Gravity Waves Driven by Diurnal Fluctuations in Mesoscale Heating

Abstract

Observed convective systems, such as mesoscale convective complexes (MCCs), often undergo repeated cycles of nocturnal growth and daytime decay especially during the summer. The gravity wave response to these pulsations is poorly understood. The motivation for this study is to understand this response and especially its sensitivity to environmental factors such as horizontal wind and static stability. A semianalytical approach is used that focuses on the roles of singularities in a complex horizontal wavenumber space. The model is linear, Boussinesq, hydrostatic, and rotating with uniform ambient conditions. Prescribed, 2D, pulsating, upright, convective heating drives the waves. The Fourier transform technique is used to unravel the response into a discrete and continuous horizontal spectrum. Many features of the response depend on a Froude number, F = πU/(DN) where U = background wind, D = depth of source region, and N = buoyancy frequency. The most efficient forcing of the gravity wave field occurs near criticality (F = 1). For typical values of U and D associated with midlatitude convective systems, the critical value of N is about one-fourth of average tropospheric values. The resulting enhanced loss of energy from the convective system due to gravity waves could limit the intensity of convective systems near criticality. At subcriticality (F < 1), the pulsating upstream response is dominated by unbalanced ageostrophic propagating modes upstream of the source region and by a balanced geostrophic mode downstream of the source. The latter advances with a speed equal to the background flow. No far-upstream response occurs for supercriticality (F > 1). The downstream response for F > 1 is dominated by the geostrophic mode in the pressure, temperature, and source-parallel wind. In addition, a vigorous ageostrophic mode advances downstream from the subcritical source region giving rise to alternating regions of rising motion and subsidence. It is hypothesized that the latter could trigger new lines of convection in the downstream subcrticial Froude number regime. Mature MCCs often develop low-level cooling in response to evaporative cooling. This cooling primarily triggers the advective inertial gravity wave mode, which propagates downstream at the background wind speed. It is shown that subcritical flows (F < 1; weak ambient flow and/or strong static stability) are tuned to strongly respond to this mode. It is suggested that the development of new convection might be suppressed near the system under subcritical conditions. Low-level cooling has little effect on the supercritical response.