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Charles Warner

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.

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Charles Warner and Richard H. Grumm

Abstract

A monsoon depression over the Bay of Bengal on 7 July 1979 has been studied using a variety of observations, in particular, cloud photographs from aircraft. Ascent in the lower troposphere was concentrated in mesoscale features of cumulus clouds covering ∼1% of the inner area (3 × 105 km2) of the depression. Across these mesoscale features discontinuities in thermal fields were found along with abrupt wind shifts.

Much of the volume of the depression featured thin fragmentary layers of stratus, implying an absence of strong vertical motion. Observed by photography, individual rising cumulus towers were of width up to a few kilometers, increasing with height; rise rates of towers reached 9 m s−1. Measured with aircraft instruments, mean updrafts in cumulus clouds were ∼2.5 m s−1. In cumulus populations scattered throughout the storm, number densities of cumulus ranged from ∼1 km−2 for fractus to ∼1 per 1500 km2 for Congestus. Congestus penetrating the 500 (300) hPa level were ∼1 per 3200 (13 000) km2. Fractional area coverage by cumulus updrafts was ∼0.5% in humilis, less in other categories. Coverage by cumulus updrafts was roughly 20 times less than coverage by inert remnants of cumulus. Cloudy ascending motion in populations of cumulus was generally on the order of hundredths of Pascals per second. It appeared to be mostly compensated by local subsidence. Great number densities of humilis were found moistening the central area following subsidence and drying.

Total cloud cover was dominated by mid-level thin fragmentary status layers and cumulus debris. There was extensive anvil cloud based at ∼400 hPa, apparently arising from cumulus.

Detailed observations were made of a cloud line growing out of the southwesterly flow south of the center of the depression. The line was followed for 3 h on GOES-1 visible imagery. It propagated faster than the low-level winds. Aircraft altimetry showed an abrupt height drop from 6097 to 6090 m at 483 hPa, over a distance of 50 km from southeast to northwest through the line. Southwesterly momentum was lifted from 900 to 600 hPa and from southeast to northwest through the line. Other colocated singularities in convection and wind fields were found.

Ascent in the lower troposphere over the depression as a whole (1066 km2) was assessed from aircraft and dropwindsonde data to be approximately −0.3 Pa s−1.

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