Buoy and satellite observations of mesoscale cellular convection during AMTEX 75

Abstract

Buoy and satellite observations of mesoscale cellular convection (MCC) over the East China Sea in the vicinity of the Kuroshio current have been made during 14–18 February 1975, as a part of the Air Mass Transformation Experiment (AMTEX). Surface observations of solar radiation from spar buoys indicate the distinct passage of open and closed MCC that formed and continued for three consecutive days during an outbreak of cold polar air over the much warmer Kuroshio. A critical air-sea temperature of −5 °C for the occurrence of MCC has been substantiated. The time required for the passage of solar radiation peaks coupled with the buoy wind speed gave a computed closed cell diameter of 28 km, comparable to estimates from satellite photographs. The horizontal component of wind beneath the cloudy portion of a closed cell, due to convection, has been estimated as 0.6 m s−1. This represents the speed at which air near the sea surface moves from the edge toward the center of a closed cell. Also, the temperature difference obtained near the sea surface between the relatively cold descending branch and the warm ascending branch is 0.2°C. Similarly, the specific humidity difference of the less moist descending air near the edge and the moist ascending air near cell center is 9% (∼0.4 g/kg). Some indications were also found in the variation of horizontal wind direction with the passage of closed cells, since wind variations at the edge of passing cells exceeded the mean sequential variability (10.6 ° compared to 9.4 °). Sensible heat flux calculations associated with closed MCC suggest that strong surface heating can be associated with closed cells, previously reported by Hubert (1966) to be a characteristic of only open cells. Finally, the results of this study should remove any disclaimers that MCC appear in satellite photography simply because of a resolution bias and that the consideration of all visible clouds actually present would remove any periodicity one might expect to see in surface observations.