By Joaquin Munoz Sabater (ECMWF) and Fernando Martin Porqueras (IDEAS)

The 18 of April 2011 a strip with unusual values of brightness temperatures was detected in Western Sahara and Morocco. This feature looked strange enough to the Quality Check team Given at ESAC that they asked US what we thought about it. Many options were quickly discarded (RFI, Instrument problem etc…). The problem was that the measurements as shown on the QC picture below pointed towards high soil moisture which is not all that expected at this period of year in this area with such a linear shape.

Fig 1 QC picture on two orbits (Ascending and Descending) on April 18th 2011 from DPGS (X pol on the left and Y on the right)

However, between the 17 and 19 April 2011 a rare precipitation event took place in Western Sahara. Precipitations up to 10 mm, locally stronger, took place in a narrow, well defined band through the desert. This was caused by the influence of a low placed in the Azores which produced unusual precipitations in the Sahara area. This precipitation band moved progressively towards the North-East direction. Figures 1-4 show the cumulated total precipitation during 48 hours, in steps of 12 hours, starting the 16 April 2011. These figures clearly show the direction and cumulative values of the precipitation band. Non-coincidentally, there is a very good correlation between SMOS measured brightness temperatures (being much colder than its surroundings) and these precipitation figures, which explain these abnormal values. Figure 5 shows the ECMWF soil moisture analysis the 18 April 2011 at 12h00, for the first layer (7cm), which also displays this geographical strip being wetter (up to 20%) than its very dry surroundings. SMOS was able to catch very well this precipitation event.

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These results demonstrates once more the good skills of the SMOS instrument to clearly catch rare precipitation events.

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