Simulations of the Saharan atmosphere—dependence on moisture and albedo

The sensitivity of an atmospheric general circulation model to changes in atmospheric water content, soil moisture content, radiation scheme and albedo have been investigated using the Meteorological Office global 11-layer model. Previous integrations had produced excessive rainfall in the Saharan region. This paper reports experiments which show the importance for a good Saharan simulation of the initial atmosphere and soil moisture specifications, of the radiation scheme and of the surface albedo specification. It is shown that a dry Sahara with realistic circulation patterns is simulated only if the surface and atmosphere are initially realistically dry and the model's longwave radiation is dependent on atmospheric water vapour content. With a moist atmosphere, introduced either initially or by rapid evaporation from a wet soil, or by using a radiation scheme without water vapour dependence, there is increased net downward radiation which enhances ascent and convective activity and leads eventually to excessive rainfall in the desert region with a reversal of the usual circulation pattern to give 700 mb westerly winds at 5°–15°N associated with a cyclonic circulation over the Saharan region which draws in moist air from the south. As expected from Charney's hypothesis on the maintenance of deserts, the use of more realistic albedos tends to transfer rainfall from regions of higher albedo (e.g. deserts) to regions of lower albedo (e.g. forests). An extension of Charney's hypothesis to include moisture is proposed in which any increase in diabatic heating, whether due to reduced albedo or increased moisture availability, must be compensated by increased cold advection; in the tropics this typically occurs through ascent of potentially colder air leading to rainfall if the air is moist enough or the ascending layer sufficiently deep. Some implications of the possibility of two distinct summer circulation patterns for the theory of past North African climate are discussed.

[1]  S. Nicholson.  Sub-Saharan Rainfall 1981–84 , 1985 .

[2]  J. Kutzbach,et al.  Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP , 1985, Nature.

[3]  J. A. Rodgers,et al.  A reappraisal of rainfall trends in the sahel , 1985 .

[4]  W. James Shuttleworth,et al.  Eddy correlation measurements of energy partition for Amazonian forest , 1984 .

[5]  A. Henderson-Sellers,et al.  Possible climatic impacts of land cover transformations, with particular emphasis on tropical deforestation , 1984 .

[6]  Y. Sud,et al.  Influence of evaporation in semi‐arid regions on the July circulation: A numerical study , 1984 .

[7]  J. W. Kidson,et al.  African mean wind changes between sahelian wet and dry periods , 1984 .

[8]  P. Lamb.  Sub-Saharan rainfall update for 1982: continued drought , 1983 .

[9]  P. Rowntree,et al.  Simulation of the atmospheric response to soil moisture anomalies over Europe , 1983 .

[10]  Y. Mintz,et al.  The sensitivity of numerically simulated climates to land-surface boundary conditions , 1982 .

[11]  R. Keshavamurty.  Response of the Atmosphere to Sea Surface Temperature Anomalies over the Equatorial Pacific and the Teleconnections of the Southern Oscillation , 1982 .

[12]  J. Kutzbach,et al.  The Sensitivity of the African-Asian Monsoonal Climate to Orbital Parameter Changes for 9000 Years B.P. in a Low-Resolution General Circulation Model. , 1982 .

[13]  Y. Sud,et al.  A study of the influence of surface albedo on July circulation in semi‐arid regions using the glas GCM , 1982 .

[14]  J. Slingo.  A study of the earth's radiation budget using a general circulation model , 1982 .

[15]  J. Shukla,et al.  Influence of Land-Surface Evapotranspiration on the Earth's Climate , 1982, Science.

[16]  B. Briegleb,et al.  Low-latitude cloudiness and climate feedback - Comparative estimates from satellite data , 1982 .

[17]  J. Otterman.  Satellite and field studies of man's impact on the surface in arid regions , 1981 .

[18]  A. T. Grove,et al.  Global Maps of Lake-Level Fluctuations since 30,000 yr B.P. , 1979, Quaternary Research.

[19]  B. Hinton,et al.  An investigation of surface albedo variations during the recent Sahel drought , 1979 .

[20]  M. Kanamitsu,et al.  Northern Summer Tropical Circulations During Drought and Normal Rainfall Months , 1978 .

[21]  S. K. Cox,et al.  Satellite inferred surface albedo over northwestern Africa , 1978 .

[22]  R. J. Reed,et al.  Diurnal Variations in Convective Activity and Precipitation During Phases II and III of GATE , 1978 .

[23]  Jack Kornfield,et al.  A Comparative Study of the Effects of Albedo Change on Drought in Semi-Arid Regions. , 1977 .

[24]  J. W. Kidson,et al.  African rainfall and its relation to the upper air circulation , 1977 .

[25]  J. Cloudsley-Thompson.  Man and the biology of arid Zones , 1977 .

[26]  Julia Walker,et al.  The effect of soil moisture on circulation and rainfall in a tropical model , 1977 .

[27]  J. Charney.  Dynamics of deserts and drought in the Sahel , 1975 .

[28]  F. S. Nakayama,et al.  The Dependence of Bare Soil Albedo on Soil Water Content. , 1975 .

[29]  J. Milford,et al.  Seasonal rainfall forecasting in West Africa , 1975, Nature.

[30]  J. Otterman,et al.  Baring High-Albedo Soils by Overgrazing: A Hypothesized Desertification Mechanism , 1974, Science.

[31]  Robert W. Burpee.  Characteristics of North African Easterly Waves During the Summers of 1968 and 1969 , 1974 .

[32]  C. Priestley,et al.  On the Assessment of Surface Heat Flux and Evaporation Using Large-Scale Parameters , 1972 .

[33]  E. Rasmusson,et al.  The general circulation of the tropical atmosphere and interactions with extratropical latitudes , 1972 .