COMPARATIVE IMPACT OF OCEAN SURFACE WIND VECTORS AND OCEAN SURFACE WIND SPEED FOR GLOBAL NWP

In this study, we compare the impact of ocean surface vector winds (OSWV) to ocean surface wind speed (OSWS) observations for global data assimilation and numerical weather prediction. Currently, OSWV are provided by scatterometers (ASCAT) and polarimetric microwave imagers (WindSat), while OSWS are provided by microwave imagers (e.g. SSMIS and AMSR-2) or GNSS-R (future research NASA CYGNSS). Forecast sensitivity observation impact (FSOI) statistics for NAVGEM (NAVy Global Environmental Model) indicate that, on average, ASCAT and WindSat wind vectors (OSVW) have slightly more than twice the impact of SSMIS wind speed observations. This result is expected, as wind vectors provide two observations, whereas wind speed provides only one.

However, this comparison does not take into account differences due sensor quality and resolution (and averaging), ability to retrieve winds in precipitating regions, and effective data coverage relative to other observing platforms. As such, it gives limited insight into the additional value provided by wind direction observations. To examine these questions in more detail, we have modified our global NAVGEM system to assimilate ASCAT wind vectors as wind speed observations. The wind vectors are processed through the same data selection and quality control algorithms, and then only the wind speed is retained. The 4DVar satellite wind processing software generates superobs for SSMIS wind speeds. Preliminary results indicate that the ASCAT wind speed observations have similar FSOI per observation as SSMIS wind speed observations.

We will present detailed results from two multi-month NAVGEM assimilation runs. The baseline run assimilates ASCAT wind vectors while the test run assimilates the ASCAT wind speed. Comparisons will include the fit of analyses and forecasts with in-situ observations and analyses from other NWP centers (e.g. ECMWF and GFS).