Horizontal Wind Divergence Research Articles

Retrieval of large‐scale wind divergences from infrared Meteosat‐5 brightness temperatures over the Indian Ocean

Over the tropics the atmospheric general circulation models usually fail in predicting horizontal wind divergence, which is closely related to atmospheric heating and to the vertical exchanges associated with convection. With the aim of forcing atmospheric models we present here a reconstruction of wind divergences based on the links between infrared brightness temperatures, convective activity, and large‐scale divergence. In practice, wind divergences are reconstructed from brightness temperatures using correlations obtained from numerical simulations performed with a general circulation model. When building those correlations, a distinction must be made between the brightness temperatures of opaque clouds and those of semitransparent clouds, only the former being directly associated with convection. In order to filter out semitransparent clouds we use radiative thresholds in the water vapor channel in addition to the window channel. We apply our approach to Meteosat‐5 data over the Indian Ocean. Comparison with wind divergences reconstructed independently from Meteosat water vapor winds partially validates our retrieval. Comparison with European Center for Medium‐Range Weather Forecasts analyses indicates that much can be gained by adding information on the wind divergence in the tropics to force an atmospheric model.

VERTICAL PROFILES OF MEAN WIND SPEED AND TURBULENT CHARACTERISTICS OF SEA BREEZES

Field observations to investigate the structure of sea breezes have been made on Fukuoka Plain. The intrusion speeds of sea-breeze front are estimated from the distributions of horizontal wind divergence. The wind velocity before and after the arrival of sea-breeze front has been measured by using a Doppler sodar, and the vertical profiles of mean wind speed, standard deviation of horizontal and vertical wind velocities and turbulent kinetic energy have been obtained. They are normalized by using the maximum wind speed and the height at a half of the maximum speed. Especially, the normalization for the wind speed profiles seems to give a good similarity.

Statistics of momentum flux estimation using the dual coplanar beam technique

We examine the statistical estimation error for the dual coplanar beam momentum flux estimator in the presence of geophysical noise. Included in our derivation are the effects of measurement noise and finite spatial correlation of the wind fluctuations. We show that there is an optimal beam separation that minimizes the statistical estimation error at a value nearly equal to the irreducible error observed by an “ideal” anemometer measuring u and w at a point. For typical mesopause parameters this beam separation is approximately ±13°. The optimal angle is dependent in part on the ratio of vertical to horizontal wind variances and is therefore variable. For optimal momentum flux measurements a system, radar or lidar, with steerable beams becomes necessary.

Another Look at the Annual and Semiannual Cycles of AtmosphericAngular Momentum

Abstract The annual variation of global atmospheric angular momentum (AAM) is dominated by its first and second harmonic components. The first harmonic is associated with maximum global AAM in winter (December– January–February) and minimum in summer, but the second harmonic is important enough to produce a distinct secondary midwinter minimum. Locally, the second harmonic has largest amplitude in the Tropics and subtropics of the upper troposphere. At present, little is known concerning the fundamental cause of this semiannual variation. The problem is investigated here by focusing on the upper-tropospheric winds, whose angular momentum is an excellent proxy of global AAM. The annual variation of the rotational part of these winds (the part that contributes to the global AAM) is diagnosed in a nonlinear upper-tropospheric vorticity-equation model with specified horizontal wind divergence and transient-eddy forcing. The divergence forcing is the more important of the two, especially in the Tropics and sub...

VHF radar observations of gravity waves at a low latitude

Abstract. Wind observations made at Gadanki (13.5°N) by using Indian MST Radar for few days in September, October, December 1995 and January, 1996 have been analyzed to study gravity wave activity in the troposphere and lower stratosphere. Horizontal wind variances have been computed for gravity waves of period (2-6) h from the power spectral density (PSD) spectrum. Exponential curves of the form eZ/H have been fitted by least squares technique to these variance values to obtain height variations of the irregular winds upto the height of about 15 km, where Z is the height in kilometers. The value of H, the scale height, as determined from curve fitting is found to be less than the theoretical value of scale height of neutral atmosphere in this region, implying that the waves are gaining energy during their passage in the troposphere. In other words, it indicates that the sources of gravity waves are present in the troposphere. The energy densities of gravity wave fluctuations have been computed. Polynomial fits to the observed values show that wave energy density increases in the troposphere, its source region, and then decreases in the lower stratosphere.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)

Dynamics of Neutral Wind in the polar region observed with two Fabry-Perot Interferometers

Optical observations were made at Ramfjord, Norway from January 10 to February 14,1997. Two types of Fabry-Perot interferometers (FPIs), Doppler-imaging and scanning, were installed at the EISCAT radar site and were used to acquire data simultaneously with radio instruments. Both FPIs can observe emissions of two different wavelengths simultaneously. We can estimate the horizontal and vertical wind in different emission layers simultaneously with high time-resolution (∼1 min). The observations on February 8 and 9, 1997, show some notable characteristics: (1) large-scale perturbations (≈ ±150 m/s) are observed in the upper thermospheric wind. They seem to begin 30 min after the onset of a magnetic substorm and to stop when the next substorm begins. (2) Clear wave-like structures are found in the horizontal wind variations. Some of them can be seen over the entire sky, and one of them is found in a restricted regions. (3) A clear wave-like structure is also found in the vertical wind in the upper thermosphere. A similar structure can be seen in the lower thermosphere, but these structures are not always in phase. This phases difference starts at the same time that horizontal winds between the two layer has their phase difference. (4) The relation between the vertical wind and the divergence of horizontal wind seems to change with time. The correlation coefficient between them changes one-hours before and on-time of a substorm on-set. This sign of the coefficient is negative in most of the time, with considering about time-lag. It means the vertical morion is caused by divergent flow of horizontal wind.

VHF radar observations of gravity waves at a low latitude

Wind observations made at Gadanki (13.5°N) by using Indian MST Radar for few days in September, October, December 1995 and January, 1996 have been analyzed to study gravity wave activity in the troposphere and lower stratosphere. Horizontal wind variances have been computed for gravity waves of period (2-6) h from the power spectral density (PSD) spectrum. Exponential curves of the form eZ/H have been fitted by least squares technique to these variance values to obtain height variations of the irregular winds upto the height of about 15 km, where Z is the height in kilometers. The value of H, the scale height, as determined from curve fitting is found to be less than the theoretical value of scale height of neutral atmosphere in this region, implying that the waves are gaining energy during their passage in the troposphere. In other words, it indicates that the sources of gravity waves are present in the troposphere. The energy densities of gravity wave fluctuations have been computed. Polynomial fits to the observed values show that wave energy density increases in the troposphere, its source region, and then decreases in the lower stratosphere.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)

Climatology of mesospheric gravity wave activity over Urbana, Illinois (40°N, 88°W)

A 5‐year (1991–1995) climatology of mesospheric gravity wave activity observed with the Urbana medium frequency (MF) radar is presented. We focus on the monthly mean horizontal wind variances associated with gravity waves with observed periods between 10 min and 8 hours. These results are compared with the seasonal characteristics from other gravity wave observations. The total wave variance, , displays a dominant semiannual variation (maxima summer/winter, minima spring/fall) below 87 km with little seasonal variation above 87 km. The zonal wave variance exceeds the meridional wave variance, , except around regions of zonal wind reversal. In all seasons the observed variance scale height is greater than that of a nondissipating wave field. A simple zero mean wind model based on linear saturation theory is found to be inadequate in reproducing the seasonal characteristics of the observed wave variances. An alternative model based on critical layer filtering by a height dependent wind field can reproduce the observed semiannual oscillation in the wave variance given a specific source input.

Errors in Wind Measurements Estimated by Five-Beam Phased Array Doppler Sodar

Abstract Estimation of the errors in sodar wind measurements using the phased array Doppler sodar operated under five-beam observation has been made. When compared with the sonic anemometer on the tower nearby, the variances or second-order moments of sodar-derived horizontal wind, σu and συ , are about twice as large as those measured at the tower, even though the mean horizontal wind u and the second moment of vertical wind σw are in reasonable agreement with the tower. The random errors in the measured wind variances, which can be separated in the five-beam detection system, are estimated to be about 0.41 m s−1 for the vertical component and 0.46 m s−1 for the tilted beam axis component. If the compensation is made for each component along the antenna beams, the horizontal wind variances averaged over 60 min agree with those values obtained by the sonic anemometer on the tower. An analysis of the power spectra suggests that the discrepancies in σu and συ are mainly caused by the excess of sodar spectra...

A comment on the geostrophic wind divergence

V VH V, = -2 (tan # + cot $1, r where VH is the horizontal divergence, V = (ug, ve) the geostrophic wind vector, r the distance from the earth’s centre and @ the latitude. Although the two terms on the right-hand side of (1) appear as a result of the earth’s sphericity, it is suggested from what follows that the tan@ term is superfluous and should be omitted. The horizontal wind divergence on a spherical surface was given by Pedlosky (1979) as I 1 a r c o s @ { i i a@ V H v = + -(v cos @)

The Damping of Potential Enstrophy in the Large-Scale Transient Eddies in the Wintertime Troposphere

Abstract On the average, there has to be a balance in the atmosphere between the rate at which the potential enstrophy of the transient eddies (TE) is generated by conversion from the time-mean flow and the rate at which it is damped by diabatic and frictional processes. In the quasi-geostrophic framework this conversion rate (and, hence, also the damping rate) can be estimated from conventional circulation statistics for temperature and horizontal wind. This estimation does not require data on vertical velocity and horizontal wind divergence, which are poorly known and play a crucial role in the TE energetics. Thus, from the data, one can presumably get more reliable information about the damping of the TE potential enstrophy than about the diabatic and frictional damping of the TE energy. An observational study is made of the amount and maintenance of the (quasi-geostrophic) potential enstrophy of the large-scale transient eddies in the troposphere over the Northern Hemisphere in February 1979 by using ...

Associations among the tropical pacific wind and sea surface temperature fields and higher latitude circulation

AbstractAn empirical study was conducted examining associations between seasonal trade wind and sea surface temperature (SST) fields in the eastern half of the tropical Pacific Ocean for the period 1949‐1979. In addition, the tropical fields were statistically related to the extratropical circulation of the Northern Hemisphere (i.e. 700 mb heights). This study differs from most prior studies in that the trade wind field was examined primarily in terms of the horizontal trade wind divergence and seasonally stratified lag relationships were computed.Seasonally stratified autocorrelations of empirical orthogonal function (EOF) amplitudes (out to four seasons) indicated significant persistence of EOF1 of SST and divergence until spring‐summer for any initial season; accordingly, spring and summer (summer and autumn) EOF1 for SST (divergence) exhibited the greatest persistence. For divergence some tendency for a reversal of sign was seen from the autumn, winter, or spring seasons to the following autumn, which may imply negative air‐sea feedback mechanisms operating.Seasonally stratified cross correlations betwen the first EOF for SST and divergence indicated a significant contemporaneous relationship. When divergence leads SST, the relationship is significant at short lags (1‐3 seasons) until spring as a target season is reached. The same is true when SST leads divergence except that a significant relationship returns with opposite sign in the subsequent summer and auiumn. Winter SST and divergence, in particular, are well correlated with prior values of the other parameter.When the tropical parameters were related to the extratropical circulation through correlation with gridded 700 mb heights, significant patterns were found only when winter 700 mb heights were the target parameter (i.e. tropical parameter leading winter 700 mb heights). Summer through winter tropical SST and divergence (EOF1) displays a good association with subsequent winter extratropical circulation in the form of an expanded/contracted circumpolar vortex. The relationship between trade wind divergence and subsequent winter circulation is significant using the prior winter (4 season lag), vanishes during spring, and returns with the opposite sign for summer through winter (lags 0‐2). Using either SST or divergence, the observed relationships are stronger at lags 1 and 2 than at lag 0.

CASE STUDIES OF NUMERICAL WIND ANALYSES

With the aid of an electronic computer, case studies of wind analyses at the 850-mb., 700-mb., 500-mb., 400-mb., 300-mb., and 200-mb. pressure levels have been made. The divergent and non-divergent wind components resulting from the u and v wind-component analyses are investigated. For the cases considered, the streamfunction fields are slightly superior to the Joint Numerical Weather Prediction operational fields, obtained initially through use of the “balance equation.” The magnitude of the horizontal wind divergence values are comparable to those obtained from the winds by previous investigators employing hand-analysis techniques. However, the divergence patterns are not sufficiently accurate for the strict requirements necessary for numerical weather forecasting.

Upper air contour patterns and associated heavy rainfall during the southwest monsoon

The necessity of dealing with the lower troposphere over India as consisting of two separate strata having opposite values of horizontal wind divergence is briefly explained. It may not be sufficient to look for areas of convergence at the lower levels areas of marked divergence at the upper levels of 500 mb and aloft may be equally, if not more, important for the understanding and forecasting of very heavy rains. The principles of deducing areas of marked horizontal divergence on qualitative reasoning based on considerations of non-geostrophic motion as well as of air flow down the gradients of vorticity and then outlined. A few cases of non-orographic phenomenal rainfall of 10-20 in 24 hours which fell over certain parts of India during the southwest monsoon of 1954 are shown to have fallen in areas of marked upper air divergence, deducible from such qualitative reasoning applied to flow patterns at 500 mb and a aloft.