Effectiveness of WRF wind direction for retrieving coastal sea surface wind from synthetic aperture radar

Abstract

Wind direction is required as input to the geophysical model function (GMF) for the retrieval of sea surface wind speed from synthetic aperture radar (SAR) images. The present study verifies the effectiveness of using the wind direction obtained from the weather research and forecasting model (WMF) as input to the GMF to retrieve accurate wind fields in coastal waters adjacent to complex onshore terrain. The wind speeds retrieved from 42 ENVISAT ASAR images are validated based on in situ measurements at an offshore platform in Japan. Accuracies are also compared with cases using wind directions: the meso-analysis of the Japan Meteorological Agency (MANAL), the SeaWinds microwave scatterometer on QuikSCAT and the National Center for Environmental Prediction final operational global analysis data (NCEP FNL).In comparison with the errors of the SAR-retrieved wind speeds obtained using the WRF, MANAL, QuikSCAT and NCEP FNL wind directions, the magnitudes of the errors do not appear to be correlated with the errors of the wind directions themselves. In addition to wind direction, terrain factors are considered to be a main source of error other than wind direction. Focusing on onshore winds (blowing from the sea to land), the root mean square errors on wind speed are found to be 0.75 m s 1 (in situ), 0.96 m s 1 (WRF), 1.75 m s 1 (MANAL), 1.58 m s 1 (QuikSCAT) and 2.00 m s 1 (NCEP FNL), respectively, but the uncertainty is of the same order of magnitude because of the low number of cases. These results indicate that although the effectiveness of using the accurate WRF wind direction for the wind retrieval is partly confirmed, further efforts to remove the error due to factors other than wind direction are necessary for more accurate wind retrieval in coastal waters.This version of the paper [24 October 2012] was modified slightly from the first version [16 July 2012]. Mainly, the notation style of the abbreviations; WRF, MANAL and NCEP FNL, was changed for an avoidance misunderstanding. In addition, excessive detail descriptions are removed from tables. This notice is included in the online and print versions to indicate that both have been corrected.

Hub Height Ocean Winds over the North Sea Observed by the NORSEWInD Lidar Array: Measuring Techniques, Quality Control and Data Management

“…Often information on air temperature, air temperature differences, humidity, boundary-layer height and other parameters are used for in-depth analysis of atmospheric boundary-layer behavior and structures. This is unfortunately not possible with this dataset except if combined with other data sources such as numerical model results, satellite data or other sources as in [38,39].…”

Validation of Sentinel-1A SAR Coastal Wind Speeds Against Scanning LiDAR

“…The wind direction over the entire SAR image deviates locally from the assumed constant 10-min average used for the retrieval. Local measurements of the wind direction have shown to have lower RMSEs compared to other methods [11,15]. Comparisons of the input wind direction with the scanning LiDAR wind directions yield up to 10 • difference.…”

“…Hasager et al [9] 149 to 197 Denmark/The Netherlands 1.27 to 1.65 Envisat Chang et al [10] 552 South Chinese Sea 2.09 Envisat Takeyama et al [11] 42 Japan 0.75 to 2.24 Envisat Chang et al [12] 522 East Chinese Sea 1.99 Envisat Takeyama et al [13] 33 to 73 Japan 0.64 to 2.34 Envisat Hasager et al [14] 875 Baltic Sea 1.17 Envisat Christiansen et al [15] 91 Denmark 1.1 to 1.8 ERS2 Hasager et al [16] 61 Denmark 0.9 to 1.14 ERS2…”

“…These RMSEs are comparable to results from validation studies in the open ocean using buoy data, scatterometer derived winds, atmospheric modelling [5,[17][18][19] that ranged between 0.65 ms −1 and 1.76 ms −1 . Large influences of the wind directions from land or from the sea with high RMSEs above 2 ms −1 were found in Takeyama et al [13] and Takeyama et al [11] while very little influence was found in Chang et al [10]. Validation studies for coastal sites in Table 1 were done with point measurements from meteorological masts and indicate that SAR can measure wind speeds in the coastal zone.…”

“…The influence of those effects can change when moving from the open ocean to the coastal zone. Table 1 shows an overview of selected past studies validating C-band SAR winds in coastal regions using meteorological masts [9][10][11][12][13][14][15][16]. Resulting Root Mean Square Errors (RMSE) ranged between 0.64 ms −1 and 2.34 ms −1 .…”