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The Saharan heat low ( SHL) is defined as a stationary thermal depression below 700 hPa located over the Sahara during the boreal summer season. This feature has a significant impact on the seasonal and intra-seasonal variability of rainfall over the Sahel. For various reasons, few observations are assimilated in operational numerical weather prediction models, especially surface sensitive observations. In this study we make use of the advanced microwave sounding unit ( AMSU-A) brightness temperatures measured at 52.8 GHz (noted A4 hereafter) and at 53.6 GHz (noted A5 hereafter) to detect the heat low and to characterize its intensity at the seasonal and intra-seasonal scales. In terms of seasonal variability, AMSU-based estimates of SHL are found in good agreement with those based on the European Center Medium-range Weather Forecast ERA-I reanalyses for the period ranging from 2000 to 2011. However, differences exist between the two datasets at intraseasonal timescale. They may be related to mismatches between the numerical weather prediction model levels and the level of maximum of sensitivity of the AMSU observations. Particular meteorological situations can also explain some differences between the two types of products, as there is a lack of assimilated observations in this region. Finally, we show that the A4 brightness temperature is a suitable proxy to provide a good estimate of the location and the intensity of the SHL from daily to seasonal timescales. [ABSTRACT FROM AUTHOR]

The Fennec automatic weather station (AWS) network consists of eight stations installed across the Sahara, with four in remote locations in the central desert, where no previous meteorological observations have existed. The AWS measures temperature, humidity, pressure, wind speed, wind direction, shortwave and longwave radiation (upwelling and downwelling), ground heat flux, and ground temperature. Data are recorded every 3 min 20 s, that is, at 3 times the temporal resolution of the World Meteorological Organization's standard 10-min reporting for winds and wind gusts. Variations in wind speeds on shorter time scales are recorded through the use of second- and third-order moments of 1-Hz data. Using the Iridium Router-Based Unrestricted Digital Internetworking Connectivity Solutions (RUDICS) service, data are transmitted in near-real time (1-h lag) to the United Kingdom, where calibrations are applied and data are uploaded to the Global Telecommunications System (GTS), for assimilation into forecast models. This paper describes the instrumentation used and the data available from the network. Particular focus is given to the engineering applied to the task of making measurements in this remote region and challenging climate. The communications protocol developed to operate over the Iridium RUDICS satellite service is described. Transmitting the second moment of the wind speed distribution is shown to improve estimates of the dust-generating potential of observed winds, especially for winds close to the threshold speed for dust emission of the wind speed distribution. Sources of error are discussed and some preliminary results are presented, demonstrating the system's potential to record key features of this region. [ABSTRACT FROM AUTHOR]