[1] In this paper, possible source mechanisms for the generation of inertia-gravity wave activity over a tropical station, Gadanki (13.5°N, 79.2°E), are investigated using a long-term data set obtained from Indian mesosphere-stratosphere-troposphere (MST) radar. The gravity wave analysis is carried out in two different height regions, namely, 4-14 and 17-21 km, representing the troposphere and lower stratosphere, respectively. Clear seasonal variation in the wave activity has been noticed in both regions with maximum (minimum) in winter (monsoon) in the troposphere. But it is maximum (minimum) in monsoon (winter) in the lower stratosphere. This kind of winter enhancement in the wave activity is not expected at this tropical site. Interestingly, the contribution of the meridional component to the total kinetic energy (E k ) is found to be dominant rather than zonal in the winter except during 1997-1998. Topography seems to be the likely source for the generation of wave activity during winter in the troposphere. The influence of this topography is also reflected in the nearby radiosonde stations, Chennai (13.0°N, 80.2°E) and Bangalore (12.9°N, 77.6°E), which are located at radial distances of 128 and 190 km from Gadanki, respectively. Although two major sources, that is, strong convection and wind shears, coexist during monsoon season, strong wind shear seems to be the likely source of the wave activity. Large interannual variability in the wave activity is also noticed from 9 years (September 1995 to December 2004) of data. Good consistency is observed between the wave activities observed from nearby (Chennai) radiosonde and Gadanki MST radar data sets. Making use of a network of radiosonde observations operated by India Meteorological Department, we also present the latitudinal variation of wave activity. From the latitudinal variations it is observed that large-scale systems can also influence the generation of the gravity wave activity over larger areas.Citation: Venkat Ratnam, M., A. Narendra Babu, V. V. M. Jagannadha Rao, S. Vijaya Bhaskar Rao, and D. Narayana Rao (2008), MST radar and radiosonde observations of inertia-gravity wave climatology over tropical stations: Source mechanisms, J. Geophys.
The global pattern of convective available potential energy (CAPE) at seasonal and diurnal time scales is discussed using 1 year of COSMIC/FORMOSAT‐3 satellite observations. The calculation of CAPE using temperature and humidity measurements of COSMIC is described. The estimated CAPE is grouped into 5 × 5 grid and is further classified into four seasons, namely, winter, spring, summer, and autumn. The CAPE magnitudes in general have high values over land as compared to oceanic region, which confirmed the consistency of CAPE calculations. The systematic migration of CAPE from Northern Hemisphere to Southern Hemisphere is observed during Northern Hemisphere summer to winter, coinciding with the movement of intertropical convergence zone. Once the seasonal pattern is established, the composite diurnal patterns of CAPE with 2 h resolution are obtained by combing all the observations in one season. Diurnal variation of CAPE has shown domination of semidiurnal variations at some latitudes (12 h) and diurnal variation (24 h) at some other latitudes. The mean removed CAPE is then subjected to Fourier analysis to extract the diurnal variation amplitudes. During the observational period, larger CAPE magnitudes are observed over the Indian Ocean during most of the seasons, comparable in magnitude to that of the land regions. As the CAPE and precipitation patterns have correlation, the present study demonstrated the capability of COSMIC/FORMOSAT‐3 to study the diurnal patterns of CAPE, which will have implications in interpreting the tropical diurnal precipitation patterns.
Abstract. The potential utility of Mesosphere-StratosphereTroposphere (MST) radar measurements of zonal, meridional and vertical winds for divulging the gravity wave vertical wave number spectra is discussed. The data collected during the years 1995-2004 are used to obtain the mean vertical wave number spectra of gravity wave kinetic energy in the tropical troposphere over Gadanki (13.5 • N, 79.2 • E). First, the climatology of 3-dimensional wind components is developed using ten years of radar observations, for the first time, over this latitude. This climatology brought out the salient features of background tropospheric winds over Gadanki. Further, using the second order polynomial fit as background, the day-to-day wind anomalies are estimated. These wind anomalies in the 4-14 km height regions are used to estimate the profiles of zonal, meridional and vertical kinetic energy per unit mass, which are then used to estimate the height profile of total kinetic energy. Finally, the height profiles of total kinetic energy are subjected to Fourier analysis to obtain the monthly mean vertical wave number spectra of gravity wave kinetic energy. The monthly mean vertical wave number spectra are then compared with a saturation spectrum predicted by gravity wave saturation theory. A slope of 5/3 is used for the model gravity wave spectrum estimation. In general, the agreement is good during all the months. However, it is noticed that the model spectrum overestimates the PSD at lower vertical wave numbers and underestimates it at higher vertical wave numbers, which is consistently observed during all the months. The observed discrepancies are Correspondence to: A. Narendra Babu (narendraalur@gmail.com) attributed to the differences in the slopes of theoretical and observed gravity wave spectra. The slopes of the observed vertical wave number spectra are estimated and compared with the model spectrum slope, which are in good agreement. The estimated slopes of the observed monthly vertical wave number spectra are in the range of −2 to −2.8. The significance of the present study lies in using the ten years of data to estimate the monthly mean vertical wave number spectra of gravity waves, which will find their application in representing the realistic gravity wave characteristics in atmospheric models.
[1] The unique facility of measuring vertical winds using Indian mesosphere, stratosphere, and troposphere (MST) radar along with zonal and meridional winds enables the study of atmospheric circulation over Gadanki (13.5°N, 79.2°E) during the Indian summer monsoon season. The mean meridional circulations during winter and monsoon seasons represent part of two different Hadley circulations. The winter Hadley cell is observed to be stable whereas the monsoon Hadley cell seems to vary and depends on the monsoon activity. During active phase of the monsoon, the Hadley cell extends to the north, and during weak phase, it extends to the south of the study region. The observed features are compared with the winds obtained from National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data. The present study emphasizes that the atmospheric circulation during monsoon season is to be studied separately for active and break phases.Citation: Roja Raman, M., V.
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