In the framework of the MAXSS project, a multi-mission (MM) wind product under tropical cyclone conditions has been generated for the period 2010-2020, i.e., a synergistic product that combines the European Center for Medium-range Weather Forecast fifth reanalysis (ERA5) output with several scatterometer and radiometer wind data adjusted to the wind scale of hurricane hunter in situ observations. The errors of the satellite and MM wind products have been estimated with triple collocation analysis, while the different spatial representation of the datasets (i.e., representativeness error r2) is accounted for and computed through spatial variance analysis. The error analysis shows that C-band scatterometers have the lowest standard deviation errors (0.9 m/s) compared to those of the Ku-band scatterometers (1.4-2.1 m/s), while radiometers have the largest errors (2.0-2.9 m/s). Finally, the analysis reveals that the MM wind product has a lower error (1.6 m/s) compared to ERA5 (2.6 m/s) under tropical cyclone conditions.

The Imaging Wind and Rain Airborne Profiler (IWRAP) is a system of two airborne Doppler radars, operating at C- and Ku-band, that observe below the aircraft. Each radar transmits two conically-scanned pencil beams at approximately 30° and 50° Earth-incidence angle. The antennas make a complete rotation once every second and sample at approximately30m range gates. Since the 2021 hurricane season, the Ocean Surface Winds Team (OSWT) at NOAA/NESDIS/STAR have been retrieving and transmitting three-dimensional atmospheric wind vectors to the ground for near-real-time use. Small biases in the horizontal wind speed and direction were observed first in the downwind legs of the hurricane flight pattern. After more investigation, these errors were apparent throughout all storms as a function of aircraft drift angle. This paper describes a methodology the OSWT use for eliminating these drift-dependent errors.

Prior to the 2022 Atlantic hurricane season, a new digital receiver and arbitrary waveform generator, the Tomorrow.io ARENA 522, was integrated with the Imaging Wind and Rain Airborne Profiler (IWRAP). IWRAP is a downward-pointing system of two Doppler radars that is routinely installed on the National Oceanic and Atmospheric Administration WP-3D Hurricane Hunter airplanes for hurricane and extratropical cyclone research. The primary research applications of IWRAP have been to observe ocean and atmospheric vector winds and their effects on the surrounding media. Sampling limitations due to a time-multiplexed switching system were overcome in this hurricane season using a frequency diversity technique, enabling full multi-beam sampling for improved atmospheric boundary layer observations.

New radar remote sensing measurements of the turbulent hurricane boundary layer (HBL) are examined through analysis of airborne (Imaging Wind and Rain Airborne Profiler; IWRAP) and spaceborne (synthetic aperture radar; SAR) data from Hurricanes Dorian (2019) and Rita (2005). These two systems provide a wide range of storm intensities and intensity trends to examine the turbulent HBL. The central objective of the work is to document the characteristics of coherent turbulent structures (CTSs) found in the eyewall region of the HBL. Examination of the IWRAP data in Dorian shows that the peak, localized wind speeds are found inside the CTSs near the eye-eyewall interface. The peak winds are typically located at lower levels (0.15 - 0.50 km), but sometimes are found at higher levels (1.0 - 1.5 km) when the CTSs are stretched vertically. A SAR overpass of Dorian’s eyewall showed ocean surface backscatter perturbations at the eye-eyewall interface that have connections to the CTSs identified in IWRAP data. Wavelet analysis, including detailed significance testing, was performed on the IWRAP and SAR data to study the CTS wavelengths and power characteristics. Both datasets showed a multi-scale structure in the wavelet power spectrum with peaks at ~ 10 km (eyewall), ~ 4 - 5 km (merger of small-scale eddies) and ~ 2 km (native scale of the CTSs). The ~ 2 km native scale of the CTSs is robust across intensity trends (rapid intensification, weakening and steady-state), storm cases and region of the storm. This information is useful for turbulence parameterization schemes used in numerical models that require the specification of a turbulent length scale.

A major gap in wind vector measurements exists in the lowest levels (below 500m) of the hurricane boundary layer (HBL). The 3-dimensional Doppler measurements from the Imaging Wind and Rain Airborne Profiler (IWRAP) on board the NOAA WP-3D aircraft are showing potential to close this gap. During the 2021 hurricane season IWRAP was configured to produce near real time measurements of the HBL winds. The IWRAP's raw data acquisition system allows for full HBL atmospheric profiling all the way down to the ocean surface using the spectral processing technique. For the first time ever, high resolution HBL wind and reflectivity profiles below 500m were retrieved from IWRAP measurements in Hurricane Ida on August 29th, 2021. The spectral processing technique utilized and subsequent wind and reflectivity profiles are presented and discussed.

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 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 modelling. 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-meter layer between 10 m to 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.

Microwave radiometers are designed to capture the Earth's electromagnetic radiation in the form of brightness temperatures. At low and medium frequencies, the relatively large footprint of microwave radiometers results in mixing land and water brightness temperatures in coastal areas and lakes. This mixing of signals, also known as land contamination, limits the usability of radiometer measurements and makes them unsuitable for geophysical retrievals up to ~ 100 km away from the coastline. In this paper, we present preliminary results of applying a land contamination correction on AMSR2 measurements to extract coastal information. The correction technique relies on calculating the land fraction within AMSR2 footprints using a high-resolution land mask and a representative antenna pattern.

The Advanced Microwave Scanning Radiometer-2 (AMSR2) on board the Global Change Observation Mission-Water (GCOM-W) launched in May 2012 by the Japan Aerospace Exploration Agency (JAXA) is acquiring electromagnetic radiation from the Earth for the purpose of monitoring its environmental and climate system. Among a suite of oceanic environmental data records (EDR), the Ocean Surface Winds Team of the Satellite Applications and Research (STAR) group at the National Oceanic and Atmospheric Administration (NOAA) has developed an all-weather wind speed (AWS) by exploiting AMSR2 brightness temperature (Tb) measurements. The new product provides wind speeds in normal and extreme weather conditions with minimal flagging and excellent accuracy. Validation results of this novel AMSR2 AWS product, represented in this paper, show a mean bias of 0 m/s and an rms error < 2 m/s when compared to numerical weather models under all weather conditions.

The Stepped Frequency Microwave Radiometer (SFMR) is a key instrument in tropical cyclones and high-latitude winter storms research. Through the observed brightness temperature (Tb) over a range of C-band frequencies, the SFMR derives wind-speed and rain rate. However, the instrument requires 5 to 10 seconds of averaging to cycle through all the frequencies, so regions of strong wind gradients and/or narrow rain features may be overlooked. The University of Massachusetts Amherst Microwave Remote Sensing Laboratory (MIRSL) developed a specialized version of the SFMR, the UMass Simultaneous Frequency Microwave Radiometer (USFMR) that operates six frequency channels simultaneously, eliminating the averaging time. In collaboration with NOAA/NESDIS/STAR we plan to use this instrument in studies of high latitude winter storms. We describe the instrument hardware, recent comparisons with operational SFMR measurements during hurricane flight in 2019, and current and planned investigation of retrieval inconsistencies in non-tropical cyclone environments. In collaboration with NOAA/NESDIS/STAR we plan to use this instrument in studies of high latitude winter storms. We describe the instrument hardware, recent comparisons with operational SFMR measurements during hurricane flight in 2019, and current and planned investigation of retrieval inconsistencies in non-tropical cyclone environments.

During the 2020 hurricane season, scientists at the National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite, Data, and Information Service (NESDIS)/Center for Satellite Applications and Research (STAR) Ocean Surface Winds Team (OSWT) in collaboration with Remote Sensing Solutions (RSS) operated the RSS Ka-band Interferometric Altimeter (KaIA) from the NOAA WP-3D aircraft, making the first comprehensive set of airborne Ka-band radar altimeter measurements of a tropical cyclone. KaIA is a nadir-looking Ka-band interferometric radar altimeter with a real-time tracker/retracker and is capable of centimetric radar altimetry. This paper shows significant wave height (SWH) retrievals from the 2020 hurricane season and compares them to wave models, and satellite and buoy observations.

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.

Operational weather analysis, forecasting, and warning utilize a wide variety of data products and tools, including satellite imagery and derived products. Satellite observations provide information where in-situ measurements are lacking or not readily available. Passive microwave satellite observations are routinely exploited by forecasters at the National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS) in the United States (U.S.) to support their weather analysis and forecasts. In this paper, we present examples of hurricane Dorian observations from Advanced Scanning Radiometer -2 (AMSR-2) on the Global Change Observation Mission (GCOM), which is part of the Japanese Aerospace Exploration Agency (JAXA). We compare NOAA AMSR-2 ocean EDR products with storm finding documented within National Hurricane Center Dorian discussions from Aug 24th through September 9th, 2019. NOAA AMSR-2 products are part of NWS forecasting product suite and are regularly used in daily operations. While Microwave Imagery product has been used the most for hurricane forecasting we examine usefulness of other ocean products by following Hurricane Dorian from its formation on August 24th to its demise on September 6th.

Beginning in 2015, scientists at the National Oceanic and Atmospheric Administration (NOAA)/NESDIS/STAR and UMass Amherst collaborated with the European Space Agency (ESA) to collect ocean surface normalized radar cross-section (NRCS) measurements at co- and cross-polarizations in extreme wind conditions from the NOAA Hurricane Hunter aircraft using a prototype antenna for the next-generation European spaceborne scatterometer. Since then, more research has been done to understand the effects of ocean-surface wind vectors on NRCS from both satellite and aircraft. Here we show the data from several seasons of flight experiments to understand the airborne measurements of NRCS in context.

In this paper we develop high wind portion of a Geophysical Model Function (GMF) for Scatsat-1 scatterometer measurements. Starting with NSCAT4 GMF the high wind portion has been modifying by utilizing measurements obtained by IWRAP instrument on board of NOAA P3 aircraft within tropical and extratropical cyclones. The Ao coefficient in NSCAT4 GMF was modeled so its slope is a linear function of a logarithm of wind speed and its first derivative of it equals to zero at saturation wind speed for particular incidence angle and measurements frequency as it was measured by IWRAP. The calibrated measured sigma0's from Scatsat-1 shows good agreement with the new GMF named NSCAT4.H at the high winds. The respective wind retrievals are higher and much better aligned with ASCAT high winds while the global winds statistic performance of ScatSat-1 winds remained unchanged. The new ScatSat-1 wind products produced by NSCAT4.H GMF has been implemented in NOAA's operational marine forecasting and warning applications and examples are presented here.

In Winter 2017, the University of Massachusetts Amherst's Imaging Wind and Rain Airborne Profiler (IWRAP) was flown on a National Oceanic and Atmospheric Administration (NOAA) WP-3D Hurricane Hunter aircraft under the direction of scientists from Center for Satellite Applications and Research (STAR) at NOAA/National Environmental Satellite, Data, and Information Service (NESDIS) over the North Atlantic ocean out of Shannon, Ireland. IWRAP is a dual-frequency, conically scanning, profiling Doppler radar initially developed by Microwave Remote Sensing Laboratory (MIRSL) at the University of Massachusetts Amherst that is routinely installed on the NOAA WP-3D research aircraft. The flight on February 6, 2017, targeted a region of high winds (greater than 30 m/s) that was also observed by the Sentinel-1B satellite's synthetic aperture radar. Sentinel-1B was configured to observe in extended wide swath mode in both VV- and VH-polarizations, whereas the IWRAP C-band radar was configured to measure all of VV-, VH-, and HH-polarizations. IWRAP and Sentinel-1B VV and VH normalized radar cross section (NRCS) at the same Earthincidence angle along the flight path match reasonably well during the entire flight, but some additional trends between aircraft and satellite can be observed. IWRAP VV-polarized NRCS generally match the CMOD5.h geophysical model function (GMF), suggesting errors in the Sentinel-1B processing chain.

Operational weather analysis, forecasting, and warning utilize a wide variety of data products and tools, including satellite imagery and derived products. Satellite observations provide information where in-situ measurements are lacking or not readily available. Passive microwave satellite observations are routinely exploited by forecasters at the National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS) in the United States (U.S.) to support their weather analysis and forecasts. In this paper, we present examples of ocean measurements and derived products from Advanced Scanning Radiometer -2 (AMSR-2) on the Global Change Observation Mission (GCOM), which is part of the Japanese Aerospace Exploration Agency (JAXA) that supported critical forecasts.

The Advanced Microwave Scanning Radiometer-2 (AMSR2) on board the Global Change Observation Mission-Water (GCOM-W) launched in May 2012 by the Japanese Exploration Agency (JAXA) is acquiring earth electromagnetic radiation for the purpose of monitoring Earth's environmental and climate system. The ocean vector winds team part of the National Oceanic and atmospheric administration (NOAA), National Environmental Satellite, Data, and Information Service (NESDIS), Center for Satellite Applications and Research (STAR), has exploited AMSR2 observations of brightness temperature (Tb) to develop an environmental data record (EDR), which includes several oceanic parameters. The purpose of this paper is to show the validation results for these geophysical parameters when compared to other active and passive microwave sensors and numerical weather models.

In this paper, airborne measurements of the ocean surface normalized radar cross-section (NRCS) taken at co- and cross-polarizations in high-wind conditions are reported. The measurements were taken in Hurricane Matthew during which it was a Category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Saturation of the co-polarized NRCS and lack of saturation in the cross-polarized NRCS is observed, consistent with previous results. The results have implications for planned and future scatterometers (e.g., MetOp-SG) that aim to increase the maximum observable wind speeds by using cross-polarized measurements.

The Stepped Frequency Microwave Radiometer (SFMR) is an instrument flown on research and reconnaissance aircraft through tropical and extratropical cyclones providing rain rate and surface wind speed estimates. Errors have been observed with the retrievals from SFMR, especially in extratropical cyclones over cold water, when compared with other sensors. In this paper, some of the SFMR wind speed errors that manifest over cold water are characterized using comparisons with in situ measurements by dropwindsondes.

We report on airborne measurements of the cross-polarized (VH) ocean surface normalized radar crosssection (NRCS) at incidence angles between 15° and 40° obtained at C-band in high-wind (> 30 m s−1) conditions. The present observations were taken in Hurricane Patricia on 23 October 2015 and extend the wind speed range of the existing cross-polarization ocean surface NRCS literature [1]–[4]. The NRCS at the smaller incidence angles decrease as wind speed increases, as expected. At the larger incidence angles, saturation of the NRCS is not observed up to at least 70 m s−1. The results have implications for planned and future scatterometers (e.g., MetOp-SG) that aim to increase the maximum observable wind speeds.

Airborne co-polarization and cross-polarization observations of ocean surface normalized radar cross section (NRCS) were conducted over the North Atlantic during January and February 2015. Observations were made using the University of Massachusetts' Imaging Wind and Rain Airborne Profiler (IWRAP) radar system and a prototype antenna for the next-generation European scatterometer aboard MetOp-SG. Both were installed on a National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft to characterize the wind response of the ocean-surface cross-polarization NRCS. During the flights, numerous constant-roll-angle circle maneuvers were performed at several different angles to collect NRCS measurements over a range of incidence angles. Surface winds at speeds between 8 and 34 ms-1were observed at incidence angles from 20° to 60° at all polarization combinations. The majority of measurements fell between 8 and 20 ms-1. Wind-direction dependence similar to copolarized NRCS was observed in the cross-polarized (VH) NRCS. The amplitude of the VH NRCS with respect to direction is less than that of copolarized NRCS at all wind speeds. Incidence angle dependence was also observed in the VH NRCS at all wind speeds. As a function of wind speed, the mean VH NRCS (A0) has a similar shape to the VV NRCS. The VH NRCS appears to not saturate at most incidence angles, unlike the VV and HH NRCS. VH and HH geophysical model functions (GMFs) were developed as functions of wind speed, incidence angle, and wind-relative azimuth for the wind speeds and incidence angles observed.

We report on airborne measurements of the sea-surface normalized radar cross-section (NRCS) at incidence angles of approximately 22° obtained at both C-band and Ku-band in high-wind (> 25ms−1) conditions. Measurements obtained over numerous research flights through tropical cyclones and high-latitude winter storms between 2011 and 2014 are composited to yield geophysical model functions in rain-free conditions. The present observations extend the results of [1], who reported high-wind NRCS for incidence angles from 30° to 50°, to a smaller incidence angle. Saturation of the mean NRCS is observed at both frequencies. In some cases the NRCS is observed to decrease with increasing wind speed beyond the saturation. The results have implications for planned and future scatterometers aiming to increase the observed swath width by extending the range of incidence angles.

Airborne dual-polarization observations of sea surface normalized radar cross section (NRCS) were conducted over the North Atlantic during January-February 2011. Observations were made using the University of Massachusetts' Imaging Wind and Rain Airborne Profiler radar system installed on the National Oceanic and Atmospheric Administration's WP-3D research aircraft during several winter storm events to determine the high-wind response of the sea surface NRCS for both horizontal and vertical polarizations. During the flights, the aircraft performed several constant-roll circle maneuvers to allow collection of NRCS over a range of incidence angles. We find consistency with prior reports in the polarization ratio observed at moderate incidence angles at the winds encountered. For larger incidence angles, we observe a measurable decrease in polarization ratio with increasing wind speed.