Jeffrey D. Hawkins

A correction for Saharan dust effects on satellite sea surface temperature measurements

Sea surface temperature (SST) retrievals obtained from NOAA advanced very high resolution radiometer (AVHRR) data can determine SST within 0.7°C on a global basis. However, retrievals have been severely limited in the presence of tropospheric aerosol particles because no adequate aerosol correction algorithm exists. High tropospheric aerosol concentrations in the atmosphere significantly increase infrared signal attenuation and prevent the retrieval of accurate satellite SSTs. This paper presents tropospheric aerosol correction algorithms derived from NOAA AVHRR SST and optical depth measurements and from drifting buoy in situ SSTs. We show that the correction algorithms perform better than the current global algorithm in an atmosphere that contains high Saharan dust concentrations and generate statistical accuracy comparable to satellite SST performance globally. These corrections would significantly increase the quantity and quality of satellite retrievals under such atmospheric conditions, and the observations would greatly benefit both global climate research and maritime operations conducted in such regions.

Circulation in the Gulf of Mexico from Geosat altimetry during 1985–1986

Using altimetry data obtained from the Geosat Geodetic Mission (April 1985 to October 1986), low-frequency sea surface height (SSH) variations are investigated in the Gulf of Mexico. SSH time series are formed using the method of Fu and Chelton and are used to calculate surface geostrophic current vectors. Spatial patterns of SSH and current vector variations enable the tracking of two major rings shed from the Loop Current. The rings drifted southwestward across the gulf and into the western boundary region at an average speed of about 3.4 cm/s. The buildup of the Loop Current was monitored, as well as the appearance of an eddy of uncertain origin in the southwestern gulf. Verification of the Geosat results are provided with surface drifters, AVHRR imagery, and hydrography.

Seasat microwave wind and rain observations in severe tropical and midlatitude marine storms

Publisher Summary This chapter presents initial results of the studies concerning Seasat measurements in and around tropical and severe midlatitude cyclones over the open ocean, and provides an assessment of their accuracy and usefulness. Sensors flown on Seasat provided complementary measurements of surface wind speed and direction, rainfall rate, significant wave height and wave length, and sea surface temperature. These measurements were made with the Seasat-A Satellite Scatterometer (SASS), the Scanning Multichannel Microwave Radiometer (SMMR), the Seasat altimeter, and the Seasat Synthetic Aperture Radar (SAR). The general tropical-storm structure is also discussed in the chapter. The progress, to date, with microwave remote sensing of tropical storms are reviewed. The nature of severe marine storms is analyzed, and microwave measurements in severe marine storms are reviewed. Seasat observations of rain rate and microwave attenuation in tropical cyclones are presented. The accuracy is assessed of derived wind, precipitation, and sea surface temperature fields in and around tropical cyclones. Initial efforts to demonstrate the usefulness of Seasat data for operational applications, such as storm warning and forecasting are also reviewed.

Atmospheric Boundary Layer and Oceanic Mixed Layer Observations in Hurricane Josephine Obtained from Air-Deployed Drifting Buoys and Research Aircraft

Abstract Three drifting buoys were successfully air-dropped ahead of Hurricane Josephine. This deployment resulted in detailed simultaneous measurements of surface wind speed, surface pressure and subsurface ocean temperature during and subsequent to storm passage. This represents the first time that such a self-consistent data set of surface conditions within a tropical cyclone has been collected. Subsequent NOAA research overflights of the buoys, as part of a hurricane planetary boundary-layer experiment, showed that aircraft wind speeds, extrapolated to the 20 m level, agreed to within ±2 m s−1, pressures agreed to within ±1 mb, and sea-surface temperatures agreed to within ±0.8°C of the buoy values. Ratios of buoy peak 1 min wind (sustained wind) to one-half h mean wind > 1.3 were found to coincide with eyewall and principal rainband features. Buoy trajectories and subsurface temperature measurements revealed the existence of a series of mesoscale eddies in the subtropical front. Buoy data revealed st...

A new automated method of cloud masking for advanced very high resolution radiometer full‐resolution data over the ocean

This paper reports on a new algorithm to remove cloud-contaminated pixels from daytime and nighttime 1-km advanced very high resolution radiometer (AVHRR) data. The technique was developed in response to Navy needs to efficiently and accurately eliminate cloud contaminated pixels from real-time satellite digital images. The remaining “cloud free” sea surface temperature (SST) pixels would be available for analysts to utilize in tracking ocean mesoscale fronts and eddies as well as input to SST and ocean thermal analysis. Initial poor results with existing cloud-masking techniques led to this effort. The method uses a series of approaches to locate cloud-contaminated pixels which include (1) use of multiple bands to detect signatures not readily available from single-channel data, (2) extraction of cloud edge information through local segmentation of the image using the cluster shade texture measure based on the gray level cooccurrence (GLC) matrix and, (3) discrimination of cloud-free from cloud-contaminated regions with an area labeling procedure. This technique is evaluated on identical data sets utilizing other experimental and operational cloud algorithms and cloud masks produced through human interpretation. This method, tested over a wide range of conditions and geographical locations, produces accurate and efficient daytime and nighttime SST data sets.

Detecting Gulf of Mexico Oceanographic Features in Summer Using AVHRR Channel 3

Abstract Efforts to monitor the Gulf of Mexico Loop Current and mesoscale ocean features using IR satellite imagery in the summertime have been significantly hindered by 1) strong surface heating that masks surface frontal gradients and 2) extremely high atmospheric water vapor attenuation that lowers effective satellite brightness-temperature values. These problems can now be addressed, provided high-quality multichannel infrared data are available during nighttime satellite passes. The National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) consists of three IR channels that include channels 3 (3.55–3.93 µm), 4 (10.3–11.3 µm), and 5 (11.5–12.5 µm). Of these, channel 3 is least affected by water vapor attenuation, making it better suited for viewing the ocean through a humid atmosphere. All satellites prior to NOAA-11, however, experienced substantial noise in channel 3 soon after launch, rendering the channel relatively useless for long-term oceanographic ...

Impact of the Geosat altimeter wet tropospheric range correction in the Greenland-Iceland-Norwegian Sea

Atmospheric water vapor data derived from the special sensor microwave imager (SSM/I) are used to make time-coincident wet tropospheric range corrections to Geosat altimeter data in the Greenland-Iceland-Norwegian (GIN) Sea. The SSM/I sensor is onboard a polar orbiting Defense Meteorological Satellite Program platform and provides water vapor data at 25-km spatial resolution and an accuracy of 0.24 g cm−2. Water vapor height corrections were calculated and analyzed along all Geosat tracks crossing the area of interest in July and September 1987 and January and March 1988. Results indicate that the horizontal spatial variations in the altimeter height corrections can sometimes equal or exceed the magnitude (10–20 cm/100 km) of true GIN Sea sea surface height gradients, especially in the summer and autumn seasons. SSM/I and Geosat data must be closely matched in time, due to the potential rapid movement of atmospheric systems through this region (35–40 km h−1). The water vapor data must be timely enough to adequately describe atmospheric conditions at the time of the altimeter pass, or significantly false signals may be introduced into the sea surface height data. Substantial differences in water vapor gradients were observed in SSM/I data sets 12 hours apart. Because of this, it is suggested that future satellite altimeter platforms include a nadir-viewing radiometer to provide time-coincident atmospheric water vapor data.