Wind scatterometer constellation captures the landfall of hurricane Ida
All three EUMETSAT ASCAT instruments measured the landfall of Ida as illustrated in the image. In addition, the Chinese HY2B and HY2C scatterometer winds processed by the OSI SAF measured Ida. The recorded 75 knot wind is among the highest wind ASCAT has measured and the ocean heat in the Gulf of Mexico indeed did its work to intensify Ida. Hence Ida’s wind damage is enormous.
KNMI led a EUMETSAT study (https://www.eumetsat.int/CHEFS ) suggesting that, following operational practices in hurricane advisories, a 75 knot ASCAT wind should really read 130 knots, 65 m/s or 235 km/h, very close to what the National Hurricane Center (NHC) warned for. So, why does the OSI SAF not increase the ASCAT winds then? Well, in the CHEFS study we found that above 15 m/s there is a large discrepancy between in-situ moored buoy winds and dropsonde winds, which remains unresolved. Hence, the needed in-situ wind speed calibration reference needs further consolidation (Polverari et al., 2021) for improved satellite-derived and model wind calibration. Earlier, moored buoys have been made suspect as calibration source in scientific literature, but during CHEFS we refuted the scientific basis for this. On the other hand, CHEFS revealed several reasons why dropsondes may be suspect. Clearly, further work is needed to consolidate the quality of moored buoy and dropsonde winds to understand the extremes. Note, while the current hurricane wind-speed-based advisory categories serve their purpose, it makes a real physical difference, whether molecules hit objects with 300 km/h or 200 km/h and hence improved calibration is needed for physical modelling of air-sea interaction, waves and storm surges.
The figure further illustrates that the 75 knot winds are off the land, which is of considerable scientific interest, as we measure winds from waves and the role of large waves in this is not so clear in extreme conditions. If anything, it appears that the cm-scale roughness out in the sea measured by scatterometers is developed well within a few tens of kms. Note that a fully developed wave sea state would take hundreds of kilometers of fetch in hurricane conditions.
More near-real-time wind monitoring plots are available at https://scatterometer.knmi.nl/tile_prod/index.php?day=0&imgt=fla , including a detailed description.