V. Sridhar a , Kyle C McDonald b and Dennis P Lettenmaier a

1. Comparisons of Sea Winds Scatterometer with a hydrologic process model for the assessment of snow melt dynamics V. Sridhara, Kyle C McDonaldb and Dennis P Lettenmaiera aDepartment of Civil and Environmental Engineering Box 352700, University of Washington, Seattle, WA 98195 bJet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109-8099

2. Topics covered • Scatterometer and Background • Objectives • Site-specific comparisons • Hydrology Model • Regions simulations and results • Conclusion

3. Scatterometer • The NASA Scatterometer (NSCAT) was the first dual-swath Ku-band scatterometer launched aboard Japan's ADEOS-Midori Satellite in August, 1996. The mission was prematurely terminated due to satellite power loss in June 1997. • Because of the success of the short-lived NSCAT Scatterometer, the QuikSCAT mission was launched atop a U.S. Air Force Titan II on June 19, 1999. • The QuikSCAT spacecraft operates in a Sun-synchronous, 800-kilometer (497-mile) near-polar orbit, circling Earth every 100 minutes, taking approximately 400,000 measurements over 93 percent of Earth's surface every day.

4. Objectives • To simulate snow surface conditions over Alaska using the Variable Infiltration Capacity (VIC) hydrologic process model at ½ degree spatial duration for Fall and Spring seasons of September 1999 through May 2001. • To compare the simulations with backscatter measurements from the Ku-band SeaWinds scatterometer for possible freeze/thaw signals.

5. Data • Daily station-observed precipitation, minimum and maximum temperature for Alaska from the National Climate Data Center (NCDC) • Daily Wind speed data (Kalnay et al., 1996) • Gridded at 0.5° resolution and used to force the model. • 0.5° vegetation and soil data from 2º x 2º global soil and vegetation dataset (Nijssen, 2001) • QuikScat scatterometer data twice daily over Alaska region, the ascending overpass time being about 8 P.M. while the descending time close to 7 A.M

6. Location map showing the study domain and the sites in Alaska used in the analysis of temperature change versus backscatter response

7. The general time series display of backscatter and hourly air temperature covering both winter and summer of year 2000 A high backscatter (~-12 dB) during snow dominant periods in Fall and Spring with a considerable drop (~-16 dB) coinciding spring snowmelt near day 125 (on X-axis) and a surge again during fall-freeze around day 275 (~-12 dB) can be seen with intermittent sharp spikes whenever excursions above 0ºC is encountered.

8. Time series comparisons of QuikScat backscatter (ascending) and hourly air temperature for day of year (2000) 80-110 for five sites All but Kobuk showed a drop of up to –4dB around DOY 105 clearly in ascending overpass signatures, matching the air temp. excursions above 0°C

9. Hydrology Model -VIC • Multiple vegetation classes in each cell and are specified by their leaf area index, root distribution and canopy resistances • Sub-grid elevation band definition (for snow) • Snow pack accumulation and ablation simulated by a 2-layer energybalance model with canopy effects • 3 soil layers used • Explicit 2-layer parameterization for ground heat flux • Energy and water budget closure at each time step • Subgrid infiltration/runoff variability • Non-linear baseflow generation

10. NOHRSC Snow cover extent in March-April, 2000 March 28 April 6 April 9 NOHRSC Snow cover extent for the period of March-April 2000 showed the presence of snow for most of Alaska. This pattern matched with the VIC simulated snow depth for the corresponding period in the region.

11. Arctic region exhibited a difference of less than 20 days and central and southern regions displayed a difference in the range of 60-100 days. • QuikScat-based assessment of landscape freeze/thaw were in broad and general agreement at a seasonal scale with the estimates using gridded maximum temperature data derived from the weather stations over a wide region Classification of frozen days using the daily maximum temperature and the QuikScat data

12. Descending backscatter, air temp. (forcing the model) and model produced snow surface temperature and snow water equivalent are shown here for April 12-16, 2000. Backscatter, air temp. and model derived snow surface temperature and snow surface water content during 12-16 Apr 2000-Early spring thaw-refreeze

13. On April 12 and April 13, modeled snow surface temperature crossed the threshold releasing liquid water over west Alaska • While model predicted liquid water was present in patches over the central region and none at all in the southwest, backscatter signal remained quite low in the southwest and no indication of drop in backscatter in Central Alaska.

14. Backscatter, air temp. and model derived snow surface temperature and snow surface water content during 13-17 May 2001:Spring melt

15. Except few patchy spots of below freezing, over all region exhibited a rise in air temperature ranging between 0ºC and 10ºC. Between May 15 and May 17 the above freezing temperatures melted the snowpack releasing significant amount of liquid water especially on the west coast. The backscatter signal is comparable over southwest but not over northcentral region where liquid water content appeared.

16. Conclusions • Modeling snow surface conditions using a hydrology model is demonstrated successfully. • QuikSCAT signals for individual sites have agreements with air temperatures but not always. • 8 P.M. QuikSCAT comparisons with VIC showed a slightly better relations than those of 7 A.M. • Existing temporal resolution of backscatter signals may not be sufficient for hydrologic applications.