Multi-scale subtropical controls on the position and intensity of the summertime West African heat low
Final Report Abstract
Within this project, five Automated Weather Stations (AWS) were maintained through the years of 2011 and 2012, during which two international FENNEC campaigns took place. Data were quality-checked, made available to the FENNEC consortium and recently published on the PANGAEA data publishing site for free access. Project Principal Investigators and Ph.D. students participated in the two fields campaigns and provided comprehensive documents on the synoptic situation during the aircraft deployments. Using the German and UK FENNEC AWS data, two articles based on the Ph.D. thesis funded through this project were published. These papers discuss the role of convective cold pools approaching the SHL from the Atlas Mountains, which are a strong orographic trigger for deep convection in Northwest Africa. Knowledge about the frequency of these events, as well as their impact on large-scale dynamics, is required to understand their contribution to the variability of the SHL and to known model uncertainties. The first aspect is addressed in the first publication that describes the development of an objective and automated method for the generation of multi-year climatologies not available before. The algorithm combines freely available standard surface observations with satellite microwave data. Representativeness of stations and influence of their spatial density are addressed by comparison to a satellite-only climatology. Applying this algorithm to data from automated weather stations (further maintained as part of this project) and manned synoptic stations in and south of the Atlas Mountains reveals the frequent occurrence. On the order of 6 events per month are detected from May to September when the SHL is in its northernmost position. The events tend to cluster into several-days long convectively active periods, often with strong events on consecutive days. These clusters appear to be controlled by variations in the synoptic-scale environment in the region. Based on simulations of two example cases using the Weather Research and Forecast (WRF) model at convection-permitting resolution, the impacts of Atlas cold pool periods on the SHL was diagnosed and published in the first ever study of this kind. Sensitivity experiments with artificially removed cold pools as well as different resolutions and parameterizations are conducted. Results indicate increases in surface pressure of more than 1 hPa and significant moisture transports into the desert over several days. This moisture affects radiative heating and thus the energy balance of the SHL. Even though cold pool events north of the SHL are less frequent when compared to their Sahelian counterparts, it is shown that they gain importance due to their temporal clustering on synoptic timescale. Together with studies focusing on the Sahel, this work emphasizes the need for improved parameterization schemes for deep convection in order to produce more reliable climate projections for the WAM.
Publications
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2013: How can a dusty cold pool change the diurnal evolution of the Saharan Atmospheric Boundary Layer? EGU General Assembly 2013, Vienna, Austria.
Kocha C., C. Flamant, J. Berckmans, A. Fink, L. Garcia-Carreras, P. Knippertz , J.-P. Lafore, F. Marnas, J. Marsham, D. Parker, P. Rosenberg, C. Ryder, P. Tulet, and R. Washington
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(2014), The impact of Atlas Mountain cold-pool events on the position and intensity of the summertime West African heat low, EGU General Assembly 2014, Vienna, Austria.
Schuster, R., A. H. Fink, P. Knippertz, J. H. Marsham, D. J. Parker, and C. Flamant
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(2015), An Objective Detection Method for Convective Cold Pool Events and Its Application to Northern Africa, Mon. Weather Rev., 143(12), 5055-5072
Redl, R., A. H. Fink, and P. Knippertz
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(2016), Convective Cold Pools over the Atlas Mountains and Their Influence on the Saharan Heat Low, 32nd Conference on Hurricanes and Tropical Meteorology, San Juan, Puerto Rico
Fink, A. H., P. Knippertz, and R. Redl
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(2016), Convective Cold Pools over the Atlas Mountains and Their Influence on the Saharan Heat Low, EGU General Assembly 2016, Vienna, Austria
Redl, R., P. Knippertz, and A. H. Fink
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(2016), Weakening and moistening of the summertime Saharan heat low through convective cold pools from the Atlas Mountains, J. Geophys. Res. Atmos., 121(8), 3907-3928
Redl, R., P. Knippertz, and A. H. Fink
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Convective Cold Pools over the Atlas Mountains and Their Influence on the Saharan Heat Low. Ph. D. Thesis, University of Cologne, 105 pp.
Redl, R.
