Dust Radiative Forcing: Indirect Impact of Drought and Land Use Change on Diurnal Temperature Range in Arid Regions

Prior studies have hypothesized that the cause for observed decline in diurnal temperature range (DTR) in the Sahel region between 1950 and 1980 is reduction of vegetation cover and soil wetness. Maximum and minimum temperatures exhibit an asymmetric response to change in land cover characteristics, with nocturnal minimum temperatures increasing at faster rate. Whereas reduction in soil moisture during the 1950-1980 time periods is due to severe drought in the region, human land utilization played an important role in decrease of vegetation cover. We propose that an indirect effect of drought and land use change, namely increase in dust emissions over the Sahel region and associated radiative forcing may have also contributed to the observed trends. Analysis of surface and satellite observations do show an increase in dust emissions from the Sahel region during the time period of 1966-2000, which has been linked to land degradation resulting from human activity.

We examined the feasibility of the hypothesis that dust radiative forcing could also contribute to asymmetric surface temperature trends using WRFChem single column model (WCSCM), constrained using surface and aircraft observations collected over Niamey, Niger. For selected case days, WCSCM was initialized using radiosonde observation collected at the Atmospheric Radiative Measurement (ARM) site in Naimey, and used to simulate boundary layer evolution over a 48 hour period. In situ observational estimate of aerosol optical depth (AOD) was used to constrain atmospheric aerosol loading. Observational estimates of Angstrom exponent was used to constrain fractional contribution from biomass burning and dust aerosols. Vertical distribution of aerosols was constrained using aircraft observed average profiles of extinction. Systematic adjustments of surface soil moisture were conducted to minimize differences between observed and modeled latent and sensible heat fluxes and 2m temperature and humidity. The simulations were then repeated assuming aerosol free atmospheric conditions and the differences between the two simulations were used to examine impact on DTR, nocturnal temperature evolution, longwave and shortwave radiative forcing.

Initial analysis conducted for five selected case days show increase in nocturnal downwelling longwave radiation and minimum temperature. Magnitude of increase varied considerably depending up the AOD, with range of ~3Wm-2 to 12 Wm-2 and 0.1 to ~1K for longwave radiation and surface temperature respectively. Analysis of aerosol observations from the AMR site for year 2006 suggest that dust is the major contributor to aerosol radiative forcing in excess of ~60% of the total number of days. The WCSCM analysis is being extended to several additional case days and the results of extended analysis will be presented. If the trends found in the initial analysis holds for the additional case days, then the proposed indirect feedback process is a viable hypothesis to explain the observed temperature trends in this region.