SAR-Derived Coastal Winds

William Pichel (NOAA/NESDIS), Frank Monaldo (JHU/APL), and Jeremy Nicoll (ASF)

The coastal waters of Alaska, as well as mountainous coastal areas elsewhere in the U.S. and Canada, are subject to wind extremes, both in velocity and spatial variability. Coastal areas of the Gulf of Alaska and Bering Sea can experience winds that vary from almost calm to over 25 m/s in only a few kilometers along the shore. This high spatial variability results from winds blowing and accelerating through mountain gaps, downslope winds from glaciers and mountains, as well as wind sheltering by islands and coastal topographic features.

Better knowledge of these coastal wind variations can improve safety of marine transportation (for example oil tankers leaving Prince William Sound), commercial and charter fishing activities, and low-flying aircraft. Many wind features in these coastal regions result from the interaction of lower tropospheric winds with coastal topography. These wind features include: mountain lee waves, wakes (mountain, island, and point), vortex streets (i.e., lines of counter-rotating vortices downstream from a mountain), barrier jets, gap flows (both upstream and downstream of the gap), and williwaws (i.e., downslope winds).

These phenomena show considerable wind variability at scales of a kilometer or two to tens of kilometers. Neither wind measurements from sparse networks of buoys nor lower-resolution satellite passive microwave radiometer or scatterometer wind measurements can resolve the details of these phenomena. High-resolution satellite, synthetic aperture radar (SAR) wind measurements from RADARSAT-1 and ENVISAT, however, show these coastal wind details, as well as details of atmospheric boundary layer phenomena and storm morphology in the open ocean (such as atmospheric frontal dynamics, storm center location and structure, convection patterns, wind rows, gust fronts, and atmospheric gravity waves).

In October 1999, the National Oceanic and Atmospheric Administration (NOAA) National Environmental Satellite, Data, and Information Service (NESDIS) in partnership with the Alaska Satellite Facility (ASF), the NOAA National Weather Service (NWS), and the U.S. Coast Guard began a near-real-time applications demonstration (called the Alaska SAR Demonstration, or AKDEMO) of SAR-derived environmental products, including SAR winds. Winds have been produced since that time in a quasioperational near-real-time fashion. Winds are produced from Quick-Look SAR image data processed by ASF within a few hours of satellite overpass time. Currently, winds are calculated at NOAA/NESDIS and The Johns Hopkins University Applied Physics Laboratory (JHU/APL). Also, ASF has begun generating SAR wind products experimentally with the goal of providing wind products more quickly to the NWS offices in Alaska. Figures 1 and 2 are examples of the wind products. Tens of thousands of these wind images have been produced since 1999, and they are available with sufficient temporal coverage to be used in studies of coastal wind climatology, which may lead to more accurate weather forecasts.

Wind products are available as PNG imagery (like Figures 1 and 2), NetCDF files, and ASCII vector information. Wind vectors are also sent directly to the NWS office in Anchorage as files that can be displayed on their interactive forecast workstations. Products are generally available within 2 to 4 hours after acquisition by ASF.

The accuracy of SAR wind measurements has been determined from comparisons with buoy winds, model winds, and satellite scatterometer winds. These comparisons generally show root-meansquare errors less than 2 m/s. Links to presentations and papers concerning the wind algorithm and accuracy of the products can be found on the NOAA web site given below. In addition, a newly completed guide to understanding SAR wind imagery and lower tropospheric wind phenomena, entitled “High Resolution Wind Monitoring with Wide Swath SAR: A User’s Guide,” is available on line at the following web site: http://fermi.jhuapl.edu/sar/stormwatch/user_guide/.

The views, opinions, and findings contained in this article are those of the authors and should not be construed as an official NOAA or U.S. Government position, policy, or decision.

These SAR wind products can be accessed by researchers, government agencies, and the general public via three web sites:
(1) NOAA: http://www.orbit.nesdis.noaa.gov/sod/mecb/sar
(2) JHU/APL: http://fermi.jhuapl.edu/sar/stormwatch/index.html
(3) ASF: http://wind.asf.alaska.edu.

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