Abstract
Abstract. The latitudinal and seasonal variations of gravity wave (GW) potential energy density (EP), kinetic energy density (EK), and total energy density (ET), i.e, the sum of potential and kinetic energy densities in the tropospheric (typically 2–10 km) and lower stratospheric (typically 18–25 km) segments have been derived from 10 years (1998–2007) of radiosonde observations over 92 United States stations in the Northern Hemisphere. The latitudinal variation of EP in the lower stratosphere is in good agreement with satellite observations. However, EK and ET in the lower stratosphere are different from satellite observations and the difference is believed to be linked with the latitudinal dependence of GW sources. Our analysis reveals that GW energy properties exhibit distinctive latitudinal and seasonal variations. The upward-propagating GW energy in the troposphere is larger than that in the lower stratosphere at low latitudes but the opposite holds true at high latitudes. The transition latitude, where the upward- propagating energies in the two altitude regions are the same, occurs at 35° N throughout the year. So striking differences between GW activity in the troposphere and lower stratosphere are not likely explained only by the background wind Doppler shifting due to strong tropospheric jets. Our analysis indicates that the region around tropopause, roughly from 10 km to 18 km, is an important source region, especially at latitudes below 35° N. Our studies strongly suggest that in order to fully understand the global GW activity in the lower atmosphere, the GW kinetic energy and its geographical and seasonal variations should be included, and more attention should be given to GWs in the troposphere and GW sources within the intermediate region, especially the upper troposphere.
Highlights
Decades of observations and simulations have revealed the key role of gravity waves (GWs) in determining the local and global dynamic and thermal structures of the middle and upper atmosphere by dissipating their energy and momentum in the background atmosphere through various complex processes
In order to study the latitudinal variation of GW activity, the calculated GW energy is divided into 5◦ bins in latitude
The vertical wavelengths for GWs in the lower stratosphere derived from present radiosonde observations are usually less than 5 km and concentrate around 2 km, which is in good agreement with satellite observations (Tsuda et al, 2000; Venkat Ratnam et al, 2004)
Summary
Decades of observations and simulations have revealed the key role of gravity waves (GWs) in determining the local and global dynamic and thermal structures of the middle and upper atmosphere by dissipating their energy and momentum in the background atmosphere through various complex processes. In modeling global scale atmospheric dynamics, the effects of GWs must be parameterized to obtain realistic mean atmospheric circulations and thermal structures. The parameterized GW effects are believed to be major sources of uncertainties in model predictability and reliability. Especially global observations, is one of the primary limitations in developing and verifying GW source parameterizations. The lower atmosphere, especially the troposphere, is believed to be the main source region of GWs propagating in the middle and upper atmosphere. Observations of lower atmospheric GWs are important in extending our knowledge of atmospheric GWs and their impacts on the background atmospheric dynamic and thermal structures
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