As of today, October 8, 2016, the coast from Florida to North Carolina is being hit by the very intense Hurricane Matthew, Category 3-4. The hurricane is now in a weakening phase, traveling North. Its impact will be experienced for days due to the significant storm surge caused by the extreme winds.
Here we take a first look at the extreme ocean surface winds observed from space by microwave sensors on polar orbiting satellites. Microwaves penetrates the clouds and allow measuring features at the ocean surface, which relate to the surface wind speeds. This is not possible using infrared or visible channels onboard geostationary satellites, which observe properties at the top of the clouds.
Measuring hurricane-level winds is challenging as the signal observed from satellite can be strongly affected by rain, and because most sensors lose sensitivity for those extreme winds. A recent breakthrough was achieved using a very low-frequency microwave sensor which retains very good sensitivity at extreme winds even in rain, the NASA SMAP radiometer (L-band, 1.4 GHz). The observed footprint has a diameter of approximately 40 Km. The SMAP winds are still considered a research dataset, and they are not yet assimilated into Numerical Weather Models, as they are not an operational product and further validation is needed.
The figure below displays the wind speed field on October 7, with the maximum winds just offshore the Florida coast. The hurricane-force winds covered an area as wide as 150 Km.
In the figure below we show how during the developing through mature phase of Hurricane Matthew (Sep 28-Oct 7), SMAP measured winds as high as 56 m/s (October 4), which are very consistent with the 10-minute maximum sustained winds reported by the National Hurricane Center (NHC) Tropical Advisory, and with the temporal evolution of the anomaly in sea-level pressure. The figure also illustrates the maximum winds observed by the European ASCAT scatterometer. ASCAT is a radar operating at a higher fequency (C-band, 5.4 GHz), which is not significantly affected by rain at high winds, except for very intense precipitation. However, as illustrated, at this frequency the sensor doesn't seem to have enough sensitivity for measuring hurricane-force winds.
The next figures show the wind field observed on October 3, 2016, when both SMAP and ASCAT had a complete view of the hurricane. Note how the ASCAT winds are significantly lower than those observed by SMAP.
Over the course of the hurricane evolution, winds observed from SMAP reached a maximum intensity of 56 m/s on October 4, when it hit Haiti, causing major devastation and numerous fatalities. The maximum observed by ASCAT on the same day was 41 m/s, indicating indeed a reduced sensitivity to high winds for the C-band scatterometer.
In conjuction with extreme winds, the hurricane featured very intense precipitation, where rain rates exceeded 10 mm/hr over an area as wide as 150 Km, as can be seen in the following panel displaying rain rates observed on October 5 by the WindSat polarimetric radiometer.
We will update this blog in the next days as more data will become available and further analysis will be performed.
A related blog on extreme winds during the Winston typhoon that hit the Fiji island in February 2016 has additional information on these satellite observations.
Acknowledgements: This work is supported by the NASA Ocean Vector Winds Science Team.