Aspect Ratios of Longitudinal Rolls and Convection Cells Observed during Cold Air Outbreaks

Open Access

Abstract

Atmospheric conditions in which cellular-roll cloud patterns are apparent, and dependence of the aspect ratios of the cells on depths of the convective layer and the stability ratio are investigated by examining satellite pictures and aerological data over eastern Asia during cold air outbreaks in the months January to March for 1975–82. Vertical profiles of temperature and relative humidity in the convective layer along the cloud bands over the ocean show an increase of static stability, relative humidity, and layer depth in the downwind direction. Longitudinal convective rolls are realized when vertical wind shear is between 10⁻³ and 10⁻² sec⁻¹, and three-dimensional open cells are realized below these values. The cloud top height is proportional to l^3/5 of the cloud-band spacing for l≲20 km and to l^2/5 for l≳20 km. The obtained aspect ratio for longitudinal rolls ranges from 5 to 18, increasing in the downwind direction at a rate of 1/110 km, while open cells are obtained in the range from 8 to 12. The Rayleigh number calculated from the observational data increases proportional to the power of the aspect ratio. The aspect ratio also depends on the stability ratio Sr (the ratio of static stability for the ascending motion to that for the descending motion); it is inversely proportional to Sr for Sr ≲ 3, while it remains constant for Sr ≳ 3. Asai's theory is modified so as to incorporate effects of eddy anisotropic diffusivities proposed by Priestley. We consider increases of the aspect ratio of longitudinal rolls in both cases—those which depend on the stability ratio and those which are independent of it. It is found that his theory explains observational results reasonably well if the ratio of eddy anisotropies is assumed to be 10–20 for the range of the present analysis.

Keywords

This publication has 0 references indexed in Scilit: