IMPACT OF ISLSCP-II INITIATIVE GLOBAL SOIL WATER STORAGE
CAPACITY OF THE ROOTING ZONE DATA ON T80 GCM
M. Ravindranath and G.R.Iyengar
Scientist-DScientist-E
mrn@ncmrwf.gov.in gopal@ncmrwf.gov.in
National Centre for Medium Range Weather Forecasting
ABSTRACT
Simulation of Asian Summer Monsoon is still a challenging task even after 4 decades of efforts using numerical weather
prediction techniques. Prediction of the strength of the Indian summer monsoon, its spatial behavior and the amount ofprecipitation is of great importance to Indian economy. Models incorporating complex surface processes including
physically based descriptions of soils and vegetation have been developed since mid 1980s. When these models are coupled
to general circulation models significant differences are reported due to land surface / atmosphere interactions in time scales
ranging from days to several weeks. The influence of different land surface parameters like vegetation cover, snow cover,ice cover, soil moisture and albedo on atmospheric circulatory patterns have been investigated by many authors. Earlier
studies experimenting with evapotranspiration at potential rate over dry surfaces in the interiors of large continents foundthat surface drying had little effect on precipitation rates in Southeast Asia and India. Further, over saturated land surface,changes in precipitation are directly related to changes in low-level moisture convergence associated with anomalous lowlevel flow. Over dry land surface, although low-level moisture convergence persists, it is unrelated to changes in tropicalprecipitation.
The T80/L18 GCM operational at National Centre for Medium Range Weather Forecasting, India incorporates a simple
land surface scheme in which soil moisture evolution is simulated fixing the bucket capacity at 150 mm. By hindsight, it isincorrect to fix the bucket capacity through the model grid since the partitioning of sensible and latent heat fluxes may beprone to error. In the present work, we used variable bucket capacity over model grid incorporating GEWEX ISLSCP-II
initiative “Global Soil Water Storage Capacity of the Rooting Zone” data to study the impact on Asian summer monsoon.
The experiment is conducted for June 2004, the first month of Asian Summer Monsoon. The global data assimilation andforecasting scheme including analysis (spectral statistical interpolation) and forecast (T80/L18 with variable bucket capacity
implementation) is run for each day to produce 5-day forecasts. Mean fields of analysis and 5-day forecasts are worked out
for operational and experimental runs. Initial diagnostics suggested changes in low level circulatory patterns, partitioning ofsensible and latent heat fluxes and rainfall. The RMSE and mean errors in temperature over Asian region showed marginal
improvement. However, the improvements were significant over Europe and North America.
Key word: Root zone soil moisture, Asian Summer Monsoon, wind field, RMSE errors

1.

Introduction

Models incorporating complex surface processes including
physically based descriptions of soils and vegetation havebeen developed since mid 1980s (Dickinson 1984, Sellers etal.1986).
When these models are coupled to generalcirculation models significant differences are reported due
to land surface / atmosphere interactions in time scales
ranging from days to several weeks. Shukla and Mintz(1982) compared the climate of a GCM with
evapotranspiration at potential rate over dry surfaces in theinteriors of large continents and found that surface drying
had little effect on precipitation rates in Southeast Asia andIndia. Kerry and Anand (1991) investigated the effects ofinteractions between the tropical circulation and continental
climate on the precipitation distribution by using idealizedboundary conditions. They found that over saturated land
surface, changes in precipitation are directly related tochanges in low-level moisture convergence associated withanomalous low-level flow. Over dry land surface, although
low-level moisture convergence persists, it is unrelated tothe changes in tropical precipitation.

Soil moisture is one the most important components of theclimate system. The summer drying phenomenon is acombination of the processes involving soil moisture,
precipitation, time of snowmelt in the spring, partitioning ofsnowmelt into the soil or runoff, water table variations andpotential evaporation due to warmer temperatures.
Complex land parameterization schemes that incorporate
biophysical control of evaporation have been developed
over the time (BATS model – Dickinson et al. (1986), theSimple Biosphere (SiB) model – Sellers et al. (1986); Xueet al.(1991)). These schemes incorporate a large number ofsurface parameters. In NWP, the sensitivity to initialconditions prohibit their use in GCMs involving medium
range weather prediction. These schemes are used with anadvantage in long-term climate simulations (Pedro Viterbo,2002).
The T80/L18 global data assimilation and forecast system
operational at National Centre for Medium Range Weather
Forecasting (NCMRWF) simulates the daily weather usingsimple bucket model with a capacity of 150 mm of soil
moisture. The disadvantage with fixed bucket scheme isthat over very dry lands like sand deserts and over very wet
lands like Amazon forest and other equatorial forest regions,partition of sensible and latent heat fluxes may be erroneous
due to large deviation of fixed bucket value from the actualvalue. The magnitude of error in the partition of fluxes, inturn, show up in the simulation of soil wetness, In thepresent work, it is attempted to adapt variable bucket
capacity over model grid for which the ISLSCP-II initiative
Root Zone Water Storage Capacity (RZWSC) data derived
using Photosynthetic ally Active Radiation (Kleidon andHeimann 1998) is used.

Design of the Experiment
2.
The global data assimilation and forecasting system
operational at NCMRWF comprises a global spectral model
adapted from National Centers for EnvironmentalPrediction(NCEP), USA. The model is implemented at T80/L-18 resolution with intermittent data assimilation
scheme involving spectral statistical interpolation (SSI) ofParrish and Derber (1992). In the assimilation cycle, 6-hforecasts of the global model are utilized to serve as firstguess for subsequent analysis. Seven-day forecasts areproduced daily on real time basis using 00UTC analysis.
The model uses a simple land-surface scheme that include(a) exchange coefficients computations based on MoninObukhov similarity theory with similarity functions ofBusinger et al. (1971) and Hicks (1976) as modified by
Carson and Richards (1978) (b) Penman-Monteith methodof evapotranspiration over land modified by Pan (1990) toinclude vegetation effects through stomatal resistance (c)prognostic surface temperature scheme of Arakawa (1972)(d) three layer scheme of Bhumaralkar (1975) for surface
and soil temperature prediction (e) interactive buckethydrology of Manabe (1969) for updating the soil moisture(f) evaporation over ocean by bulk method and (g)Charnocks roughness length computation over ocean.
The time dependent prognostic equations for estimation of
surface temperature of Arakawa (1972) is:

Potential Evapotranspiration over land LEp as given by Pan(1990):

where, σ Ta4 is upward longwave radiative flux at air
temperature Ta,

The components of LEa - the latent heat flux are:
ρ - density of air in the first model layer
L - latent heat constant of vaporization
Ch – turbulent exchange coefficient
V – Wind speed in the first model layer
qs (Ts) – saturation mixing ratio at surface temperature (skintemperature)qa – mixing ratio of air in the first model layer
The actual evapotranspiration is β times LEp.
Excessive dryness of land surface with positive systematic
error in surface temperature and lowered specific humidity
at the first model layer have been recurring features ofNCMRWF global model forecasts over the years. It isespecially significant over north and central Indian regions.
By altering the size of the soil reservoir it was attempted toinfluence surface temperature and evaporation sinceevaporative fraction has a direct bearing on soil moistureavailability and RZWSC in the formulation. Soil moistureis initialized by normalization as follows:
SOILM*(new_bucket_capacity)/(control_bucket_capacity)
The experiment is conducted for June, 2004 the first monthof Asian Summer Monsoon.

where (1-α ) S↓ is downward short wave radiative flux, α isthe albedo, L↓ is downward longwave radiative flux, σ Ts4
is upward longwave radiative flux at the surfacetemperature Ts,
where Ho = - cpρ Ch(θ (Z) - θ (Zo)), LEo = β LEp and β =fraction of soil moisture availability.

Discussion and Results
3.
The discussion is focused only on the Indian region due tospace constraints although significant impacts are seenglobally. We have a good idea on the performance ofcontrol T80 GCM; hence the experimental results arepresented in comparison to control. RMSE and other scoresare presented for different regions of the world to highlightthe influence of RZWSC data on land surface processes.

where W is the available soil moisture, Wc = 0.75 Wfc, Wfc
is the field capacity(=150 mm) and LfEx is heat used
for melting snow/ice.

The difference between RZWSC and operational bucket
capacity is shown in Fig.1. All over the Indian region,
RZMSC is higher except over NW arid regions where it islower by about 100 mm. Along the west coast and overeastern reach of monsoon trough, RZWSC is very highranging from 400 to 700 mm. The initial soil moisture inexperimental analysis (Fig.2) was very high nearing bucket
capacity along Western Ghats during June 2004. Over NW
arid regions the bucket fill was 100 mm less while over therest of India it was 100 mm above control. Significant
changes were seen over other parts of the world also. In theexperimental analysis, stronger wind compared to control
blew against northern parts of Western Ghats though themagnitude of raise in speed was low at less than 1 m/s(Fig.3). Anti-cyclonic bias over central parts of India andmild cyclonic tendency over northern parts of WesternGhats were other striking features.

from day-1 to day-5 forecast over Asia whereas theexperimental run for the same region marginally reduced
the error by 0.1 deg (Fig.9). Over Europe, the error inexperimental forecast temperature was reduced by 0.2 to 0.5
deg.
The most significant feature was that in theoperational run, the errors over Europe were more
pronounced than over Asia whereas experiment brought
down the errors over Europe below the Asian values. The
experiment did not bring about any change worth of
consideration in the wind field RMSE errors over Asia andEurope (Fig.10).

In day-3 forecast (Fig.4), the converging Bay of Bengal
branch of monsoon current blowing towards NE andadjoining regions had become stronger. The northern plains
experienced westerly bias and the west coast, the gatewayof SW monsoon current, exhibited cyclonic tendency by andlarge. Anti-cyclonic tendency over central parts prevailed
at reduced magnitude.

4.

Conclusionsa.

b.
Day-3 forecast rainfall (Fig.5) suggested an increase of 5-20
cm over NE region. West coast too experienced rise in
rainfall but of low order. Heavy drop of the order of 5-15cm was witnessed over central parts adjacent to monsoon
trough.

c.

Latent heat flux was decreased by 40 w/m**2 over central
Indian region where anti-cyclonic tendency prevailed inday-3 forecast (Fig.7). Rise up to 30 w/m**2, indicating
increased evaporation, featured over the whole of Bay ofBengal, north Indian ocean and Gujarat regions wheremerging of the monsoon current branches and cyclonic
tendency were seen respectively. The NE region thatexperienced maximum boost in rainfall had witnessed
marginal fall in latent heat flux below zero.
Day-3 forecast surface temperature (Fig.8) was rised byabout 1.5 deg over central parts of India and parts of northIndia where eastern end of monsoon trough pass through. Afall of the order of -0.5 deg was experienced all overGujarat and southernmost parts of India. Maximum fall of–1 deg was confined to a small pocket over Gujarat. It isinteresting to note that the NE areas were subjected tomarginal rise of 0.5 deg even though the total rainfall wasamplified significantly. Mixed response was seen overoceans where the air temperature values were rangedbetween –0.5 to 0.5.
The rise and fall in the initial soil moisture over different
parts of India had not diametrically influenced the forecast
rainfall. In fact increased rainfall was seen over the drier
regions in the experiment. Wetter regions did not coincidewith increased rainfall activity consistently.

d.

Increased rainfall activity had resulted frommagnified wind speed hitting against Western
Ghats and convergence over north Bay of Bengal
and adjoining NE region rather than due toincreased soil moisture as evidenced from themarginally reduced latent heat flux.
There was no definite signal as to the influence ofincreased soil moisture on the simulation of Asiansummer monsoon rainfall as both the regions ofincreased rainfall activity – the Western Ghats andthe NE region – witnessed strong wind activity.
The global distribution RZMSC had discernible
effect on the wind fields in simulating the Asiansummer monsoon. Over central Indian region, themonsoon activity during June, 2004 was below
normal resulting in poor rainfall. This feature wasmagnified in the experimental simulation leading
to increased surface temperature and reduced latentheat. Thus, almost all the biases of operational
forecast have magnified by the use of RZMSC data.
No improvement in the spatial activity of Asiansummer monsoon was brought out by the use ofRZMSC.
Global distribution of RZMSC could not
significantly influence the RMSE errors overtropics but showed significant improvement inRMSE errors over Europe and North America.

References
Arakawa, Tech.Rep 7, 103, Dept. of Met, Univ.Cal , 1972.
Bhumaralkar, JAM, 14, 1246, 1975.
Businger et al., JAS, 28, 181, 1971.
Carson and Richards, BLM, 14, 67, 1978.
Charnock, QJRMS, 81, 639-648, 1955.
Dickinson, Amr.Geop.Uni Geophys. Monogr, 29, 58, 1984.
Dickinson et al., NCAR/TN-275+STR, 69, 1986.
Hicks, QJRMS, 102, 535, 1976.
Kerry and Anand, JC, 4, 873-889, 1991
Kleidon and Heimann, Global change biology, 4, 275-286, 1998.
Manabe, MWR, 97, 739, 1969.
Pan , MWR, 118, 2500, 1990.

Parrish and Derber, MWR, 120, 1747-1763, 1992
The RMSE errors against radiosonde data for two fields temperature and wind - at 850-hPa level were shown inFig.9 and Fig.10. In the legend, ‘A’ stands for Asia and ‘E’stands for Europe.
In the operational run, the error in
forecast temperature increased from 2.4 deg. to 3.9 deg.

Pedro Viterbo, Meteorological Training Course Lecture Series, 4,ECMWF, 2002.
Sellers et al. JAS, 43, 505, 1986.
Shukla and Mintz, Science, 215, 1498-1507, 1982.
Xue et al, JC, 4, 345-364, 1991

Fig.1 Difference between EXP and CTL
Soil Reservoir Capacity (mm)Fig.2 As in Fig.1 but for initial soil moisturefor June 2004

Fig.3 As in Fig.1 but for day-3 total forecast
rainfall (cm) for June 2004

Fig.4 As in Fig.3 but for day-5 forecast

Fig.5 As in Fig.2 but for wind at 850 hPa

Fig.6 As in Fig.1 but for day-3 forecast

Fig.7 As in Fig.1 but for latent heat flux(w/m**2)

Fig.8 As in Fig.1 but for surface temperature

5RMSE

4OPER-A
EXP-A
OPER-E
EXP-E

3.5
3
2.5
2
12

3DAYS

45Fig.9 RMSE errors in temperature at 850 hPa

RMSWVE

4.51098765432
10OPER-A
EXP-A
OPER-E
EXP-E

123DAYS

4

5Fig.10 RMSE errors in wind at 850 hPa