MESOSCALE SPECTRA OF THE FREE ATMOSPHERIC MOTION IN MID-LATITUDE SUMMER－UNIVERSALITY AND CONTRIBUTION OF THUNDERSTORM ACTIVITIES

Meridional and vertical wind velocities of the free atmosphere were observed continuously in mid-latitude summer of 1981 by using Platteville ST radar in the eastern Colorado plains in order to obtain the mesoscale spectra. Power spectra were obtained for both meridional and vertical components at heights of 3.3－7.9 km for meridional and 3.3－17.7 km for vertical. Results show that the “-5/3 law” is a good fit to “meridional” spectra for wave periods ranging from ~3 hr to 2 days which are consistent with other published observations and give further evidence to the existence of a universal -5/3 law in mesoscale atmospheric motions. Results also show that for wave periods shorter than 3 hr (to about 10 min), the spectra obviously depart from the –5/3 law and reflect the significant contribution of thunderstorm activities which frequently happen in the mid-latitude summer. Mesoscale spectra of vertical velocity show some characteristics of gravity waves. The mechanism of the observed spectra is discussed.