Salem El-Nimri

An Improved C-Band Ocean Surface Emissivity Model at Hurricane-Force Wind Speeds Over a Wide Range of Earth Incidence Angles

An improved microwave radiometric ocean surface emissivity model has been developed to support forward radiative transfer modeling of brightness temperature and geophysical retrieval algorithms for the next-generation airborne Hurricane Imaging Radiometer instrument. This physically based C-band emissivity model extends current model capabilities to hurricane-force wind speeds over a wide range of incidence angles. It was primarily developed using brightness temperature observations during hurricanes with coincident high-quality surface-truth wind speeds, which were obtained using the airborne Stepped-Frequency Microwave Radiometer. Also, other ocean emissivity models available through the published literature and the spaceborne WindSat radiometer measurements were used.

Hurricane Wind Speed Measurements in Rainy Conditions Using the Airborne Hurricane Imaging Radiometer (HIRAD)

This paper describes a realistic computer simulation of airborne hurricane surveillance using the recently developed microwave remote sensor, the hurricane imaging radiometer (HIRAD). An end-to-end simulation is described of HIRAD wind speed and rain rate measurements during two hurricanes while flying on a high-altitude aircraft. This simulation addresses the particular challenge which is accurate hurricane wind speed measurements in the presence of intense rain rates. The objective of this research is to develop baseline retrieval algorithms and provide a wind speed measurement accuracy assessment for future hurricane flights including the NASA GRIP hurricane field program that was conducted in summer of 2010. Examples of retrieved hurricane wind speed and rain rate images are presented, and comparisons of the retrieved parameters with two different numerical hurricane models data are made. Special emphasis is provided on the wind speed measurement error, and statistical results are presented over a broad range of wind and rain conditions over the full measurement swath (earth incidence angle).

Restrictions on the field of view for an undersampled 1-D synthetic thinned aperture radiometry

Traditional radio astronomy formulation of the field of view (FOV) applied to a non-Nyquist spatially sampled synthetic thinned aperture radiometers appear to overestimate the FOV. Utilizing the current design of the Hurricane Imaging Radiometer (HIRad) as a baseline instrument, a simple analytical instrument simulator is developed to investigate the different methods of determining the FOV. Analytically, this value is found to be 70deg. However the usable range seems to extend only to 61deg. The presence of aliased grating lobes is easily seen in the synthesized antenna patterns. Beam efficiency plots with threshold levels can be used to determine the reduced FOV. Error analysis is performed utilizing several sample images of Hurricane Katrina.

Development of an ocean surface emissivity model for wide swath imaging of wind speed to hurricane force

The Hurricane Imaging Radiometer, HIRad, is a new instrument for making wind and rain observations in hurricanes. It is being developed by NASA to give NOAA improved capability in forecasting hurricane intensity and track. HIRad is being designed to measure ocean surface wind speed up to greater than 70 m/sec. over a swath out to plusmn60 deg. Current surface emissivity models are not adequate for both high winds and a large swath so a HIRad model is being developed. NOAA, Stepped Frequency Microwave Radiometer brightness temperature measurements are being used in this development This paper presents a review of the HIRad wind speed model development and the preliminary results for both V-pol and H-pol.

A Wide-Swath, Hurricane Imaging Radiometer For Airborne Operational Measurements

In recent years, the contributions of microwave remote sensing to hurricane numerical forecasting have increased significantly, particularly with the Stepped Frequency Microwave Radiometer, SFMR, measurements of wind speed and rain rate. The Hurricane Imaging Radiometer, HIRad, is a new instrument concept that improves on the SFMR by imaging surface wind speed and rain rate over a ± 45 deg. swath. It is compatible with high altitude jet aircraft and unpiloted aerial vehicles, and has potential for space- borne use. This paper provides a brief description of the HIRad concept, the status of the HIRad microstrip patch array technology, and a review of a physically based radiative transfer model developed for HIRad modeling and simulations.

Hurricane Imaging Radiometer Wide Swath Simulation for Wind Speed and Rain Rate

There is a strong national interest in the observation of ocean surface winds with high spatial and temporal resolution for understanding tropical cyclones and their effects on weather and climate and in forecasting storms making landfall. Current satellite and aircraft based remote sensing capability is limited in wind speed dynamic range and in the ability to retrieve wind information in the presence of rain, or in temporal and spatial coverage, respectively. The hurricane imaging radiometer (HIRAD) is capable to capture all the hurricane features and dynamics from a high altitude aircraft preserving high resolution measurements. A detailed description of the methods used in simulating the HIRAD instrument surface sampling of wind speed, in intense rain, from various aircraft platforms with realistic operational flight patterns through a time evolving hurricane will be provided in this paper. A noise model used to simulate the effects of rain for various observation path lengths over the swath will also be described. Results will demonstrate the extent of spatial and temporal coverage available from currently available aircraft platforms.

Hurricane Imaging Radiometer wind speed and rain rate retrieval: [Part-1] development of an improved ocean emissivity model

A new microwave radiometric ocean surface emissivity model has been developed to support the analysis and design of the new airborne Hurricane Imaging Radiometer, HIRAD. This radiative transfer model extends current ocean surface emissivity capabilities to higher wind speeds and incidence angles. This model utilizes a variety of empirical data sources many of which were collected in hurricanes.

Sea surface salinity roughness correction at L-band for Aquarius instrument

Aquarius/SAC-D is a joint NASA/CONAE (Argentine Space Agency) Earth Science satellite mission to measure the global sea surface salinity (SSS). The prime remote sensor is an L-band microwave radiometer to measure ocean blackbody emission (brightness temperature, Tb), which depends upon the sea surface temperature and SSS. The application of L-band radiometry to measure SSS is a difficult task, and there are many Tb corrections that must be made correctly to obtain accurate SSS data. One of the major error sources is the effect of ocean roughness that “warms” the ocean Tb. The Aquarius baseline approach uses the coincident radar scatterometer to provide this ocean roughness correction through the correlation of radar backscatter with the excess ocean emissivity without directly measuring the surface wind speed. This paper provides an alternative approach using a theoretical Radiative Transfer Model (RTM) driven by numerical weather forecast model for ocean surface wind vector. The theoretical basis of our algorithm is described and results are compared with the AQ baseline scatterometer method.

An ocean roughness correction algorithm for retrieving salinity on Aquarius

Aquarius/SAC-D is a joint NASA/CONAE (Argentine Space Agency) Earth Sciences satellite mission to measure the global sea surface salinity (SSS) using an L-band radiometer to measure ocean blackbody emissions also known as brightness temperature (Tb). The application of L-band radiometry to measure SSS is a difficult task, and there are many corrections that must be made correctly to obtain accurate SSS data. One of the major error sources is the effect of ocean roughness that “warms” the ocean brightness temperature (Tb). The Aquarius baseline approach uses the radar scatterometer to provide this ocean roughness correction through the correlation of radar backscatter with the excess ocean emissivity without directly measuring the surface wind speed. This paper provides an alternative approach using a theoretical Radiative Transfer Model (RTM) driven by numerical weather forecast model surface winds. The theoretical basis of our algorithm is described and results are compared with the AQ baseline scatterometer method.

Improved hurricane active/passive simulated wind vector retrievals

Microwave scatterometers are the standard for satellite ocean vector winds (OVW) measurements, and they provide the major source of global ocean surface winds observations for scientific and operational applications. A major challenge for Ku-band scatterometry missions is to provide reliable retrievals in the presence of precipitation, particularly in extreme ocean wind events that are usually associated with intense rain. This paper explores the advantages of combining dual frequency (C- and Ku-band) scatterometer measurements and passive microwave observations to improve high wind speed retrievals. For this study, a conceptual design proposed by the Jet Propulsion Laboratory for a Dual Frequency Scatterometer (DFS) to fly onboard the future Japan Aerospace Exploration Agency (JAXA) GCOM-W2 mission with the Advanced Microwave Scanning Radiometer (AMSR) was adopted. A computer simulation that combines the DFS and AMSR measurements was used to develop an artificial neural network OVW retrieval algorithm. The Weather Research and Forecasting (WRF) numerical weather model of Hurricane Katrina (2005) was used as the nature run (surface truth), and simulated OVW retrievals demonstrate that this new technique offers a robust option to extend the useful wind speed measurements range beyond the current operating scatterometers for future satellite missions.