ALBERT

Description: Precipitation downscaling improves the coarse resolution and poor representation of precipitation in global climate models and helps end users to assess the likely hydrological impacts of climate change. This paper integrates perspectives from meteorologists, climatologists, statisticians, and hydrologists to identify generic end user (in particular, impact modeler) needs and to discuss downscaling capabilities and gaps. End users need a reliable representation of precipitation intensities and temporal and spatial variability, as well as physical consistency, independent of region and season. In addition to presenting dynamical downscaling, we review perfect prognosis statistical downscaling, model output statistics, and weather generators, focusing on recent developments to improve the representation of space-time variability. Furthermore, evaluation techniques to assess downscaling skill are presented. Downscaling adds considerable value to projections from global climate models. Remaining gaps are uncertainties arising from sparse data; representation of extreme summer precipitation, subdaily precipitation, and full precipitation fields on fine scales; capturing changes in small-scale processes and their feedback on large scales; and errors inherited from the driving global climate model.

Description: Retrievals of aerosol optical depth (AOD) from MODIS, and of sea surface temperature (SST) from TMI are analyzed jointly with the output of a numerical model for the period 2000-2006 to determine the impact of Saharan dust on the eastern subtropical North Atlantic SST. Simultaneously with, or shortly after strong dust outbreaks, a decrease in SST of 0.2 degrees to 0.4 degrees C can be observed in the microwave SST data set, which is consistent with an independent estimate of SST decrease simulated here by a local mixed layer model. However, low wind conditions and a shallow mixed layer are required to reach this response, and it is therefore unlikely that a clear response of SST to dust lasting more than a few days can be seen in the microwave SST observations. An inspection of microwave SST observations suggests that about 30% of SST variance could be explained by dust-induced cooling in our study region that is not represented in existing AVHRR SST fields nor represented in reanalysis centers-provided surface heat fluxes. On longer time scales, a comparison between observed SST fields and simulated SST, using an eddy-permitting model of the North Atlantic, suggests a cooling of about 0.5 degrees C on the local SST on sub-seasonal to interannual time scales which is significantly correlated and consistent with a dust-induced cooling. However, while supportive of the hypothesis that Saharan dust lead to a reduction in SST, the eddy-resolving model results are not by themselves conclusive. Moreover, the effects of dust-induced cooling on simulations of the ocean circulation, on atmospheric forecasts and on climate simulations remains to be investigated in future studies.