Comparisons of Sea Winds Scatterometer with a hydrologic process model for the assessment of

snow melt dynamics

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

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

Topics coveredTopics covered

• Scatterometer and Background

• Objectives

• Site-specific comparisons

• Hydrology Model

• Regions simulations and results

• Conclusion

• Scatterometer and Background

• Objectives

• Site-specific comparisons

• Hydrology Model

• Regions simulations and results

• Conclusion

ScatterometerScatterometer • 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.

• 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.

ObjectivesObjectives

• 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.

• 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.

DataData• 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

• 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

Location map showing the study domain and the sites in Alaska used in the analysis of

temperature change versus backscatter response

Location map showing the study domain and the sites in Alaska used in the analysis of

temperature change versus backscatter response

The general time series display of backscatter and hourly air temperature covering both winter and summer of year 2000

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.

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.

Time series comparisons of QuikScat backscatter (ascending) and hourly air temperature for day of year (2000) 80-110 for five sites

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

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

Hydrology Model -VICHydrology

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

• 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

NOHRSC Snow cover extent in March-April, 2000

NOHRSC Snow cover extent in March-April, 2000

March 28March 28 April 6April 6 April 9April 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.

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.

Classification of frozen days using the daily maximum temperature and the QuikScat data Classification of frozen days using the daily maximum temperature and the QuikScat data

•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

•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

Backscatter, air temp. and model derived snow surface temperature and snow surface water content during 12-16 Apr 2000-Early spring thaw-refreeze Backscatter, air temp. and model derived snow surface temperature and snow surface water content during 12-16 Apr 2000-Early spring thaw-refreeze

•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.

•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.

•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.

•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.

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

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.

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.

ConclusionsConclusions

• 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.

• 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.