Vertical structure over the Bay of Bengal during the monsoon onset

The thermodynamic and dynamic structure of the atmosphere over the Bay of Bengal during the monsoon onset of 1999 will be described. The data to be presented are from radiosondes launched 6-8 times per day during the Joint Air-Sea Interaction Experiment (JASMINE), which took place in the eastern Indian Ocean and Bay of Bengal in May 1999 on the NOAA ship the R/V Ronald H. Brown.

Preliminary results indicate CAPE values in the range of 2-2.5 x 10^3 J/kg for air parcels lifted from below 200m. Calculations of CIN indicate very low potential energy barriers exist both in convectively active and suppressed periods, likely owing to the high sea surface temperatures and boundary layer moisture values which exist in the Bay of Bengal region at this time.

Wind profiles indicate very little vertical shear is present during suppressed periods, and tropospheric winds are less than 10 m/s to a depth of approximately 400 mb. During the active period of the onset, zonal winds were westerly and increased to between 10 and 20 m/s. The easterlies aloft also increase in speed, creating a vertical zonal shear at these times between the upper and mid-levels. Overall, the meridional wind variability is not as significant as that for the zonal winds between the active and suppressed periods.

C-band radar data will also be presented to place the vertical structure observed in the context of the convective activity and structure.