Cold Pools and Boundary Layer Turbulent Fluxes under Precipitating Shallow Cumulus

Dynamics of Madden Julian Oscillation (DYNAMO) field experiment was conducted over the Topical Indian Ocean during 2011 boreal winter. NOAA's WP-3D Orion aircraft (WP-3D) conducted oceanic and atmospheric measurements during the special observation period of DYNAMO. Prior to the onset of MJO, wide spread presence of shallow cumulus clouds were observed in the study domain (8°S-EQ and 72°E-81°E). Dedicated aircraft sortie was made on 19th November 2011 to study the tropical marine cold pool structure and boundary layer turbulent fluxes under shallow cumulus convection. On this day, five vertically stacked level legs were conducted with each having 4-5 measurement levels. High frequency (25 Hz) measurements of temperature, humidity as well as wind components along with the measurements of radiometric sea surface skin temperature and liquid water content were performed during a total of ~6 hour flight period. C-band radar mounted on the lower fuselage and Doppler tail radar provided unprecedented details of the shallow convection. Average sampling levels of the vertically stacked legs were at 60 m, 120 m, 350 m, 475 m and 650 m. The lowest three levels were well inside the boundary layer while the next two levels were at the cloud base and within the cloud, respectively. Boundary layer profiling by the aircraft was also performed between the successive vertical stacks.

Precipitating cumuli were found to produce cold pools with an average cooling of up to 1.5°C in surface air temperature and sea surface skin temperature. Increased wind speed and change in wind direction was also noticed in cold pools in comparison with the surroundings. Boundary layer was stably stratified in the cold pool. Fluxes of momentum, sensible heat and latent heat calculated using the eddy correlation method will be presented to illustrate the energy and water vapor transport in this type of boundary layer in the DYNAMO domain. Sensible heat flux show considerable difference in cold pool region in comparison with surrounding. Near surface sensible heat was negative due to the evaporative cooling. We also discuss the mean vertical structure and the horizontal variability of this cumulus-topped boundary layer.