Aircraft Investigation of Mesoscale Cellular Convection During AMTEX 75

Abstract

The 1 s data set collected by the NCAR Electra research aircraft in the presence of closed mesoscale cellular convection (MCC) has been examined for the purpose of determining the convective wind field and horizontal profiles of temperature and specific humidity. Two flight legs during AMTEX (16 February 1975) were selected for study: one at the 100 m level (case 1) and the other at the 970 m level (case 2), 30 m below cloud base Two closed cells with diameters of 39,0 and 33.0 km were traversed in case 1. Filtered virtual temperature data showed a double-cycle variation, with an average difference between warm and cool regions of 0.32 K. The difference in specific humidity between moist cell center and dry cell wag was ?1.0 g kg?1. The vertical velocity power spectrum showed no MCC-scale energy. Horizontal wind velocity data indicated convergence toward cell center with mesoscale wind velocity components ?15 m s?1. The data for case 2 encompassed two closed cells, with traverse lengths ?32 km each. The difference between dry cell wall and moist cell center was ?0.7 g kg ?1 Filtered vertical velocity showed downward motion in cell walls and upward motion near cell center, the difference being ?1.0 m s?1 Filtered virtual temperature data indicated a weak single-cycle variation. Horizontal convective velocities were 0.75 m s?1 A physical explanation of the double-cycle temperature profile at the 100 m level is offered based on the combined effects of 1) warm air entrainment from an overlying inversion layer; 2) radiative cooling at cloud-top level, side-wall mixing and evaporative cooling, and subsequent (partial) moist adiabatic descent of air; and 3) sensible heating at cell center due to the warm sea surface.