GCM Simulations of tropical rainfall

The aspect of GCM simulations of most relevance to studies of the impact of tropical vegetation changes is rainfall. An assessment has therefore been made of the GCM simulations of rainfall. The observed precipitation according to Jaeger (1976) (from Randall 1982) is shown in figure 3a, c.

In addition to the models listed in table 1, the long established OSU model and the ECMWF medium-range forecast model are included here. The OSU model (Schlesinger and Gates 1980) is a two-layer model with 4° latitude by 5° longitude grid. The boundary-layer parametrization uses constant flux layer assumptions like the GISS model. The model albedos are from Posey and Clapp (1964) and there is a one-layer ground temperature simulation. The hydrology has recently been changed from that described by Carson to one like the GISS model's (Ghan et al. 1982). The version of the ECMWF model used by Tiedtke (1983) for two 50-day experiments is a 1 7/8° by 1 7/8° model with 15 layers. The physics include a one-layer soil treatment but with diffusive linking of both moisture and temperatures to prescribed deep soil values (Tiedtke et al. 1979). The surface and boundary layer fluxes are based on Monin-Oboukhov similarity theory and are dependent on thermal stability and mixing lengths.

In order to relate them to tropical deforestation, the following summary of each model's success in predicting the observed rainfall pattern is restricted to northern South America, Africa, and southern Asia.

AES:	(January only) wet areas too far north over all three continents with excessive rain north of the equator over west and east Africa and parts of Asia
ECMWF:	good agreement in shape both in February and July. Too wet over north east South America and southern India in February and over all India in July, when the African rainbelt is also too intense and a few degrees too far north
EERM:	generally too dry with most of the tropical land below 3 mm/day rainfall
GISS:	too wet over North Africa and southern Asia in January. In July (8° by 10° model) rain too far north over Africa and Arabia with eastern South America and Africa also too wet
GLAS 1980:	(see fig. 3) good generally; rain too far north over Africa in February
GLAS 1982:	African rains too extensive to north and south in January. July African and Indian rainfall about 10° too extensive to the north
LMD:	rain belts too narrow and intense but generally well-centred, except dry area over north India in July
MO:	good agreement in January except Somalia and south India too wet; generally right July pattern over South America and Africa but too dry, northern India also too dry
NCAR:	northern South America and southern Asia too wet both in January and July; ahara too wet in July
OSU:	patterns generally good (except western Sahara wet in July); rain too intense in January over South America and especially Africa
GFDL:	wet over South-East Asia in January and Sahara in July; most maxima Rhomboidal 30) too intense
GFDL:	July pattern good except northern India too dry; India rather wet in (250 km grid point) January; South American maxima too intense January and July.

Overall, the spectral models, especially those of lower resolution, perform less well than the grid point models. There is less evidence of a dependence of quality on resolution for grid point models, with the 4° by 5° GLAS 1980 model probably the best. However, some aspects of that model's simulation in the tropics are less realistic: for example, the upper troposphere is too warm and the flow tends to be too westerly, especially in the upper troposphere near, and south, of the equator in July (Randall 1983).

FIG. 3a. February observed precipitation, taken from Jaeger (1976). The contour interval is 1 mm d^-1

FIG. 3b. February simulated precipitation (GLAS). The contour interval is 1 mm d^-1 except in congested regions.

FIG. 3c. July observed precipitation, taken from Jaeger (1976). The contour interval is 1 mm d^-1.

FIG. 3d. July simulated precipitation (GLAS). The contour interval is 1 mm d1 except in congested regions.