Retrieval of Snow Water Equivalent Using Passive Microwave Brightness Temperature Data By:...
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Transcript of Retrieval of Snow Water Equivalent Using Passive Microwave Brightness Temperature Data By:...
Retrieval of Snow Water Equivalent
Using Passive Microwave Brightness
Temperature Data
By:Purushottam Raj Singh & Thian Yew GanDept. of Civil & Environmental Engineering
University of Alberta, Canada
RESEARCH OBJECTIVE
To Develop new SWE retrieval algorithms
using Passive Microwave Brightness
Temperature Data of SSM/I Sensor for a
prairie like environment of North America
*SWE = Snow Water Equivalent (cm)
INTRODUCTION
• Snow: Dominant source of Water Supply, contributes up to 70% in many parts of Canada
• Seasonal Variation of SWE: Critical to an effective management of Water Resources
• Snow course & snow gauge data: Point measurements & Limited
• Airborne Data for SWE: Expensive
PASSIVE MICROWAVE RADIOMETRY
• Passive Microwave (PM): can penetrate clouds & provide information during night
• Daily PM data available on a global basis
• Satellite Microwave data: To retrieve SWE Chang et al.,1976; Goodison et al.,1986; etc.
• Basis of microwave detection of snow: Redistribution of upwelling radiation (RTM, SM)
STUDY SITE
• Red River Basin (120,000 Km^2)
• Elevation Range: 237-552m
Figure 1. The Red River basin study area of eastern North Dakota and northwestern Minnesota.
DATA• Airborne SWE Data(88, 89 & 97)->NWS
• SSM/I Brightness Temperature
Year SSM/I Ascending/Descend Source
1988: DMSP(F8) 6:13 18:13 -> NSIDC
1989: DMSP(F8) 6:13 18:13 -> NSIDC
1997: DMSP(F10) 22:24 10:24 -> MSFC
1997: DMSP(F13) 17:46 5:46 -> MSFC
• Other DataLand Use/Cover & DEM(30 arc”) -> USGS
Temperature & Precipitation -> HPCC
Total Precipitable Water(1 deg.) -> TOVS
0
50
100
150
200
Nov Dec Jan Feb Mar AprMonth
Sno
wfa
ll(c
m)
• Airborne SWE Data: ->NWS
Year 1988 1989 1997
# of Airborne Data: 65 241 192
# of Gridded Data: 52 175 197
# of Dry Snow Cases: 16 121 119
Mean SWE(cm) 3.43 9.25 13.55
• Cumulative Snowfall1997
1989
1988
Cumulative snowfall in cm.
Selection Criteria for Dry Snow Cases
• V37<250°K; V19-V37=>9 °K ! Goodison et al.,’86
• V37-H37 => 10 °K ! Walker & Goodison’93
• P_factor > 0.026
• V37 > 225 °K (DMSP-F8)
• P_factor < 0.041 (F10/F13)
Where,
V37: 37GHz Vertical Polarization Brightness Temperature(°K)P_factor or polarization factor = (V37-H37)/(V37+H37)
! From Present Study
RETRIEVAL ALGORITHMS
Goodison et al.,’94: SWE=K1+K2(V19-V37) ..(1)
Chang et al.,’96: SWE=K3+K4(H19-H37)(1-AF) ..(2)
Proposed: (a) Conventional Regression (b) PPR
(a) SWE = K5(V19-H37) + K6(AMSL) + K7(1-AF) +
K8(1-AW)TA + K9 (TPW) ..(3)
)4..()x(yy)b( Tmm
o
m
M
1m
• Projection Pursuit Regression (PPR):)4()x(
M
1myy T
mm
o
m
)5()x(yE
2
Tmm
Mo
1mmy
)6()y(Var
)y,x,,,(LU
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
1 2 3 4 5 6 7 8 9
Number of Terms of PPR, Mo
Un
exp
lain
ed V
ari
an
ce, U
Figure 2.Calibration Results: Fraction of unexplained variance (U) versus the number of terms (Mo) for the PPR model using selected dry snow cases, ascending set of SSM/I data of 1989.
Figure 3. Scatterplots of SWE from Airborne Gamma Ray Vs. SWE Retrieved from SSM/I using Existing (Eq. 1) and Proposed (Eq. 3: Multi-variate Regression) Algorithms.
DISCUSSION OF RESULT
Figure 4. Scatterplots of SWE from Airborne Gamma Ray Vs. SWE Retrieved from SSM/I using Existing (Eq. 2) and Proposed (Eq. 4: Projection Pursuit Regression) Algorithms.
DISCUSSION OF RESULT (Contd.)
Necessity to Add Shift-Parameter(or “offset’)
• Shift-Parameter (SP) required at validation stage.
• Existing retrieval algorithms: show some improvement with SP.
• SP depends on the overall SWE of each year.
• Example: (Number encircled are SP for Calibration Year)
Year: 1988 1989 1997
Mean SWE(cm) 3.43 9.25 13.55
Shift-Para(1) -5.00 0.00 + 4.00
Shift-Para(2) 0.00 +5.00 + 9.00
Figure 5. Distinct patterns of inter-annual SWE retrieved from exist- ing algorithms (Eqs. 1 & 2) when plotted against one of the proposed algorithm (Eq.3). Marked improvement with Shift Parameter (SP).
DISCUSSION OF RESULT (Contd.)
Reason behind Shift-Parameter
• Snowfall, temperature gradient & snow metamorphism process vary from year to year
• Scatter-induced darkening is not a function of Scattering albedo alone. It is also a function of Snow-Depth (England, 1975).
• Also Retrieval algorithms of statistical nature are biased towards the mean.
* Scattering of TB by snow grains within the dielectric layer gives rise to Scattering albedo
Figure 6. Scatter-induced darkening (TBo) versus scattering albedo (o) for various thickness (D) of dry fresh snowpack at 273 K, a case of free space microwave wave-length (o) of 10 cm (adapted from England, 1975).
Reason behind Shift-Parameter (contd.)
CONCLUSIONS
• Reasonably accurate SWE retrieved from SSM/I data from different satellites using Proposed algorithms and calibration techniques like Projection Pursuit Regression (PPR) & multi-variate regression.
• Introduce a Shift-parameter (SP) to retrieval algorithms. Magnitude of SP depends on the overall SWE difference between calibration & validation years.
• Introduce new criteria for selecting dry snow cases that are affected by depth-hoar, and/or large water bodies.
FOR FURTHER DETAILS ON THIS POSTER PRESENTATION
Singh, P. R., and Gan, T. Y. (2000), Retrieval of snow water equivalent using passive microwave brightness temperature data. Remote Sensing of Environment. 74(2):275-286.
Thank You