Uruguay had its hottest February on record in 2016.
29 October 2020
31 January 2016
By Paulina Tedesco (INUMET ) and José Prieto (EUMETSAT)
February is a summer month in Uruguay. Typical air temperatures in Montevideo range from 18 to 28 °C (averages of minimum and maximum daily values). February 2016 had seven days of precipitation, and 70% humidity. Summer thunderstorms were frequent.
A comparison between precipitation data, as measured by gauges, and satellite data offers a good correlation. For instance, precipitation correlates for February 2016 at the level of 0.72 with the minimum infrared brightness temperature (BT) at the pixel where gauges are located, and presumably at higher level for shorter periods of time.
Satellites like Meteosat-10, in the infrared and solar domains, provide good proxies for precipitation estimates in the absence of gauge data, with the benefit of a much more dense mesh/coverage than the gauge network. So, satellite data could be used to assess precipitation in areas without gauges.
In the case of Uruguay, 252 stations, with more than 23 days of precipitation, were selected from INUMET, INIA and INMET (Brazil) networks to perform this comparison.
Two main satellite proxies were identified for precipitation:
- Minimum brightness temperature (BT) at 10.8 µm.
- Maximum albedo at 0.8 µm.
The statistical regression considered, in addition, less obvious predictors from the maximum BTs at channel 3.9 µm and at 10.8 µm.
As a result, satellite data explained 70% of in-situ measured precipitation. Figure 1 illustrates the interpolated precipitation field from gauges and the average temperatures from the station readings.
Figure 2 shows satellite data, consisting of maximum values in the month at around 17:00 UTC for every pixel on Uruguay's territory and surroundings.
The statistical regression for precipitation and temperature on four satellite variables as predictors resulted in a high degree of explained variance for the case of precipitation. The strongest correlation occurs for precipitation data and the corresponding pixel’s minimum 10.8 µm brightness temperature. Colder tops correlate with higher precipitation (Figure 3).
However, regression was not satisfactory for the temperature estimates. A future study will have a refined approach for the proxies based on daily data (rather than grouped as monthly data) and on the use of average satellite values, better than max or min values.
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