Coupling Strength between Soil Moisture and Precipitation

1
Soil Moisture - Atmosphere Coupling in the
Rossby Centre Regional Atmospheric Model during
the South American Monsoon
Anna A. Sörensson (1), Claudio G. Menéndez (1),
Ulf Hansson (2), Patrick Samuelsson (2), Ulrika
Willén (2) (1) Centro de Investigaciones del Mar
y la Atmósfera, CONICET/UBA, Buenos Aires,
Argentina (2) Rossby Centre, Swedish
Meteorological and Hydrological Institute,
Norrköping, Sweden
Introduction
The soil moisture memory could influence on the
intraseasonal variability of precipitation during
the development of the South American Monsoon
System (SAMS) and potentially contributes to
atmospheric variability and seasonal
predictability. We explore the interaction
between soil moisture and atmosphere during the
SAMS of 92-93 through a) calculating the coupling
strength and b) studying the effect of initial
dry and wet soil moisture conditions, using the
Rossby Centre regional atmospheric climate model
(RCA3). The model has been adapted to the South
American region through a series of tunings and
by incorporating the surface database Ecoclimap.
Coupling Strength between Soil Moisture and
Precipitation
Tunings and the Land-Surface Database Ecoclimap
Experiment designTwo 10-member ensembles -
Period November 1992 March 1993 Ensemble W
the soil moisture is calculated by the model at
each time step - the soil is interactive with the
atmosphere Ensemble S All ensemble members are
forced, at each time step, to maintain the same
space-time varying series of soil moisture. The
soil moisture field is a boundary condition for
the atmosphere - the atmosphere does not feed
back upon the soil moisture.
Annual cycle of Precipitation, Amazon region
2001-2002
Annual cycle of Precipitation, La Plata Basin
2001-2002
To increase the RCA3 performance in simulating
the South American climate, the land-surface
database Ecoclimap was incorporated and a set of
tunings were performed.
Sm(xi,yj)
Annual cycle of T2m, Amazon region 2001-2002
The monsoon precipitation was improved for the
Amazon and Plata Basin regions although large
negative winter bias prevails.
t
Since soil moisture is a boundary condition for
ensemble S but not for ensemble W, the difference
O(S) O(W) isolates the fraction of atmospheric
variability that is explained by the soil
moisture and is used as a measure of the coupling
strength.
Coupling Strength CS
t
The spring T2m bias of Amazon region was removed
by Ecoclimap.
Initialization dates
ResultsWe present the coupling strength between
soil moisture and evaporation and soil moisture
and precipitation. The precipitation coupling
strength depends to some (unknown) degree on the
evapotranspiration coupling strength but also on
e.g. model parameterizations of boundary layer
and moist convection. The precipitation coupling
strength is proposed by various authors (e.g.
GLACE project) to be highest in transition zones
regions that are neither arid nor humid.
CS
Coupling Strength Soil Moisture
EvapotranspirationJanuary -93
Evaporation coupling strength binned according
to Soil Water Availability (SWA)
In arid regions the atmosphere is dry and
rainfall easily evaporates to the atmosphere.
High coupling strength
In transition zones, the coupling to
evapotranspiration is intermediate AND the
atmosphere can get unstable by a less amount of
added water vapor. Higher coupling strength
HOT SPOTS
SWA
Initial soil moisture of ensemble DRY
CS
Precipitation coupling strength binned according
to Soil Water Availability (SWA)
Coupling Strength Soil Moisture
PrecipitationJanuary -93
Initial soil moisture of ensemble WET
SWA
Initial Soil Moisture the effect of a dry/wet
winter on SAMS
In humid regions, the air is wet and the soil is
close to field capacity, and 1. rainfall will
not change the soil moisture to such a high
degree.2. The evapotranspiration will be limited
by the wet atmosphere Low coupling strength
We explored the influence of anomalous soil
moisture initial conditions on the intraseasonal
development of the 1992-93 SAMS on a monthly
timescale. Two ensembles of five members each
were realized, one with anomalously dry and the
other with anomalously wet land surface initial
conditions over the whole domain. The simulated
period is August 1992 March 1993.
In arid regions the atmosphere is too dry to get
unstable. Lower coupling strength
Precipitation
Winds and temperature 850hPa

• The choice of land surface database and
  parameters like soil depth and leaf area index is
  important for the for the performance of RCA3
  over South America. With the database Ecoclimap
  and a set of tunings to the atmosphere, the
  surface temperature and the summer precipitation
  was improved for important South American
  regions, including La Plata Basin.
• La Plata Basin seems to be a region where the
  precipitation is partly controlled by the soil
  moisture in January. Results were similar for
  other months. The coupling strength of
  evapotanspiration is very high for la Plata
  Region, which indicates that the soil probably is
  too dry in RCA3 in this region, perhaps due to
  the winter precipitation deficit.
• A very dry winter with a very dry soil in
  August, induced continental scale circulation
  changes through changed Bowen Ratio and deep
  convective areas. Moisture fluxes from the
  Atlantic increased and initiated the monsoon
  earlier than for a simulation with wet initial
  conditions.

Conclusions