Showing papers by "Paul J. Valdes published in 2001"

Modeling the Impact of Land Surface Degradation on the Climate of Tropical North Africa

Abstract: Degradation of the land surface has been suggested as a cause of persistent drought in tropical north Africa. A general circulation model is used to assess the impact of degradation of five regions within tropical north Africa. Idealized degradation scenarios are used since existing observations are inadequate to determine the extent and severity of historical degradation. It is found that the impact of degradation varies between the regions. The greatest effects are found from degradation of the Sahel or West Africa, which result in substantial reduction of precipitation over the degraded area. Both surface evaporation and atmospheric moisture convergence are reduced. In the Sahelian case the precipitation reduction extends well to the south of the area of changed land surface. The occurrence of easterly wave disturbances is not altered by degradation, but the mean rainfall from each event is reduced. Degradation of an area in eastern north Africa results in smaller reductions of precipitation and moisture convergence. Finally, degradation of a southern area next to the Gulf of Guinea has little effect on precipitation because of a compensatory increase of moisture convergence. The simulated rainfall reduction following degradation of the Sahel is comparable to observed changes in recent decades, suggesting that degradation may have contributed to that change.

Dust transport to Dome C, Antarctica, at the Last Glacial Maximum and present day

Abstract: The Antarctic polar ice-core records show large changes in the concentration of dust over glacial-interglacial timescales. This paper explores how much of this variation is due to changes in the transport of dust from arid regions to the ice cores. Back trajectories, initialised from the site of Dome C ice-core, Antarctica, are calculated using an offline trajectory code forced by modelled winds, output from the UKMO Unified Model running under present day and LGM boundary conditions. As well as comparing the present day and LGM back trajectories, their seasonal and interannual variability is also explored. The results suggest that the Patagonian provenance of the Dome C dust can be understood in terms of the atmospheric transport whereas the total change in dust concentration requires changes to the sources or sinks.

Validation of ECMWF (re)analysis surface climate data, 1979–1998, for Greenland and implications for mass balance modelling of the ice sheet

Abstract: Climate (re)analysis products are potentially valuable tools, when properly verified, for helping to constrain the surface mass balance of the Greenland Ice Sheet (GIS). Monthly surface fields from European Centre for Medium-Range Weather Forecasts (ECMWF) operational- and re-analyses spanning 1979-1998 were validated using in situ data (surface air pressure and temperature, precipitation, cloud cover, short-/all-wave radiation, and wind speed/direction). These validation data are from coastal or near-coastal Danish Meteorological Institute (DMI) synoptic stations, inland Greenland Climate Network (GC-Net) and University of Wisconsin Automatic Weather Stations (AWSs), and two energy balance stations near the southern ice margin. The ECMWF analyses closely reproduce the seasonal patterns and interannual variations of much of the in situ data. Differences in the mean values of surface air pressure and temperature can mainly be ascribed to orography errors in the analyses' schemes, compared with the latest available accurate digital elevation model. Much of the GIS margin as modelled by ECMWF was too cold, on average by 4°C, and ECMWF precipitation averaged some 136 of the DMI station values. The misrepresentation of the (relatively) steep ice-sheet margin, which tends to be broadened and systematically over-elevated by several hundred metres, orographically reduced temperature and enhanced precipitation there in the ECMWF models. The cloud-cover comparison revealed not dissimilar annual mean cloud covers (ECMWF - 8) but the ECMWF analyses had too little cloud and were too 'sunny' during the critical summer melt-season. ECMWF-modelled surface albedo in summer was I� 11 lower than GC-Net values, which was mainly responsible for the disagreement of modelled surface short-wave radiation fluxes with observations. Model albedo and cloud errors need to be rectified if the analyses are to be used effectively to drive energy balance models of Greenland snowmelt. ECMWF wind speed averaged 66 (62) of the DMI station (AWS) values. The validation results provide useful insights into how one can best improve the ECMWF Greenland climate data for use in glaciological and climatological studies. Copyright © 2001 Royal Meteorological Society.

Patterns and Variations of Snow Accumulation over Greenland, 1979-98, from ECMWF Analyses, and Their Verification

Abstract: ECMWF monthly climate analyses were used to retrieve surface temperature, precipitation, evaporation/sublimation, and in conjunction with a simple meteorological model, snow accumulation over Greenland for 1979–98. The modeled snow accumulation is compared with existing observational maps of Greenland accumulation and mass balance, and with widely distributed coincident and contemporaneous ice-core data, primarily from NASA's Program in Arctic Regional Climate Assessment (PARCA) program. ECMWF-derived accumulation gave more realistic values for substantial (2–3-km elevation) areas of the “intermediate” inland plateau than previous methods, although the estimates are still (∼20%–30%) too low over central Greenland. Overall, this ECMWF-driven model provides a reasonable first-order depiction, among the best currently available, of snow accumulation and its interannual variations. Mean annual ECMWF precipitation (snow accumulation) for the whole island (Greenland area >2 km) was 0.314 (0.260) m yr−1...

Modelling middle pliocene warm climates of the USA

Abstract: The middle Pliocene warm period represents a unique time slice in which to model and understand climatic processes operating under a warm climatic regime. Palaeoclimatic model simulations, focussed on the United States of America (USA), for the middle Pliocene (ca 3 Ma) were generated using the USGS PRISM2 2° x 2° data set of boundary conditions and the UK Meteorological Office’s HadAM3 general circulation model (GCM). Model results suggest that conditions in the USA during the middle Pliocene can be characterised as annually warmer (by 2° to 4° C), less seasonal, wetter (by a maximum of 4 to 8 mm/day) and with an absence of freezing winters over the central and southern Great Plains. A sensitivity experiment suggests that the main forcing mechanisms for surface temperature changes in near coastal areas are the imposed Pliocene sea surface temperatures (SST’s). In interior regions, reduced Northern Hemisphere terrestrial ice, combined with less snow cover and a reduction in the elevation of the western cordillera of North America, generate atmospheric circulation changes and positive albedo feedbacks that raise surface temperatures. A complex set of climatic feedback mechanisms cause an enhancement of the hydrological cycle magnifying the moisture bearing westerly wind belt during the winter season (Dec., Jan., Feb.). Predictions produced by the model are in broad agreement with available geological evidence. However, the GCM appears to underestimate precipitation levels in the interior and central regions of the southern USA.

Sensitivity of the Northern Hemisphere climate of the Last Glacial Maximum to sea surface temperatures

Abstract: The role of prescribing sea surface temperature in paleoclimate atmospheric simulations has been investigated by comparing Last Glacial Maximum AGCMs experiments using different SSTs data sets as well as coupled atmosphere/oceanic mixed layer models. Changes in the SSTs and sea-ice margin generate different patterns of zonal asymmetries in the atmospheric circulation that are responsible for reorganisation of heat and moisture transport, leading to important variations of Northern Hemisphere regional climates, particularly in winter. Additional sensitivity experiments have been carried out to isolate the individual role of North Pacific and North Atlantic SSTs anomalies. We found that changes in North Pacific SSTs have a much stronger impact over all the northern continental surfaces, including Europe and Siberia, than changes in the North Atlantic SSTs. As these SSTs anomalies are of the order of the typical errors generated by coupled ocean-atmosphere models, this suggests that these more complete models will likely still have problems in simulating the regional climate change at the LGM.