ASCAT new 6.25 wind product

Space-borne scatterometers regularly provide a large number of measurements of near surface wind in places where conventional observations are scarce. Their wind speed and direction data are reported as the equivalent neutral-stability wind at 10 m height above the sea surface. In the inner coastal region, wind estimates from scatterometers are restrained by the distance to shore since backscatter measurements are strongly affected by the presence of land. Nevertheless, these products allow obtaining good quality data of winds over the sea surface as close as 15-20 km from the coast line (Verhoef et al., 2012).

Recently, an ASCAT scatterometer on a 6.25 km grid with a true spatial resolution of about 17 km has become available (Vogelzang et al., 2017). This product was developed by KNMI in the Netherlands in the framework of the Numerical Weather Prediction Satellite Application Facility (NWPSAF).

ASCAT-6.25 data were obtained using the full resolution ASCAT L1B product from EUMETSAT Earth Observation portal as input. ASCAT-6.25 product was generated employing ASCAT wind data processor (AWDP) version 2.4 (AWDP-2.4) and was generated with an aggregation radius of 7.5 km, which offers the best possible compromise between resolution and accuracy (Vogelzang et al., 2017). At present, the AWDP developed by NWPSAF allows deriving ocean vector winds on a 25 km, 12.5 km, or 6.25 km grid, but the latter is currently not distributed operationally and has to be generated individually by users.

In the ASCAT-A pass shown in fig. 4.1, the synoptic wind is from the north-east and downstream Madeira Island the change in wind field pattern is clear. Upstream the wind is less intense but there is no clear change in wind direction. At this time (11:18 UTC), the foehn-like conditions were not detected anymore at Funchal weather station. However, this ASCAT wind product is only valid 15-20 km from the coast line, so the upstream flow deceleration could still be present near the island.

Figure 4.1: ASCAT-A 6.25x6.25 wind product on 5 August at 11:18UTC.


In the HRV RGB image on August 5, 2016 at 11:00UTC (fig. 4.2), some low cloudiness can be seen upstream and near the eastern part of Madeira Island, indicating some humidity at low levels, as this was the first event and the flow has less eastward component (drier), which developed better in the following days. In the stagnant area associated with the blocking mechanism, the flow is cool and moist and the upwind can be warm and dry, so the clear skies over the island at 11:00 UTC can indicate that the blocking mechanism may be happening or has just happened (see In-situ observations in chapter III).

Figure 4.2: HRV RGB on 5th August 2016 at 11:00UTC.

In the ASCAT-B pass on 8 August (fig. 4.3), the second event, the synoptic wind is from north/north-east and the changes in wind field direction and speed upstream of Madeira Island are clear. The area of light winds is consistent with a possible stagnant area associated with the blocking mechanism. It is also possible to see the flow going around the island.

Figure 4.3: ASCAT-B 6.25x6.25 wind product on 8 August at 11:09UTC.

In the HRV RGB image on 8th August 2016 at 11:00UTC (fig. 4.4), dry air at all levels was settled and there were clear skies over the island.


Figure 4.4: HRV RGB on 8th August 2016 at 11:00UTC.

Later that day, in the ASCAT-B pass shown in fig. 4.5 at 21:12 UTC, the synoptic wind is from the north-east and the changes in wind field direction and speed upstream of Madeira Island are again clear. The area of light winds can be detected and the flow is going around the island.

Figure 4.5: ASCAT-B 6.25x6.25 wind product on 8 August at 21:12UTC


The Night Microphysics RGB image on 8th August at 21:00 UTC (fig. 4.6) shows almost clear skies which is consistent with the synoptic conditions - remember that this RGB has lower resolution than the HRV RGB. At this time, the fires were already occurring and it is possible to see some purple pixels over the island which may indicate the hot spots (remember that 3.9 µm is one of the channels present in this RGB).

Figure 4.6: Night Microphysics RGB on 8th August at 21:00 UTC