (1975) and Rossow and Williams (1979) but with relatively modest Prandtl numbers. The latitudinal profile of zonal wind resembles that for uniform absolute linear momentum. Removal of the cloud decreases static stability, increases vertical convective mixing, and almost completely eliminates equatorial superrotation. Equatorial winds >60 m sec-1 also occur at Venus' rotation period, but only near the upper boundary in the absence of stratospheric drag. In the slowly rotating regime, equatorial winds increase as surface drag increases. Our calculations represent the first GCM simulation of strong superrotation without unrealistic momentum sources. The results suggest that superrotation is not inevitable on slowly rotating planets but is likely if diabatic heating produces a statically stable thermal structure.

Retrieval of a wind profile from the Galileo Probe telemetry signal

Ultrastable oscillators onboard the Galileo Probe and Orbiter will permit very accurate determinations of the frequency of the Probe’s telemetry signal as the Probe descends from a pressure level of several hundred mb to a level of about 20 bars. Analysis of the time-varying frequency can provide, in principle, a unique and important definition of the vertical profile of the zonal wind speed in the Jovian atmosphere. In this paper, we develop a protocol for retrieving the zonal wind profile from the Doppler shift of the measured frequency; assess the impact of a wide range of error sources on the accuracy of the retrieved wind profile; and perform a number of simulations to illustrate our technique and to assess the likely accuracy of the retrieval.

Na Doppler/temperature lidar: Initial mesopause region observations and comparison with the Urbana medium frequency radar

Meteoric ablation in the upper atmosphere is the source of a dense layer of atomic Na between about 80 and 105 km altitude. Peak densities are typically several thousand atoms/cm3, and the total abundance varies seasonally at mid‐latitudes with the maximum in winter. The Na layer is a sensitive tracer of the thermal and density structure of this region of the atmosphere and can be probed with exceptional accuracy and resolution using resonance fluorescence lidar techniques. We report the first observations of vertically resolved Na density, temperature, and wind profiles in the mesopause region using a new four‐frequency Doppler/temperature lidar. The initial zonal wind and temperature observations were obtained with a resolution of 30 min and 1.5 km and rms accuracies of 12 m/s and 2 K. These measurements were conducted simultaneously with the medium frequency (MF) radar at the Urbana Atmospheric Observatory (40°N, 88°W) during the night of March 14–15, 1991. The mean temperature profile exhibits a well‐defined mesopause at 95.5 km with a temperature of 181 K. The mean zonal wind profile exhibits a strong 58 m/s westward jet at 97 km which appears to be related to the mesopause. During the 6‐hour and 45‐min observation period the Na Doppler winds, averaged between 88 and 98 km, exhibit the same general trends as the MF radar apparent and true winds in this same region. However, the lidar and radar velocity estimates differ by as much as 40 m/s at some times during the night.

Observations of a Reversal in Long-Term Average Vertical Velocities near the Jet Stream Wind Maximum

Analysis of vertical velocity measurements made for four days each month over the period from 1986 to 1988 by the MU radar in Japan shows a reversal in direction near the peak in the zonal wind profile during the winter months. More specifically, the reversal is noted during periods when the peak horizontal wind speeds 60 m s−1. The vertical velocities associated with the circulation have magnitudes of 10-20 cm s−1, and the depth of the circulation is of the order of several kilometers. In 6 out of 14 cases when the feature was observed, the direction of the vertical circulation, although not the magnitude, could be explained by adiabatic ascent or subsidence along the average potential temperature surface slopes for the observation intervals. The direction of the circulation was such that it would tend to produce cooling and heating for the ascent and subsidence, respectively, that would tend to strengthen or at least maintain the jet. In the remaining eight cases, the direction of the vertical circulation could not be explained by the slope of the time-averaged potential temperature surfaces alone since the combination of the horizontal winds and the slopes of the isentropic surfaces would have led to a prediction of a circulation directly opposed to that observed. Thus, either the local tendency in the time-averaged potential temperature must have been significant, structure with scales smaller than the rawinsonde station separation must have been present, or diabatic effects may have played a role in the dynamics of the vertical velocity feature.