Low-level humidity boundary across Europe.
21 October 2020
14 July 2006
by Jochen Kerkmann and HansPeter Roesli (EUMETSAT)
As discussed in the case study Monitoring airmass/moisture boundaries with MSG (4-5 October 2005) , the dust RGB product, besides monitoring dust storms and tracking thin, high-level clouds, can also be used for the detection of low-level humidity boundaries. An interesting case for this application is shown below.
The RGB product (upper left image) shows a mostly cloud-free situation over the UK, Northern Germany and the Benelux countries (with a pale blue to magenta colour) and a large area of convective clouds over Southern Europe (with a strong blue 'background' colour). The reason for this colour difference can be attributed to an airmass/moisture boundary that crosses Europe from west to east at about 49 degrees latitude North (see Interpretation ). North of this line (49°N) there is mainly dry air as a result of the earlier passage of a cold front (that can be seen over Eastern Poland), whereas south of this line moist air is present in the lower layers of the troposphere.
The existence of the moisture boundary is confimed by the ECMWF analysis fields (see e.g. Dust RGB with abs. humidity 850 hPa, ECMWF analysis ). Across the moisture boundary the relative humidity at 850 hPa drops from around 80% on the moist side to about 20% on the dry side of the boundary. In parallel, the brightness temperature difference IR10.8–IR12.0 drops from around +3 K to 0 K, which causes the change of the colour discussed above.
ECMWF analysis surface dew point (18:00 UTC)
ECMWF analysis rel. Humidity 925 hPa (18:00 UTC)
ECMWF analysis rel. Humidity 850 hPa (18:00 UTC)
ECMWF analysis rel. Humidity 700 hPa (18:00 UTC)
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