PS-01 Differences in the atmospheric boundary layer characteristics

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                Differences in the atmospheric boundary layer characteristics
                 between wet and dry spells of the Indian summer monsoon

             A. Sandeep1, T. Narayana Rao1, C. N. Ramkiran2 and S.V.B. Rao2

            1 National Atmospheric Research Laboratory, Gadanki – 517 112, India
                   2 Sri Venkateswara University, Tirupati – 517 502, India
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The atmospheric boundary layer (ABL) is strongly affected by exchanges of heat, moisture and
momentum with the underlying land or sea surface and these turbulent exchanges driven by a
combination of buoyancy and shear largely determine the variations of wind, temperature and
humidity close to the surface. Also, depending on the surface properties (wet vs. dry) and
synoptic conditions (sunny vs. rainy day and cold vs. warm air advection), the ABL
characteristics may change even within the season Understanding such variations in
fundamental variables of ABL (such as its vertical extent, winds and turbulence) and the
processes responsible for those variations is, thus, essential for air quality assessment,
meteorological forecasting and quantification of two-way transport between the residual and
surface layers. For the first time, the differences and similarities in atmospheric boundary layer
(ABL) characteristics, in particular its height, evolution and wind field, between two contrasting
spells of Indian summer monsoon have been studied using measurements from UHF wind
profilers and an instrumented 50-m tower at Gadanki. The observed differences are discussed in
light of various forcing mechanisms, in particular the effect of soil moisture on the surface
energy balance and ABL. The differences in ABL height, its evolution and the wind field
between spells are quite pronounced. Wet spells not only have shallower ABL but also their
growth is delayed by 1-4 hours when compared with that of in dry spell. Availability of soil
moisture in abundance during the wet spell (higher than during dry spell by a factor of 2) reduces
the buoyancy flux, as most of the net radiation is converted into latent heat flux, and thereby not
only limits the ABL height but also delays the start of ABL growth. The low-level jet (LLJ) is
stronger during the dry spell and has a larger diurnal range than during the wet spell. The highest
occurrence and magnitude of LLJ apparent at 1.5 km during early morning hours shift
progressively with height and time till the afternoon, following the ABL evolution. The weaker
LLJ during the wet spell is attributed to its southward migration from the mean position (15°N).

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