Abstract
The climatic effects of dust aerosols in North Africa and South/East Asia have been investigated using an atmospheric general circulation model, NCEP/GCM/SSiB (Simplified Simple Biosphere Model) and the three-dimensional aerosol data simulated by the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model. GCM simulations show that due to the scattering and absorption of solar radiation by dust particles, surface temperature decreases over both regions, accompanied by a reduced sensible heat flux. However, precipitation responses are different in these two regions. Due to differences in dust location and the associated heating with respect to the rainfall band and circulation, the effect of dust could either enhance or suppress precipitation. Over the North Africa region where dust particles are mainly located to the north of rainfall band, heating of the air column by dust particles forces a stronger ascent motion over dust layers, which induces an anomalous subsidence (or a weakened upward motion) and suppressed cyclonic circulation to its south where precipitation reduces. Furthermore, both humidity and cloud decrease due to the heating in the middle troposphere (semi-direct effect). In South/East Asia, dust particles are located in the upper troposphere over the major rainfall band during the monsoon season, especially Southwest India and the coastal area of Bay of Bengal. Heating of the air column increases upward motion and strengthens cyclonic circulation. Humidity also increases due to the draw-in of the low level moist air. Therefore, cloud and precipitation increase over South/East Asia associated with dust effect. During the pre-monsoon season, when dust particles are located to the north of the monsoon rainfall band, the heating effect results in shifting precipitation northward. The heating of air column due to dust particles, not surface cooling, plays the major role in precipitation changes. The anomalous upward motion over dust regions will induce a subsidence to its south and subsequently reduce precipitation over that region. Therefore, the responses of circulation and precipitation to aerosol forcing depend on the relative location of dust aerosols with respect to rainfall band, which may explain the fact that contradictory results exist regarding whether the aerosol effect would enhance or suppress precipitation. The dust induced change in precipitation is actually more of redistribution rather than the simple action of increase or decrease.

A GCM Investigation of Dust Aerosol Impact on the Regional Climate of North Africa and South/East Asia

This study investigates the impact of burned areas on the surface energy balance and monthly precipitation in northern Africa as simulated by a state-of-the-art regional model. Mean burned area fraction derived from MODIS date of burning product was implemented in a set of 1-year long WRF-NMM/SSiB2 model simulations. Vegetation cover fraction and LAI were degraded daily based on mean burned area fraction and on the survival rate for each vegetation land cover type. Additionally, ground darkening associated with wildfire-induced ash and charcoal deposition was imposed through lower ground albedo for a period after burning. In general, wildfire-induced vegetation and ground condition deterioration increased mean surface albedo by exposing the brighter bare ground, which in turn caused a decrease in monthly surface net radiation. On average, the wildfire-season albedo increase was approximately 6.3 % over the Sahel. The associated decrease in surface available energy caused a drop in surface sensible heat flux to the atmosphere during the dry months of winter and early spring, which gradually transitioned to a more substantial decrease in surface evapotranspiration in April and May that lessened throughout the rainy season. Overall, post-fire land condition deterioration resulted in a decrease in precipitation over sub-Saharan Africa, associated with the weakening of the West African monsoon progression through the region. A decrease in atmospheric moisture flux convergence was observed in the burned area simulations, which played a dominant role in reducing precipitation in the area, especially in the months preceding the monsoon onset. The areas with the largest precipitation impact were those covered by savannas and rainforests, where annual precipitation decreased by 3.8 and 3.3 %, respectively. The resulting precipitation decrease and vegetation deterioration caused a drop in gross primary productivity in the region, which was strongest in late winter and early spring. This study suggests the cooling and drying of atmosphere induced by burned areas caused the strengthening of subsidence during pre-onset and weakening of upward atmospheric motion during onset and mature stages of the monsoon leading to a waning of convective instability and precipitation. Monthly mid-tropospheric vertical wind showed a strengthening of downward motion in winter and spring seasons, and weakening of upward movement during the rainy months. Furthermore, precipitation energy analysis revealed that most of precipitation decrease originated from convective events, which supports the hypothesis of reduced convective instability due to wildfires.

Impact of burned areas on the northern African seasonal climate from the perspective of regional modeling

Accurate assessments of the CO2 fluxes between the terrestrial ecosystems and the atmosphere are pressingly needed for the climate change and carbon cycle studies. The Collatz et al. parameterization of leaf photosynthesis-stomatal conductance has been widely applied in land surface parameterization schemes for simulating the land surface CO2 fluxes. The study in this paper developed an analytical solution approach for the Collatz et al.'s parameterization for stable solution and computational efficiency. This analytical approach is then applied to the simplified biosphere model (SSiB), enhancing its capability of simulating land surface CO2 fluxes. The enhanced SSiB model is tested with field observation data sets from two Amazonian field experiments (ABRACOS missions and Manaus Eddy Covariance Study). Simulations of the land surface fluxes of latent heat, sensible heat and soil heat by the enhanced SSiB agree very well with observations with correlation coefficients being larger than 0.80. However, the correlation coefficient for the daily means Of CO2 fluxes is only 0.42 for the Manaus data set. A day-time, square wave in the simulated CO2 flux diurnal curves is found. The discrepancies between simulation and observation were found to be the results of incorrect parameter setup or improper leaf to canopy scaling strategy. A modification to the scaling strategy improves significantly the accuracy of the photosynthesis-stomatal conductance model. (C) 2002 Elsevier Science B.V. All rights reserved.

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G J Collatz

Papers

An analytical approach for estimating CO2 and heat fluxes over the Amazonian region - eScholarship