Meridional Momentum Flux Research Articles

Momentum Transport in Organized Shallow Cumulus Convection

Abstract This study investigates momentum transport in shallow cumulus clouds as simulated with the Dutch Atmospheric Large Eddy Simulation (DALES) for a 150 × 150 km2 domain east of Barbados during 9 days of EUREC4A. DALES is initialized and forced with the mesoscale weather model HARMONIE–AROME and subjectively reproduces observed cloud patterns. This study examines the evolution of momentum transport, which scales contribute to it, and how they modulate the trade winds. Daily-mean momentum flux profiles show downgradient zonal momentum transport in the subcloud layer, which turns countergradient in the cloud layer. The meridional momentum transport is nontrivial, with mostly downgradient transport throughout the trade wind layer except near the top of the surface layer and near cloud tops. Substantial spatial and temporal heterogeneity in momentum flux is observed with much stronger tendencies imposed in areas of organized convection. The study finds that while scales < 2 km dominate momentum flux at 200 m in unorganized fields, submesoscales carry up to 50% of the zonal momentum flux in the cloud layer in organized fields. For the meridional momentum flux, this fraction is even larger near the surface and in the subcloud layer. The scale dependence of the momentum flux is not explained by changes in convective or boundary layer depth. Instead, the results suggest the importance of spatial heterogeneity, increasing horizontal length scales, and countergradient transport in the presence of organized convection.

Seasonal variations of gravity‐wave parameters over the northern and southern Tibetan plateau radiosonde observatory

AbstractSeasonal variations of gravity‐wave parameters over the northern and southern Tibetan Plateau are studied using high vertical‐resolution L‐band radiosonde data during the period 2014 to 2019. The wave parameters include wavelength, frequency, phase speed, group velocity, perturbation energies and momentum fluxes. The synoptic effects on the gravity‐wave parameters are discussed and assessed with the Pearson correlation. The tropospheric vertical wavelengths as estimated from the zonal wind perturbation were found to show a pronounced decrease over the southern Tibetan Plateau in the warm season. The tropospheric (stratospheric) horizontal wavelength is shortest (longest) in summer, and the stratospheric horizontal wavelength is longer than the tropospheric horizontal wavelength. The ratio of the intrinsic frequency to the Coriolis parameter is 2.5–5 (2–4.5) with a summer (winter) maximum in the troposphere (lower stratosphere). The ratio is generally higher in the north than in the south. The typical value of the mean tropospheric (stratospheric) intrinsic‐phase speed is 3.5–6 (2–11) m·s−1. The mean intrinsic‐phase speed in the south is mostly larger than that in the north. The tropospheric zonal ground‐based group velocities are dominantly eastward in all seasons. The stratospheric zonal ground‐based group velocity shifts from eastward in summer to westward in winter. In the troposphere, the seasonal cycles of the kinetic energy (KE) and potential energy (PE) exhibit regional differences. The tropospheric vertical fluctuation energy (VE) is independent of both season and region. The stratospheric KE, PE and VE are smaller in the warm season. The negative zonal momentum flux and positive meridional momentum flux are strongest in winter and summer.

Global Distributions of Tropospheric and Stratospheric Gravity Wave Momentum Fluxes Resolved by the 9-km ECMWF Experiments

Abstract Based on 20-day control forecasts by the 9-km Integrated Forecasting System (IFS) at the European Centre for Medium-Range Weather Forecasts (ECMWF) for selected periods of summer and winter events, this study investigates global distributions of gravity wave momentum fluxes resolved by the highest-resolution-ever global operational numerical weather prediction model. Two supplementary datasets, including 18-km ECMWF IFS experiments and the 30-km ERA5, are included for comparison. In the stratosphere, there is a clear dominance of westward momentum fluxes over the winter extratropics with strong baroclinic instability, while eastward momentum fluxes are found in the summer tropics. However, meridional momentum fluxes, locally as important as the above zonal counterpart, show different behaviors of global distribution characteristics, with northward and southward momentum fluxes alternating with each other especially at lower altitudes. Both events illustrate conclusive evidence that stronger stratospheric fluxes are found in the ECMWF forecast with finer resolution, and that ERA5 datasets have the weakest signals in general, regardless of whether regridding is applied. In the troposphere, probability distributions of vertical motion perturbations are highly asymmetric with more strong positive signals especially over latitudes covering heavy rainfall, likely caused by convective forcing. With the aid of precipitation accumulation, a simple filtering method is proposed in an attempt to eliminate those tropospheric asymmetries by convective forcing, before calculating tropospheric wave-induced fluxes. Furthermore, this research demonstrates promising findings that the proposed filtering method could help in reducing the potential uncertainties with respect to estimating tropospheric wave-induced fluxes. Finally, absolute momentum flux distributions with proposed approaches are presented, for further assessment in the future.

Seasonal evolution of winds, atmospheric tides, and Reynolds stress components in the Southern Hemisphere mesosphere–lower thermosphere in 2019

Abstract. In this study we explore the seasonal variability of the mean winds and diurnal and semidiurnal tidal amplitude and phases, as well as the Reynolds stress components during 2019, utilizing meteor radars at six Southern Hemisphere locations ranging from midlatitudes to polar latitudes. These include Tierra del Fuego, King Edward Point on South Georgia island, King Sejong Station, Rothera, Davis, and McMurdo stations. The year 2019 was exceptional in the Southern Hemisphere, due to the occurrence of a rare minor stratospheric warming in September. Our results show a substantial longitudinal and latitudinal seasonal variability of mean winds and tides, pointing towards a wobbling and asymmetric polar vortex. Furthermore, the derived momentum fluxes and wind variances, utilizing a recently developed algorithm, reveal a characteristic seasonal pattern at each location included in this study. The longitudinal and latitudinal variability of vertical flux of zonal and meridional momentum is discussed in the context of polar vortex asymmetry, spatial and temporal variability, and the longitude and latitude dependence of the vertical propagation conditions of gravity waves. The horizontal momentum fluxes exhibit a rather consistent seasonal structure between the stations, while the wind variances indicate a clear seasonal behavior and altitude dependence, showing the largest values at higher altitudes during the hemispheric winter and two variance minima during the equinoxes. Also the hemispheric summer mesopause and the zonal wind reversal can be identified in the wind variances.

Planetary geostrophic Boussinesq dynamics: Barotropic flow, baroclinic instability and forced stationary waves

AbstractMotions on planetary spatial scales in the atmosphere are governed by the planetary geostrophic equations. However, little attention has been paid to the interaction between the baroclinic and barotropic flows within the planetary geostrophic scaling. This is the focus of the present study, which utilizes planetary geostrophic equations for a Boussinesq fluid supplemented by a novel evolution equation for the barotropic flow. The latter is affected by meridional momentum flux due to baroclinic flow and drag by the surface wind. The barotropic wind, on the other hand, affects the baroclinic flow through buoyancy advection. Via a relaxation towards a prescribed buoyancy profile the model produces realistic major features of the zonally symmetric wind and temperature fields. We show that there is considerable cancellation between the barotropic and the baroclinic surface zonal mean zonal winds. Linear and nonlinear model responses to steady diabatic zonally asymmetric forcing are investigated, and the arising stationary waves are interpreted in terms of analytical solutions. We also study the problem of baroclinic instability on the sphere within the present model.

Inertia‐Gravity Waves Revealed in Radiosonde Data at Jang Bogo Station, Antarctica (74°37′S, 164°13′E): 1. Characteristics, Energy, and Momentum Flux

AbstractCharacteristics of inertia‐gravity waves (IGWs) at high latitude in Antarctica are investigated using radiosondes launched daily at Jang Bogo Station (74°37′S, 164°13′E), a new Antarctic station that has been operating since 2014, in the troposphere (z = 2–7 km) and lower stratosphere (z = 15–22 km) for 25 months (December 2014 to December 2016). The vertical propagation of IGWs exhibits strong seasonal variations in the stratosphere, with an enhancement (reduction) in downward (upward)‐propagating IGWs from May to mid‐October. In the troposphere, both upward‐ and downward‐propagating IGWs have similar occurrence rates without seasonal variations. The intrinsic phase velocity of IGWs mostly direct to the west (isotropic), while the ground‐relative phase and group velocities are dominant in the east and southeast (northeast), respectively, in the stratosphere (troposphere). The intrinsic frequency, vertical wavelength, and horizontal wavelength of IGWs averaged in the troposphere (stratosphere) are 3.57f (1.93f; where f is the Coriolis parameter), 1.48 (1.48) km, and 63.06 (221.81) km, respectively. The wave energy in the stratosphere has clear seasonal variations with large values in autumn and spring, while that in the troposphere is smaller without obvious seasonal variations. Zonal and meridional momentum fluxes averaged in the stratosphere (troposphere) are −0.008 (−0.0018) and −0.0005 (0.001) m2/s2, respectively. The momentum flux of downward‐propagating IGWs in the stratosphere is mostly positive in both zonal and meridional directions, whereas the directional preference is not obvious in the troposphere. In Part 2, sources of the observed IGWs in the troposphere and stratosphere will be examined.

Multiyear Observations of Gravity Wave Momentum Fluxes in the Midlatitude Mesosphere and Lower Thermosphere Region by Meteor Radar

AbstractMultiyear high‐frequency gravity wave (GW) momentum fluxes and variances in the mesosphere and lower thermosphere region are revealed using four meteor radars along 120°E longitude at Northern Hemisphere midlatitudes for the first time, which are located at Mohe (53.5°N, 122.3°E), Beijing (40.3°N, 116.2°E), Mengcheng (33.3°N, 116.5°E), and Wuhan (30.5°N, 114.2°E), respectively. The seasonal and latitudinal variations of GW momentum fluxes in the midlatitude are discussed. The directions of the monthly mean zonal momentum fluxes are mostly against the background mean zonal winds, which agree well with the selective filtering mechanism. The seasonal variations of meridional momentum fluxes have similar trends over all four stations. The latitudinal variations in the seasonal variation of GW momentum fluxes are mainly due to the latitudinal variations of background winds and GW sources. The unexpected eastward momentum fluxes in winter over Beijing are likely caused by the secondary GWs propagating eastward from the source region over the Tibetan Plateau (25°–40°N, 70–100°E). The GW variances show a V‐shaped structure indicating annual and semiannual variations over four stations in zonal component. A quasi‐4‐month oscillation was observed over Mohe, Mengcheng, and Wuhan in meridional component. The background winds play decisive roles in these GW variance structures.

Measurement of momentum flux using two meteor radars in Indonesia

Abstract. Two nearly identical meteor radars were operated at Koto Tabang (0.20° S, 100.32° E), West Sumatra, and Biak (1.17° S, 136.10° E), West Papua, in Indonesia, separated by approximately 4000 km in longitude on the Equator. The zonal and meridional momentum flux, u′w′ and v′w′, where u, v, and w are the eastward, northward, and vertical wind velocity components, respectively, were estimated at 86 to 94 km altitudes using the meteor radar data by applying a method proposed by Hocking (2005). The observed u′w′ at the two sites agreed reasonably well at 86, 90, and 94 km during the observation periods when the data acquisition rate was sufficiently large enough. Variations in v′w′ were consistent between 86, 90, and 94 km altitudes at both sites. The climatological variation in the monthly averaged u′w′ and v′w′ was investigated using the long-term radar data at Koto Tabang from November 2002 to November 2013. The seasonal variations in u′w′ and v′w′ showed a repeatable semiannual and annual cycles, respectively. u′w′ showed eastward values in February–April and July–September and v′w′ was northward in June to August at 90–94 km, both of which were generally anti-phase with the mean zonal and meridional winds, having the same periodicity. Our results suggest the usefulness of the Hocking method.

Gravity wave momentum fluxes from MF and meteor radar measurements in the polar MLT region

AbstractAnnual cycles of horizontal winds and gravity wave (GW) momentum fluxes in the mesosphere/lower thermosphere (MLT) are presented for the medium frequency Doppler radar at Saura (SMF radar, located at 69°N, 16°E) for the first time. Four year mean wind and momentum flux fields for 2008 through 2011 clearly show the coupling and interactions between GWs and the mean flow especially in the summer months. GW breaking at mesopause heights results in momentum flux divergence and affects the wind field by forcing a reversal of the wind profile in summer. Height‐time cross sections for the individual years (2008 to 2011) illustrate the year‐to‐year variation of horizontal winds and the vertical fluxes of zonal and meridional momentum. They show similar annual patterns from year to year which are more consistent in the summer months than during winter and have maximum absolute values in 2009. Furthermore, the precise SMF radar measurements give an excellent possibility to evaluate momentum flux estimates from the colocated meteor radar at Andenes. Both radars have different capabilities, and different techniques are applied to derive momentum fluxes. They show comparable results for the 4 year mean annual cycles of horizontal winds and momentum fluxes especially in summer. This holds for both structure and magnitudes in the overlapping heights, where the SMF radar data provide a wider vertical coverage. The best agreement is found for the zonal components of both radars, whereas there are some larger discrepancies in the meridional components, especially in the vertical flux of meridional momentum.

Short period gravity wave momentum fluxes observed in the tropical troposphere, stratosphere and mesosphere

Using long-term data (1998–2008) collected from mesosphere–stratosphere–troposphere (MST) radar and Rayleigh Lidar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, vertical flux of the momentum from troposphere to mesosphere associated with the gravity waves of periods in the range 20min to 2h is investigated for the first time. The emphasis is on seasonal variability of mean zonal and meridional momentum fluxes in mesosphere and troposphere and vertical flux of horizontal momentum in the stratosphere. At tropospheric altitudes of 11–16km large enhancement in flux is noticed during equinoxes. In the lower mesosphere in the altitude region 58–62km the maximum values of flux (∼2.8m2/s2) observed are pragmatic in winter and spring. Interestingly, the vertical flux of horizontal momentum estimated from lidar is in the range of those estimated from radar data in the overlap altitude region, though the estimates are from two different techniques. In the mesosphere, large variations with altitude in zonal momentum flux are noticed with a magnitude ∼0–4m2/s2 in summer. In winter and summer the zonal wind direction is opposite to the momentum flux direction between 73 and 80km and in equinoxes zonal wind follows the momentum flux. The meridional fluxes in the mesosphere are higher in equinoxes (∼10–12m2/s2).

Momentum flux determination using the multi-beam Poker Flat Incoherent Scatter Radar

Abstract. In this paper, we develop an estimator for the vertical flux of horizontal momentum with arbitrary beam pointing, applicable to the case of arbitrary but fixed beam pointing with systems such as the Poker Flat Incoherent Scatter Radar (PFISR). This method uses information from all available beams to resolve the variances of the wind field in addition to the vertical flux of both meridional and zonal momentum, targeted for high-frequency wave motions. The estimator utilises the full covariance of the distributed measurements, which provides a significant reduction in errors over the direct extension of previously developed techniques and allows for the calculation of an error covariance matrix of the estimated quantities. We find that for the PFISR experiment, we can construct an unbiased and robust estimator of the momentum flux if sufficient and proper beam orientations are chosen, which can in the future be optimized for the expected frequency distribution of momentum-containing scales. However, there is a potential trade-off between biases and standard errors introduced with the new approach, which must be taken into account when assessing the momentum fluxes. We apply the estimator to PFISR measurements on 23 April 2008 and 21 December 2007, from 60–85 km altitude, and show expected results as compared to mean winds and in relation to the measured vertical velocity variances.

Drake Antarctic Agile Meteor Radar first results: Configuration and comparison of mean and tidal wind and gravity wave momentum flux measurements with Southern Argentina Agile Meteor Radar

A new generation meteor radar was installed at the Brazilian Antarctic Comandante Ferraz Base (62.1°S) in March 2010. This paper describes the motivations for the radar location, its measurement capabilities, and comparisons of measured mean winds, tides, and gravity wave momentum fluxes from April to June of 2010 and 2011 with those by a similar radar on Tierra del Fuego (53.8°S). Motivations for the radars include the “hotspot” of small‐scale gravity wave activity extending from the troposphere into the mesosphere and lower thermosphere (MLT) centered over the Drake Passage, the maximum of the semidiurnal tide at these latitudes, and the lack of other MLT wind measurements in this latitude band. Mean winds are seen to be strongly modulated at planetary wave and longer periods and to exhibit strong coherence over the two radars at shorter time scales as well as systematic seasonal variations. The semidiurnal tide contributes most to the large‐scale winds over both radars, with maximum tidal amplitudes during May and maxima at the highest altitudes varying from ∼20 to >70 ms−1. In contrast, the diurnal tide and various planetary waves achieve maximum winds of ∼10 to 20 ms−1. Monthly mean gravity wave momentum fluxes appear to reflect the occurrence of significant sources at lower altitudes, with relatively small zonal fluxes over both radars, but with significant, and opposite, meridional momentum fluxes below ∼85 km. These suggest gravity waves propagating away from the Drake Passage at both sites, and may indicate an important source region accounting in part for this “hotspot.”

Seasonal variation of planetary wave momentum flux and the forcing towards mean flow acceleration in the MLT region

Monthly and seasonal variations of vertical flux of zonal and meridional momentum (u′w′ and v′w′) for the prominently present planetary waves (2–4 day and 6–8 day period) in the Mesospheric Lower Thermospheric (MLT) region are estimated by making use of the horizontal and vertical wind information obtained from SKiYMET Meteor Wind radar observations at Trivandrum (8.5°N, 77°E), during the period April 2006–April 2007. Substantial transport of momentum is observed during equinox and winter season over the station. The momentum flux values estimated from three dimensional wind velocities obtained from SKiYMET radar are compared with that calculated by the indirect method of cross-spectra from radar measured zonal winds and temperature from the SABER onboard TIMED satellite and found to be in good agreement. An attempt is made to calculate the forcing towards the mean flow by the 2–4 day and 6–8 day planetary waves in the 82–98km height region. The forcing by 6–8 day wave towards the mean flow acceleration is found to be more during winter and vernal equinox and by 2–4 day is more during summer and both the equinoxes. The quantitative estimation of forcing towards the mean flow acceleration show generally 30–80% contribution from both 2–4 day wave and 6–8 day wave, which varies with season also. The study presented in this paper is the first of its kind, quantitatively estimating the momentum flux values of short period planetary waves in the MLT region at an equatorial station and their forcing towards the mean flow.

Effect of Tropical Waves on the Tropical Tropopause Transition Layer Upwelling

Abstract An initial-value problem is employed with a GCM to investigate the role of the convectively driven Rossby and Kelvin waves for tropopause transition layer (TTL) upwelling in the tropics. The convective heating is mimicked with a prescribed heating field, and the Lagrangian upwelling is identified by examining the evolution of passive tracer fields whose initial distribution is identical to the initial heating field. This study shows that an overturning circulation, induced by the tropical Rossby waves, is capable of generating the TTL upwelling. Even when the heating is placed in the eastern Pacific, the TTL upwelling occurs only over the western tropical Pacific, indicating that the background flow plays a crucial role. The results from a Rossby wave source analysis suggest that a key feature of the background flow is the strong absolute vorticity gradient associated with the Asian subtropical jet. In addition, static stability is relatively weak over the western Pacific, suggesting that this may also contribute to the TTL upwelling in that region. The background flow also modulates the internal Kelvin waves in such a manner that the coldest region in the TTL (resembling the observed “cold trap”) occurs over the western tropical Pacific. As a consequence, the upwelling air, induced by the meridional momentum flux of the Rossby wave, passes through the cold trap generated by the Kelvin wave. Since in reality the background flow is shaped by the convective heating, the climatological western tropical Pacific heating is ultimately responsible for both the TTL upwelling and the cold trap; however, both processes are realized indirectly through its impact on the background flow and the generation of the tropical waves.

Momentum flux of stratospheric gravity waves generated by Typhoon Ewiniar(2006)

The momentum flux of stratospheric gravity waves generated by Typhoon Ewiniar (2006) is examined using a Weather Research and Forecasting (WRF) model. In the stratosphere, zonal momentum flux with a positive sign by eastward-propagating waves is significant during the northward moving of the typhoon, while both zonal and meridional momentum fluxes with positive signs are significant during the typhoon decaying stage in which the typhoon moves northeastward. The magnitude of the momentum flux is greater during the mature stage of the typhoon than the decaying stage, and the phase speeds of the dominant momentum flux are less than 30 m s−1 with a peak at 10–16 m s−1. Positive momentum flux decreases with height overall in the stratosphere for both zonal and meridional components. The resultant gravity-wave drag forcing plays a role to decelerate the easterly background wind in the stratosphere. This drag forcing is relatively large above z = 40 km and below z = 20 km, and lower stratospheric wave drag is expected to affect the typhoon dynamics by modifying the background wind shear and inducing the secondary circulation in the troposphere.

MF radar observations of the diurnal tide over Syowa, Antarctica (69° S, 40° E)

Abstract. Characteristics of the diurnal tide in the Antarctic mesosphere and lower thermosphere (MLT) are investigated using 10 years of medium frequency (MF) radar data from Syowa Station (69° S, 39.6° E). Seasonal variations and height dependence of the diurnal amplitude and phase of zonal and meridional winds are mostly consistent with previous studies using the other Antarctic station data. The meridional momentum flux due to the diurnal tide shows a seasonal variation clearly different between above and below 90 km, which has never been reported in the literature. Finally, a cause of some discrepancy in the characteristics of the diurnal tide between the observation and simulation (i.e., GSWM-02) is discussed. It implies that the realistic representation of gravity waves in the simulation is crucial for realistic modeling of the diurnal tide.

Estimation of Gravity Wave Momentum Flux and Phase Speeds from Quasi-Lagrangian Stratospheric Balloon Flights. Part II: Results from the Vorcore Campaign in Antarctica

The stratospheric gravity wave field in the Southern Hemisphere is investigated by analyzing observations collected by 27 long-duration balloons that flew between September 2005 and February 2006 over Antarctica and the Southern Ocean. The analysis is based on the methods introduced by Boccara et al. in a companion paper. Special attention is given to deriving information useful to gravity wave drag parameterizations employed in atmospheric general circulation models. The balloon dataset is used to map the geographic variability of gravity wave momentum fluxes in the lower stratosphere. This flux distribution is found to be very heterogeneous with the largest time-averaged value (28 mPa) observed above the Antarctic Peninsula. This value exceeds by a factor of ∼10 the overall mean momentum flux measured during the balloon campaign. Zonal momentum fluxes were predominantly westward, whereas meridional momentum fluxes were equally northward and southward. A local enhancement of southward flux is nevertheless observed above Adélie Land and is attributed to waves generated by katabatic winds, for which the signature is otherwise rather small in the balloon observations. When zonal averages are performed, oceanic momentum fluxes are found to be of similar magnitude to continental values (2.5–3 mPa), stressing the importance of nonorographic gravity waves over oceans. Last, gravity wave intermittency is investigated. Mountain waves appear to be significantly more sporadic than waves observed above the ocean.

Meridional Momentum Flux and Superrotation in the Multiscale IPESD MJO Model

Abstract The derivation of the meridional momentum flux arising from a multiscale horizontal velocity field in the intraseasonal, planetary, equatorial synoptic-scale dynamics (IPESD) multiscale models of the equatorial troposphere is presented. It is shown that, because of the balance dynamics on the synoptic scales, the synoptic-scale component of the meridional momentum flux convergence must always vanish at the equator. Plausible Madden–Julian oscillation (MJO) models are presented along with their planetary-scale meridional momentum fluxes. These models are driven by synoptic-scale heating fluctuations that have vertical and meridional tilts. Irrespective of the sign of the synoptic-scale meridional momentum flux (direction of the tilts) in each of the four MJO examples, the zonal and vertical mean meridional momentum flux convergence from the planetary scales always drives westerly winds near the equator: this is the superrotation characteristic of actual MJOs. The concluding discussion demonstrates that equatorial superrotation occurs when the planetary flow due to the vertical upscale momentum flux from synoptic scales reinforces the horizontally convergent flow due to planetary-scale mean heating.

Seasonal variations of the gravity wave momentum flux in the Antarctic mesosphere and lower thermosphere

Airglow imager and dynasonde/imaging Doppler interferometer (IDI) radar wind measurements at Halley Station, Antarctica (75.6°S, 26.6°W) have been used to estimate the seasonal variation of the vertical fluxes of horizontal momentum carried by high‐frequency atmospheric gravity waves. The cross‐correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2000 and 2001. These were combined with wind velocity variances from coincident radar measurements to estimate the daily averaged upper limit of the vertical flux of horizontal momentum due to gravity waves. The resulting momentum flux at the Na airglow altitudes, while displaying a large day‐to‐day variability, showed a marked rotation from the northwest to the southeast throughout the winter season. Calculations show that this rotation is consistent with seasonal changes in the wind field filtering of gravity waves below the Na airglow region. The calculations also indicate that while the magnitude of the meridional wind is small, this filtering leads to the observed seasonal changes in the meridional momentum flux.

Momentum flux associated with gravity waves in the low-latitude troposphere

Abstract. The vertical fluxes of horizontal momentum at tropospheric heights are calculated for four days, 25–28 August 1999. The mean zonal wind during these days show the presence of strong westward wind at the upper troposphere. Both the symmetric beam radar method and the power spectral method of evaluation of vertical flux of zonal and meridional momentum shows nearly the same result for quiet conditions. The temporal evolution of the momentum flux is estimated for a day with strong zonal shear and convection. These results indicate that on 28 August 1999, the strong downward vertical wind in the lower altitude range is associated with upward vertical flux of zonal momentum, and strong upward vertical wind is associated with downward vertical flux. Similarly, the strong shear in zonal wind is associated with the increase in negative values in vertical flux in the upper troposphere. Analysis of the role of wave periods in the transport of momentum flux indicates that the vertical momentum flux magnitude is not evenly distributed in all wave periods, but instead it peaks at certain wave periods in the range 10 to 100 min.Key words. Meteorology and atmospheric dynamics (convective process; tropical meteorology; precipitation)