Kinematic Analysis and Air-Sea Heat Flux Associated with Mesoscale Cellular Convection during AMTEX 75

Abstract

Several excellent examples of open and closed mesoscale cellular convection (MCC) were observed in DMSP satellite imagery during the second field period of the Air Mass Transformation Experiment (AMTEX), conducted 14–28 February 1975 over the East China Sea in the vicinity of the Kuroshio current. The kinematic fields of horizontal divergence, vertical motion and vorticity of the large-scale flow producing the MCC and the associated heat flux at the air-sea interface were determined to starch for observational evidence as to the cause of circulation direction in the convection cells. None of these kinematic fields had features unique to open and closed cells, with predominantly downward vertical motion and diverging horizontal flow in the convecting layer. From the study of the kinematic fields determined for 14 cases of open cells and 20 cases of closed Cells a physical depiction has been made of the distribution of the vertical motion in the vertical plane of a mesoscale cell during its formation. Critical values of daily averaged air-sea temperature difference and surface wind speed as necessary conditions for the occurrence of MCC due to heating from below have been determined to be 1) sea surface temperature at least 5°C greater than air temperature and 2) surface wind speed in excess of 5 m s⁻¹. Calculations of the sensible heat flux yielded a threshold value of ∼70 W m⁻² for the onset of MCC. Similarly, the threshold value for the total heat flux was ∼200 W m⁻². Calculations also showed that both open and closed cells occurred in the cold air outbreak when the sea to air energy flux (latent plus sensible) was either strong (∼1200 W m⁻²) or weak (∼200 W m⁻²).