Comparison and Evaluation of Scatterometer (SCR) observed wind data wi

th buoy wind data

Xinzhong Zhang

Remote Sensing

December 8th, 2009

Outline

•Abstract

•Introduction

•SCR observation issues

•Data Processing & Results

•Future work

Abstract

Abstract

Seawinds scatterometer (SCR) wind data on QuikSCAT are compared with buoy observed wind data in this study. Based on this comparison, we can evaluate the accuracy and reliability of SCR wind data. Furthermore, factors like SST, ocean current, rain effect etc., which could probably affect the SCR wind data accuracy are briefly reviewed.

Introduction

Introduction

•Why is this kind of comparison important?

•How does SCR wind observation work?

•How does buoy-based wind observation work?

•Potential factors to influence scatterometer wind data.

•Data description (SCR data & Buoy wind data).

•Why is this important?

•Increasing “popularity” of SCR wind data

•Errors INDEED exist within those SCR wind data

•Calibration is needed to improve SCR wind data accuracy

•How does SCR wind observation work?

•Bragg scattering

•EMPIRICAL model function to relate backscatter cross section to Relative wind velocity

•How does buoy-based wind observation work?

Anemometer

•Anemometer installed at the top

•Wind speed and direction are averaged over periods of 8 or 10 minutes

12 m

10 m

6 m 3 m < 2 m

•Potential factors to influence SCR wind data.

•SST (Liu, W. T., 1984)

•Air-sea temperature difference

•Atmospheric density stratificaiton (Liu, 1984; Wu, 1991)

•Underneath ocean currents (Kelly et al., 2001)

•Rain (Weissman et al.,2002)

•Data description

Seawinds data on QuikScat (ftp://ftp.ssmi.com/qscat/qscat_wind_vectors/ ):• Original Orbit Wind Vector Data (swath data), which are not averaged

spatially between ascending and descending swaths.• Each orbit has one set of data, where those data are gridded into win

d vector cells, 76(cross orbit) * 1624 (along orbit)

Buoy wind data (http://www.ndbc.noaa.gov/):• Continuous wind data (10 minutes period’s average)• 10 minutes’ time interval• Original wind data at the height of buoy anemometer, which need to b

e converted in to standardized height (10m) in order to be comparable to the satellite wind data (wind at 10m height)

SCR observation issues

•Satellite observed RELATIVE velocity

(wind relative to the sea surface)

•Ocean current effect to the QuikScat observed wind:

(Xu, Y., R. B. Scott. 2008)

•Rain effect to the QuikScat observed wind:

(Weissman, et al. 2001).

•SST effect (Left figure) and Atmospheric stability effect (Right figure) to the QScat observed wind:

(Liu, W. T., 1984)

Data Processing & Results

1. Select buoy station 44009, located right outside of the Delaware Bay.

#44009

Anemometer height: 5 m above site elevation

Fortunate thing: Its distance away from the coast is >25 km.

2. Choose time periods of interest.

Here in this study, the periods of Jan. 1st-10th, Apr. 1st-10th, Jul. 1st-10th, Oct. 1st-10th

from 2005, 2006, 2007, 2008 years are chosen, so that both available satellite data and buoy data could be compared within those chosen periods.

25km

3. Choose collocation time and distance ranges for QuikScat observation and buoy observation

QuikScat wind cells closest to the buoy locations within

25 km and buoy observations closer to the QuikScat winds

within 30 minutes are chosen.

Satellite Swath

Buoy location

4. Convert buoy wind data to the equivalent neutral wind

speed at the standardized height (10m), using the method

of Liu and Tang (1996).

Wind at 5m

Wind at 10m

Results

For all collocated data available (193 data points)

Results

For all collocated data available (193 data points)

Results

Jan1-10 Apr1-10 Jul1-10 Oct1-10

R_wspeed 0.9121 0.9497 0.8875 0.9613

R_wdir 0.8356 0.8061 0.6233 0.5966

For 4 years averaged dataPOOR!!

Should be due to the inappropriate processing method

Future work

Jan1-10 Apr1-10 Jul1-10 Oct1-10

Wspeed_Qscat

minus

Wspeed_Buoy

0.0913 -0.3823 0.3551 -0.1603

Mean wind direction

272.3416 13.9619 139.093 273.320

Find out:

Does the sign (+ OR -) of the difference give any information about the ocean current below the surface wind?

Need Current data to verify!

Future work

Choose more interesting buoys and compare again….

• Dickinson, S., K.A. Kelly, M.J. Caruso, and M.J. McPhaden (2001): A note on comparisons between TAO buoy and NASA scatterometer wind vectors. J. Atmos. Oceanic Tech., 18, 799–806.

• Freilich, M. H., and R. S. Dunbar (1999), The accuracy of the NSCAT 1 vector winds: Comparisons with National Data Buoy Center buoys, J. Geophys. Res., 104(C5), 11,231–11,246.

• Xu, Y., R. B. Scott. Subtleties in forcing eddy resolving ocean models with satellite wind data. Ocean Modelling 20 (2008) 240–251

• Liu, W.T., Tang, W., 1996. Equivalent neutral wind, JPL Publication 96-17, Jet Propulsion Laboratory, Pasadena, 16 pp.

• LIU, W. T., 1984, The effects of the variations in sea surface temperature and atmospheric stability in the estimation of average wind speed by SEASAT-SASS, J. Phys. Oceanogr., 14, 392 –401.

• Satheesan, K. Sarkar, A. Parekh, A. Kumar, M. R. Kuroda, Y., Comparison of wind data from QuikSCAT and buoys in the Indian Ocean. International Journal of Remote Sensing. 2007, VOL 28; Vol 10, pages 2375-2382

• Weissman, D. E., M. A. Bourassa, and J. Tongue, Effects of rain-rate and wind magnitude on SeaWinds scatterometer wind speed errors, J. Atmos. Oceanic Technol., submitted,2001.

Reference