AUGUST 2006 V O N A H N E T A L . 523 Operational Impact of QuikSCAT Winds at the NOAA Ocean Prediction Center JOAN M. VON AHN STG, Inc., and NOAA/NESDIS/ORA, Camp Springs, Maryland JOSEPH M. SIENKIEWICZ NOAA/NWS/NCEP/OPC, Camp Springs, Maryland PAUL S. CHANG NOAA/NESDIS/ORA, Camp Springs, Maryland (Manuscript received 26 April 2005, in final form 19 December 2005) ABSTRACT The NASA Quick Scatterometer (QuikSCAT) has revolutionized the analysis and short-term forecasting of winds over the oceans at the NOAA Ocean Prediction Center (OPC). The success of QuikSCAT in OPC operations is due to the wide 1800-km swath width, large retrievable wind speed range (0 to in excess of 30 msϪ1), ability to view QuikSCAT winds in a comprehensive form in operational workstations, and reliable near-real-time delivery of data. Prior to QuikSCAT, marine forecasters at the OPC made warning and forecast decisions over vast ocean areas based on a limited number of conventional observations or on the satellite presentation of a storm system. Today, QuikSCAT winds are a heavily used tool by OPC fore- casters. Approximately 10% of all short-term wind warning decisions by the OPC are based on QuikSCAT winds. When QuikSCAT is available, 50%–68% of all weather features on OPC surface analyses are placed using QuikSCAT. QuikSCAT is the first remote sensing instrument that can consistently distinguish ex- treme hurricane force conditions from less dangerous storm force conditions in extratropical cyclones. During each winter season (October–April) from 2001 to 2004, 15–23 extratropical cyclones reached hur- ricane force intensity over both the North Atlantic and North Pacific Oceans. Due to QuikSCAT, OPC forecasters are now more likely to anticipate the onset of hurricane force conditions. QuikSCAT has also revealed significant wind speed gradients in the vicinity of strong sea surface temperature (SST) differences near the Gulf Stream and shelfbreak front of the western North Atlantic. These wind speed gradients are most likely due to changes in low-level stability of the boundary layer across the SST gradients. OPC forecasters now use a variety of numerical guidance based tools to help predict boundary layer stability and the resultant near-surface winds. 1. Introduction for the Safety Of Life At Sea (SOLAS). Wind warning categories are based on the Beaufort wind speed scale The NOAA Ocean Prediction Center (OPC) is re- as described by Bowditch (2005): gale, 34–47 kt (17.2– sponsible for issuing marine wind warnings and fore- 24.4 m sϪ1); storm, 48–63 kt (24.5–32.6 m sϪ1); and hur- casts of winds and seas for the extratropical high seas ricane force, 64 kt or greater (32.7 m sϪ1 or greater). In and offshore waters of the Atlantic and Pacific Oceans. Bowditch (2005), winds speeds are given in whole knots OPC wind warnings and forecasts, in part, fulfill the whereas meters per second are continuous and given to United States requirement to provide marine warnings the nearest tenth. Since wind speeds are given in knots and forecasts under the 1974 International Convention for all OPC graphical and text products, that conven- tion will be maintained throughout this paper (conver- sion to m sϪ1 will follow in parentheses and may not Corresponding author address: Joan M. Von Ahn, 5200 Auth exactly match the values above due to the rounding Road, Camp Springs, MD 20746. applied by Bowditch). OPC wind warnings are broad- E-mail: [email protected] cast directly to mariners at sea and are used to make WAF934 524 WEATHER AND FORECASTING VOLUME 21 decisions regarding both safe and economic opera- to the two 600-km-wide swaths. NSCAT also provided tions. a wide range of retrieved wind speeds that extended OPC forecasters issue these wind warnings for vast well into the storm force category. For the first time open-ocean areas of the North Pacific and North At- forecasters were able to see retrieved winds over entire lantic Oceans from the subtropics to the Arctic. A va- storm systems and differentiate between gale and storm riety of cyclone activities occur across these waters in- force winds (Atlas et al. 2001). Unfortunately, the sat- cluding meteorological “bombs” during the autumn and ellite suffered a catastrophic failure in July 1997. In winter (Sanders and Gyakum 1980) and tropical cy- 1999, in response to the loss of NSCAT, NASA clones undergoing extratropical transition during the launched the Quick Scatterometer Satellite with a Sea- summer and autumn. Accurate and timely observations Winds scatterometer (henceforth referred to as Quik- of meteorological conditions are necessary for OPC fore- SCAT) on board (Atlas et al. 2001). The QuikSCAT casters to make rapid and accurate warning decisions. near-real-time winds were accessible to OPC forecast- Although conventional observations from buoys and ers shortly after launch through Internet access. In Oc- voluntary observing ships (VOSs) are extremely useful tober 2001, QuikSCAT winds were introduced to the to marine forecasters, their distribution is sparse and OPC operational National Centers Advanced Weather mostly limited to trade routes or continental waters. Interactive Processing System (N-AWIPS) (desJardins Over the past 12 yr, forecasters have come to rely et al. 1991) workstations and became fully integrated more and more on remotely sensed data to help fill in into OPC operations. In N-AWIPS, QuikSCAT winds the gaps inherent in conventional observations. OPC can be displayed as an overlay or underlay and com- forecasters have used the winds derived from Special pared to observations, numerical model analysis and Sensor Microwave Imager (SSM/I) available from the forecast fields, and conventional satellite imagery. Defense Meteorological Satellite Program (DMSP) se- Several characteristics of QuikSCAT have made it a ries of satellites. However, they are of limited value. very popular tool for OPC forecasters. The data are SSM/I retrievals consist of wind speed only and not the available to the forecasters in near–real time, on aver- full wind vector. The operational SSM/I winds available age within 90 min–3 h of data acquisition. The very from NOAA/National Environmental Satellite, Data, wide 1800-km swath width provides ocean vector winds and Information Service (NESDIS) are processed using over 90% of the world oceans daily, and gives OPC the retrieval algorithm developed by Goodberlet et al. forecasters two swaths for both the North Atlantic and (1989). These winds have an upper retrievable limit North Pacific each day. QuikSCAT retrieves wind within the gale warning category [less than 48 kt (24.5 speeds to 58.3 kt (30 m sϪ1) (near hurricane force) with msϪ1)]. Therefore, forecasters using SSM/I wind an accuracy of Ϯ3.9 kt (2 m sϪ1) (Shirtliffe 1999), al- speeds can only distinguish between the lowest warning though OPC forecasters have often observed Quik- category and nonwarning winds. Perhaps a larger hin- SCAT winds in excess of 63 kt (32.7 m sϪ1) in associa- drance is that SSM/I is not able to retrieve wind speeds tion with extratropical cyclones (Von Ahn et al. 2004). in areas of liquid cloud and precipitation, which are of Although QuikSCAT wind retrievals in areas of mod- very high interest to marine forecasters as they often erate to heavy rain can be contaminated, this does not contain high winds (Atlas et al. 2001). significantly detract from its use to forecasters in the Scatterometer-derived winds have been available to extratropics. OPC forecasters for various periods over the last 10 yr. In this paper we have attempted to quantify the im- The European Space Agency’s European Remote Sens- pact of QuikSCAT winds on the OPC analysis and ing Satellites-1 and -2 (ERS-1 and ERS-2) winds were warning process. OPC operational QuikSCAT display used by forecasters with minimal success as the swath capabilities will be discussed in section 2. The results of width was narrow (500 km) and therefore the chances several impact studies will be presented and discussed of retrieving wind vectors over a particular area of in- in section 3. Section 4 addresses data concerns. The terest were small (Katsaros et al. 2001). In 1996, the capability to detect hurricane force conditions will be National Aeronautics and Space Administration discussed in section 5. QuikSCAT winds have also re- (NASA) Scatterometer (NSCAT) was launched on vealed strong wind speed gradients across the oceanic board Japan’s Advanced Earth Observing Satellite-I thermal fronts of the western Atlantic. The impact on (ADEOS-I) and provided 90% coverage of the ocean the forecasters’ ability to see the sensitivity of near- areas within a 2-day period. OPC forecasters used surface winds to the adjoining ocean surface tempera- NSCAT data routinely and for the first time were able ture will be discussed in section 6. Summary and con- to view ocean vector winds over large ocean areas due clusions will be given in section 7. AUGUST 2006 V O N A H N E T A L . 525 FIG. 1. Screen capture of N-AWIPS workstation display of QuikSCAT winds. Wind vectors are plotted as conventional wind barbs in knots. Rain-flagged winds are depicted in white (color bar in upper left); non-rain-flagged winds are depicted in colors according to preset wind speed categories (color bar in upper right). White lines with time stamp (in 1-min intervals) at each end of the swath indicate the time (UTC) of data acquisition. These attributes can all be edited from within the QuikSCAT attribute window (QSCT) (right side of figure). 2. OPC operational QuikSCAT display capabilities product generation mode. As an example, forecasters are able to overlay the most recent pass of winds from The operational forecast centers of the National Cen- QuikSCAT with conventional satellite imagery while in ters for Environmental Prediction (NCEP) use the N- the process of producing an analysis of surface features AWIPS workstations to display observations, output or preparing a high seas or offshore warning bulletin.