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Gravity waves in the atmosphere can be generated by deep convection as well as by orography. High resolution radiosonde data form the basis of a study to find whether convectively generated gravity waves are energetically important in the midlatitudes.
Several years of data from stations in the UK are used. Convective available potential energy (CAPE) is used as an indicator of the likely occurrence of deep convection. Gravity waves are detected through their effect on the ascent rate of radiosondes (typical vertical velocity perturbations are 1 ms-1) and by examining the hodograph (u-v diagram) using the Stokes parameter technique for finding the polarization properties. Cases of correspondence between high values of CAPE and the existence of waves in the stratosphere are described. Shear is determined for the ascents. In the resulting climatology it is found that high values of CAPE occur in about 10% of all the gravity wave cases in the radiosonde ascents.
The energy content of gravity waves is determined using a spectral analysis. Using the total data set it is found that the energy content of gravity waves on average is NOT as important during convection as during strong shear events. This can also be seen in the associated calculations of the momentum transfer.
These two results suggest that convectively generated gravity waves represent a physical process in the midlatitudes that should not be neglected.
For the spectral analysis the high resolution radiosonde data was analysed following the work of Allen and Vincent (1995) and Vincent et. al. (1997). Climatologies of energy and of direction of propagation in both the horizontal and vertical directions which can be determined from high resolution radiosonde data are a valuable tool to do this.
Figure 1 : Energy density for high CAPE cases (solid) and low CAPE cases
(dashed), for the total ascents for Camborne from 1990-1996 and
for high (solid) and low (dashed) shear cases.
Figure 2 : Energy density for high CAPE cases (solid) and low CAPE cases
(dashed), for the total ascents for Camborne from 1990-1996 and
for high (solid) and low (dashed) shear cases.