Thursday, 2 August 2001

A numerical study on characteristics of internal gravity waves in the stratosphere induced by mesoscale convective system

Three numerical simulations are performed using a 2-dimensional (x-z), non-hydrostatic cloud model in order to examine characteristics of the stratospheric internal gravity waves induced by mesoscale convective system and understand mechanisms for generating those waves. In control simulation (CTR), an organized mesoscale convective system is simulated under warm rain microphysics and wind and thermodynamic environmental conditions favorable to typical mid-latitude squall lines. The convective system in CTR reaches a quasi-stationary state after t=4 hr, and convective cells are regenerated in the convective system every about 18.05 min. Spectral analysis shows that the stratospheric internal gravity waves induced by the convective system have dominant vertical wavelengths of 6.8 - 10 km, horizontal wavelengths of 10 - 100 km, and periods of 10 - 60 min. Even though period of 17.19 min associated with the regeneration of convective cells is dominant in frequency spectra of gravity waves, there also exists a spectral peak on 60.17 min. In order to understand possible mechanisms to characterize the convectively generated gravity waves in the stratosphere, two dry simulations are made, DRYN simulation with vertical momentum forcing and DRYH simulation with diabatic heating/cooling forcing extracted from the CTR for every 1 min. In DRYM, the vertical momentum forcing can generate gravity waves whose temporal and spatial scales are similar to those by convective cells in the CTR. But the momentum forcing fails to generate waves with long period (~ 60 min) and large horizontal wavelength (~ 150 km). Contrary, the diabatic forcing can generate the stratospheric gravity wave spectra similar to those in the CTR. The gravity waves with a short period produced by DRYM propagate westward relative to the ground slowly, while those with a long period and large horizontal wavelength simulated by DRYH propagate westward relative to the ground with high phase speed. It shows that the slowly varying spatial structure of the diabatic forcing with the dominant periodicity of about 60 min can explain the gravity waves with the period of 60 min and the horizontal wavelength of 150 km.

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