MST Radar Observations of Gravity Waves and Turbulence near Thunderstorms

Abstract

The effect of deep convection on the intensities of gravity waves and turbulence during the summer at White Sands, New Mexico, is investigated using 50-MHz mesosphere–stratosphere–troposphere (MST) radar observations and surface weather reports. Radar data taken at 3-min intervals from the summers of 1991 through 1996 (with occasional gaps of varying length) are used to construct hourly means, medians, and standard deviations of wind speed, spectral width (σ2turb), and backscattered power calibrated as the refractivity turbulence structure constant (C2n). The hourly variance of the vertical velocity σ2w is used as an indicator of high-frequency gravity wave intensity. Surface observations taken near the radar site are used to identify periods marked by convection at or near the radar. During cases in which no convection is reported, the median hourly σ2w is nearly constant with altitude (about 0.04 m2 s−2 below and 0.03 m2 s−2 above the tropopause). Values of σ2w, C2n, and σ2turb are significantly enhanced from no-convection cases to thunderstorm cases. Largest increases are about 12 dB relative to the no-convection cases at about 11 km for σ2w, about 9.5 km for σ2turb, and about 7.5 km for C2n. The relatively lower height for the maximum of C2n is likely due to the influence of humidity advected upward during convection on the mean gradient of the refractive index. The probability density distributions of C2n and σ2turb near their levels of maximum enhancement are unimodal, with the modes steadily increasing with increasing proximity of convection. However, the probability density distribution of σ2w is bimodal in all instances, suggesting that there can be enhanced wave activity even when visible convection is not present and that the presence of a thunderstorm at the station does not necessarily indicate greatly enhanced wave activity.