Core Structure of a Bay of Bengal Monsoon Depression

View More View Less
  • 1 University of Virginia, Charlottesville, VA 22903
© Get Permissions
Full access

Abstract

Summer MONEX aircraft flight level and dropwindsonde data have been used to examine the central core structure of a mature Bay of Bengal monsoon depression on 7 July 1979. Continuous aircraft data including cloud photographs were obtained at three flight levels.

The depression sloped toward the southwest with height, with buoyant cloudy ascent within cold air to the south and west of the axis and warm subsidence of clear air to the northeast of the axis. Marked departures from gradient balance were found. In the northeast subgradient subsiding flow was found throughout the lower troposphere. In the southeast, new cloud base, supergradient flow accompanied areas of convergence; near 700 hPa, the overlying flow was subgradient and subsident; near 400 hPa, marked variations in flow were related to clouds above flight level.

Near cloud base, convergence reached magnitudes of approximately -2 × 10-4 s-l in areas of ∼300 km2 to the south and west of the center (ascent reaching roughly -1 Pa s-1) with divergence and subsidence of similar magnitude in and to the northeast of the center. These two regimes were separated by cloud arcs in the boundary layer below a subsidence inversion.

Northward out of the cold region of cloudy ascent in the westerlies south of the depression center, flow at 700 hPa subsided at ∼0.1 Pa s-1 around the cast side of the depression.

A cumulonimbus anvil spread at 150 hPa over the area of convergence and ascent west of the centers of circulation at low levels. Airborne radar showed ∼1% area coverage by echoes of reflectivity ≳ 35 dB(Z) (rainfall rate ≳ 6 mm h-1), with average rainfall ∼0.5 mm h-1. The flow configuration was consistent with ascent in cumulus convection, coupled with compensating dry adiabatic subsidence.

Abstract

Summer MONEX aircraft flight level and dropwindsonde data have been used to examine the central core structure of a mature Bay of Bengal monsoon depression on 7 July 1979. Continuous aircraft data including cloud photographs were obtained at three flight levels.

The depression sloped toward the southwest with height, with buoyant cloudy ascent within cold air to the south and west of the axis and warm subsidence of clear air to the northeast of the axis. Marked departures from gradient balance were found. In the northeast subgradient subsiding flow was found throughout the lower troposphere. In the southeast, new cloud base, supergradient flow accompanied areas of convergence; near 700 hPa, the overlying flow was subgradient and subsident; near 400 hPa, marked variations in flow were related to clouds above flight level.

Near cloud base, convergence reached magnitudes of approximately -2 × 10-4 s-l in areas of ∼300 km2 to the south and west of the center (ascent reaching roughly -1 Pa s-1) with divergence and subsidence of similar magnitude in and to the northeast of the center. These two regimes were separated by cloud arcs in the boundary layer below a subsidence inversion.

Northward out of the cold region of cloudy ascent in the westerlies south of the depression center, flow at 700 hPa subsided at ∼0.1 Pa s-1 around the cast side of the depression.

A cumulonimbus anvil spread at 150 hPa over the area of convergence and ascent west of the centers of circulation at low levels. Airborne radar showed ∼1% area coverage by echoes of reflectivity ≳ 35 dB(Z) (rainfall rate ≳ 6 mm h-1), with average rainfall ∼0.5 mm h-1. The flow configuration was consistent with ascent in cumulus convection, coupled with compensating dry adiabatic subsidence.

Save