Interaction of low-level flow with the Western Ghat Mountains and Offshore Convection in the Summer Monsoon

Bob Grossman and Dale Duran

MWR, 112, 652-672

During the monsoon season between June and September, the unbroken Western Ghats chain acts as a barrier to the moisture laden clouds. The heavy, eastward-moving rain-bearing clouds are forced to rise and in the process deposit most of their rain on the windward side. Rainfall in this region averages 3,000–4,000 mm (120–160 in) with localized extremes touching 9,000 mm (350 in). The eastern region of the Western Ghats which lie in the rain shadow, receive far less rainfall averaging about 1,000 mm (40 in) bringing the average rainfall figure to 2,500 mm (150 in).

Principal areas of offshore convection in Monsoon

Area addressed in paper

Geography Typical summer monsoon flow

Triggering of the summer monsoon:

Traditional thinking: Sensible heating over desert regions produces divergence aloft and a surface low which draws air into continent.

Alternate explanation: Monsoon is first triggered by heating associated with latent heat release associated with deep convection triggered upstream of topographic barriers like the Western Ghats

Purpose of paper: To show that the Western Ghats are capable of contributing to the production of deep convection well offshore by gentle lifting of potentially unstable air as it approaches the coast.

Hypothesis: Lifting is due to decelleration of air as it approaches ridge of high pressure generated by blocking of flow by the Western Ghats

22 June 1979 –Surface Pressure

No offshore convection west of Ghats on this day

24 June 1979 –Surface Pressure

Offshore convection west of Ghats on this day

Not a lot of difference in the surface pressure fields in these analyses

Note pressure ridge…upstream blocking of flow?

Occurrence of “highly reflective clouds” determined from Satellite visible imagery for June 1979

Assumption: These are indicative of the presence of deep convection rather than anvils of older convection

Percent occurrence

Standard deviation

The boundary layer flights from which the study was derived

Area of deep convection

Downstream boundary layer profile

Upstream boundary layer profile

Surface fluxes

Conditions at time of flight based on a TIROS-N satellite Mosaic (no GOES at that time)

Data from Electra (on stripcharts and microfilm):

1s averages

Horizontal velocityPressureAltitudeTemperatureMoistureUp and down IR and Shortwave radiation

Dropwindsondes

Satellite data

Radar images on paper

What they had to work with

Wind Speed

Equiv. Pot. Temp

Important points:

West to East increase in ABLvalues of e – ABL is warming and moistening(Temp 0.5K, Moisture 1 g/kg)

Cumulus congestus affected e in the middle of the section

SST increased 2.5K, 1.7 K in last 250 km

Wind direction constant in vertical by 240 deg at upstream profile and 255 at downstream profile

Wind speed continually decreases toward India

Characteristics of the cross section

Cloud structure

Lifting condensation level

Pressure (lots of corrections applied make this fishy

Wind speed

Vertical velocity calculated based on 1-D divergence of wind

t

u

s

p

z

uw

z

uw

l

uuuu

i

TDDUU

1

Horizontal momentum equation averaged over a volume along a trajectory from the upstream profile to the downstream profile

Horizontal advection

Verticaladvection Friction

(fluxDivergence)

PGF Local timederivative

With a lot of hand-waving and no error analysis

Argues that calculations are OK based on the fact that increased with timet

u

Mountain wave model – uses the Lilly and Klemp (1979) model for flow over a mountain.

Problem: Requires atmosphere where the stability and wind are constant with height

Actual case: Winds are westerly below 4 km, easterly above.

Solution: Assume linear solution with upward propagating waves absorbed at the wind reversal level (critical level).

Is this really the way it works??

Solution does also not include effects of rotation, 3-D non uniformities, surface friction, and convection

Final approach: Average soundings in upstream area of Arabian Sea and determine if soundings on convective days differ from those on non-convective days (based on satellite images)

Final approach: Average soundings in upstream area of Arabian Sea and determine if soundings on convective days differ from those on non-convective days (based on satellite images)

cooler

significantly drier

Less CAPE

Dry air from non-convective soundings in mid levels(more air off deserts)