Ocean wind plays a fundamental role in the Earth's climate system by transporting heat, moisture, and gases globally while mediating their exchange with the ocean. How, and at what scales these processes occur is an active area of research, and modeling studies suggest that the strongest vertical transport and air-sea exchanges occur at small scales. This paper introduces results from the S-MODE campaign, during which a Doppler Scatterometer mapped 2D ocean vector wind on scales between 200 m and 100 km. While in an average sense, DopplerScatt wind data match classical k(-5/3) spectra, atmospheric rolls, convection, and underlying ocean currents all drive unique wind spectra that are inconsistent with the average. Frequent observations of strong atmospheric divergence imply small-scale vertical transport reaching >10 km/day. These results are not consistent with Kolmogorov and two-dimensional turbulence theories due to anisotropy and divergence.

Ocean images collected by astronauts onboard the Apollo spacecraft more than 50 years ago revealed a large number of ocean eddies, with a size between 1 and 20 km. Since then, satellite infrared, ocean color, sun glitter and synthetic aperture radar images, with high spatial resolution, have confirmed the ubiquitous presence of these small eddies in all oceans. However, observing the dynamical characteristics and evolution of these eddies has remained challenging. An experiment was recently carried out in the California Current system using the new airborne Doppler Scatterometer (National Aeronautics and Space Administration-Jet Propulsion Laboratory DopplerScatt) instrument that observes surface velocities. Here, with DopplerScatt, we mapped a 30 x 100 km domain over multiple days to unveil numerous 1-20 km ocean eddies, called submesoscale eddies, that evolve over a period of a few hours. The strong interactions between eddies generate horizontal velocity divergence, implying vertical velocities reaching 250 m day-1 at 40 m depth. The velocity field also produces horizontal dispersion of particles over a distance of 50 km within 12 h, which rapidly fills the turbulent eddy field. These observations suggest that submesoscale ocean turbulence may profoundly affect the vertical transport of heat, carbon, and important climatic gases between the atmosphere and the ocean interior, as well as the horizontal dispersion of tracers and particles. As such, submesoscale ocean eddies are a critical element of Earth's climate system.

Accurate high and extreme sea surface wind observations are essential for the meteorological, ocean, and climate applications. To properly assess and calibrate the current and future satellite-derived extreme winds, including those from the C-band scatterometers, building a consolidated high and extreme wind reference data set is crucial. In this work, a new approach is presented to assess the consistency between moored buoys and stepped-frequency microwave radiometer (SFMR)-derived winds. To overcome the absence of abundant direct collocations between these two data sets, the reprocessed Advanced Scatterometer (ASCAT)-A winds at the 12.5-km resolution, from 2009 to 2017, have been used to perform an indirect SFMR/buoy winds' intercomparison. The ASCAT/SFMR analysis reveals an ASCAT wind underestimation for winds of above 15 m/s. SFMR measurements are calibrated using GPS drop-wind-sondes (dropsondes) data and averaged along-track to represent ASCAT spatially. On the other hand, ASCAT and buoy winds are in good agreement up to 25 m/s. The buoy high-wind quality has been confirmed using a triple collocation approach. Comparing these results, both SFMR and buoy winds appear to be highly correlated with ASCAT at the high-wind regime; however, they show a very different wind speed scaling. An SFMR-based recalibration of ASCAT winds is proposed, the so-called ASCAT dropsonde-scale winds, for use by the extreme wind operational community. However, further work is required to reconcile dropsonde (thus, SFMR) and buoy wind measurements under extreme wind conditions.

The airborne stepped frequency microwave radiometer (SFMR) provides the measurements of 10-m ocean surface wind speed in high and extreme wind conditions. These winds are calibrated using the surface-adjusted wind estimates from the so-called dropsondes. The surface-adjusted winds are obtained from layer-averaged winds scaled to 10-m altitude to eliminate the local surface variability not associated with the storm strength. The SFMR measurements and, consequently, the surface-adjusted dropsonde winds represent a possible reference for satellite instrument and model calibration/validation at high and extreme wind conditions. To this end, representativeness errors that those measurements may introduce need to be taken into account to ensure that the storm variability is correctly resolved in satellite retrievals and modeling. In this work, we compare the SFMR winds with the dropsonde surface-adjusted winds derived from the so-called WL150 algorithm, which uses the lowest 150-m layer between 10 and 350 m. We use nine years of data from 2009 to 2017. We focus on the effects of the layer altitude and thickness. Our analysis shows that the layer altitude has a significant impact on dropsonde/SFMR wind comparisons. Moreover, the averaged winds obtained from layers thinner than the nominal 150 m and closer to the surface are more representative of the SFMR surface wind speed than the WL150 speeds. We also find that the surface-adjusted winds are more representative of 10-km horizontally averaged SFMR winds. We conclude that for calibration/validation purposes, the WL150 algorithm can introduce noise, and the use of actual 10-m dropsonde measurements should be further investigated.

Physical understanding and modeling of Ka-band ocean surface backscatter is challenging due to a lack of measurements. In the framework of the NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE) mission, a Ka-Band Ocean continuous wave Doppler Scatterometer (KaBODS) built by the University of Massachusetts, Amherst (UMass) was installed on the Woods Hole Oceanographic Institution (WHOI) Air-Sea Interaction Tower. Together with ASIT anemometers, a new data set of Ka-band ocean surface backscatter measurements along with surface wind/wave and weather parameters was collected. In this work, we present the KaBODS instrument and an empirical Ka-band wind Geophysical Model Function (GMF), the so-called ASIT GMF, based on the KaBODS data collected over a period of three months, from October 2019 to January 2020, for incidence angles ranging between 40° and 68°. The ASIT GMF results are compared with an existing Ka-band wind GMF developed from data collected during a tower experiment conducted over the Black Sea. The two GMFs show differences in terms of wind speed and wind direction sensitivity. However, they are consistent in the values of the standard deviation of the model residuals. This suggests an intrinsic geophysical variability characterizing the Ka-band surface backscatter. The observed variability does not significantly change when filtering out swell-dominated data, indicating that the long-wave induced backscatter modulation is not the primary source of the KaBODS backscatter variability. We observe evidence of wave breaking events, which increase the skewness of the backscatter distribution in linear space, consistent with previous studies. Interestingly, a better agreement is seen between the GMFs and the actual data at an incidence angle of 60° for both GMFs, and the statistical analysis of the model residuals shows a reduced backscatter variability at this incidence angle. This study shows that the ASIT data set is a valuable reference for studies of Ka-band backscatter. Further investigations are on-going to fully characterize the observed variability and its implication in the wind GMF development.

The Ka-band wind scatterometry is a relatively new methodology to retrieve ocean surface winds. Modeling the Ka-band ocean surface backscatter is challenging, especially because of the lack of in-situ measurements. In the framework of the NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE) mission, a new data set of ocean surface backscatter has been collected. These measurements were obtained from a Ka-band Doppler scatterometer (KaBODS) located on the Woods Hole Oceanographic Institution (WHOI) Air-Sea Interaction Tower (ASIT). In this work we present our analysis and findings on the KaBODS backscatter measurements based on the development of a wind empirical backscatter model. We show that the data are characterized by a large variability, which is mainly due to intrinsic geophysical effects. The source of this geophysical variability is currently under investigation.

How strong does the wind blow in a hurricane? This proves a question that is difficult to answer, but has far-reaching consequences for satellite meteorology, weather forecasting and hurricane advisories. In the EUMETSAT CHEFS project, KNMI, ICM and IFREMER worked with international colleagues to address this question to prepare for the EPS-SG SCA scatterometer, which introduces C-band cross-polarization measurements to improve the detection of hurricane-force winds. To calibrate the diverse available satellite, airplane and model winds, in-situ wind speed references are needed. Unfortunately, these prove rather inconsistent in the wind speed range of 15 to 25 m/s, casting doubt on the higher winds too. Should we trust dropsondes at high and extreme winds or perhaps put more confidence inthe moored buoy references? This dilemma will be presented to initiate a discussion with the international community gathered at IGARSS '21.

A particularly pressing requirement in the Ocean Surface Vector Wind (OSVW) community is to obtain reliable extreme winds in hurricanes (> 30 m/s) from wind scatterometers, since extreme weather classification, surge and wave forecasts for societal warning are a high priority in nowcasting and in numerical weather prediction (NWP). A main goal of the EUMETSAT C-band High and Extreme-Force Speeds (CHEFS) study is therefore to consolidate an in-situ wind reference for assessing scatterometer high and extreme-force wind capabilities. Scatterometers have proven to have very good performances when retrieving low to moderate winds. However, measuring high and extreme winds is still challenging as vicarious calibration is needed and calibrated in situ reference winds are scarce. Moored buoy data are usually used as absolute reference to calibrate the scatterometer Geophysical Model Functions (GMF), however, for very high and extreme winds above 25 m/s, moored buoys may not be reliable. Moreover, controversy exists in the OSVW satellite community on the quality of moored buoys above 15 m/s rather than 25 m/s. Hence, the quality of buoy winds between 15 m/s and 25 m/s is thoroughly evaluated. The buoy wind performance, estimated with triple collocation analyses of buoy, ASCAT and ERA5 winds, shows that the quality of buoy wind vectors up to 25 m/s is within 2 m/s, indicating that buoy winds can indeed be used for wind scatterometer GMF calibration in the mentioned wind range. The NOAA hurricane hunters fly into hurricanes to drop sondes, and thus obtain wind profiles in the lowest few kilometers of hurricanes, and operate dedicated microwave instrumentation on aircraft to obtain detailed wind patterns in hurricanes, such as the Stepped-Frequency Microwave Radiometer (SFMR). Ideally, local dropsonde winds may be statistically used to calibrate SFMR as they have similar spatial representation (“footprint”). SFMR, in turn, after spatial aggregation to scatterometer footprints, may be used to calibrate satellite scatterometers and radiometers in overflights. The so-called WL150 algorithm is operationally used to estimate 10-m surface winds from dropsonde wind profiles. The measured radiosonde 10-m winds are a more direct calibration resource for the 10-m surface wind than WL150 estimates. However, an improved assessment of the position processing of the sonde near the surface, where its deceleration is maximum, is needed. The air mass density needs to be considered to calibrate scatterometer winds in hurricanes, as these mainly occur at low pressures and hence low air mass density, i.e., so-called stress-equivalent winds should be used for comparison. Finally, ASCAT winds show sensitivity to high winds, but lack good GMF calibration due to the lack of a consolidated in-situ wind reference. The saturation of the GMF at extreme winds is somehow compensated by the high calibration stability of the ASCAT instrument. As a result, further backscatter calibration refinements will support the retrieval of good-quality ASCAT winds in extreme conditions. In addition, GMF development and wind retrieval studies will be useful to improve high and extreme winds, in particular after a consolidated in-situ wind reference has been established.

EUMETSAT supported the CHEFS study, which provided resources to KNMI, ICM and IFREMER to lead and progress on the ocean winds community issue of an in‐situ reference for satellite and model calibration of extreme winds. Such study is only possible with access to uniformly reprocessed wind data sets which were obtained from NOAA/NESDIS/STAR OSWT dropsondes and SFMR, ICOADS buoys, ECMWF buoy archive and ERA5 winds, ESA Sentinel‐1 SAR and RadarSat SAR VV and VH data

International Ocean Vector Winds Science Team Meeting (IOVWST), 29-31 May 2019, Portland, Maine

7th International Conference on Meteorology and Climatology of the Mediterranean (MetMed), 4-6 March 2019, Palma

42nd Photonics & Electromagnetics Research Symposium (PIERS), 17-20 December 2019, Xiamen, China

China France Oceanography SATellite (CFOSAT) pre-launch workshop, 8-9 October 2018, Brest, France

European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Satellite Conference, 17-21 September 2018, Tallinn, Estonia

European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Satellite Conference, 17-21 September 2018, Tallinn, Estonia

ISSI-BJ/ISSI Joint Workshop on Tropical and Subtropical Cyclones with Improved Satellite Observations, May 2018, Beijing, China

European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteorological Satellite Conference, 17-21 September 2018, Tallinn, Estonia