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Ann. Geophys., 26, 1671-1680, 2008
www.ann-geophys.net/26/1671/2008/
© European Geosciences Union 2008
Long-term MST radar observations of vertical wave number spectra of gravity waves in the tropical troposphere over Gadanki (13.5° N, 79.2° E): comparison with model spectra
A. Narendra Babu1, K. Kishore Kumar2, G. Kishore Kumar1, M. Venkat Ratnam3, S. Vijaya Bhaskara Rao1, and D. Narayana Rao4
1Sri Venkateswara University, Tirupati, India
2Space Physics Laboratory, Vikram Sarabhai Space Center, Thiruvananthapuram, India
3National Atmospheric Research Laboratory, Gadanki, India
4RISH, Kyoto University, Japan
Abstract. The potential utility of Mesosphere-Stratosphere-Troposphere (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 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.
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