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					  SOME EXPERIENCES ON SATELLITE
 RAINFALL ESTIMATION OVER SOUTH
            AMERICA.


                       Daniel      Vila1,   Inés   Velasco 2




 1   Sistema de Alerta Hidrológico - Instituto Nacional del Agua y de Ambiente
Autopista Ezeiza - Cañuelas km 1.60 - (1402) Ezeiza - Buenos Aires - Argentina
                         TE/FAX: +54 -11 - 4480 - 9174
                          2   Universidad de Buenos Aires
                                                                      Photo:Iguazu Falls
OVERVIEW

• Results of the study of the South American version of
NOAA/NESDIS “Hydro-Estimator” satellite rainfall estimation
technique in selected regions of the Del Plata River basin.
• Brief algorithm description and correction methodologies:
constant rate integration and local bias correction.
• Verification methods.
• Case studies: Salado River Basin (Pcia de Buenos Aires,
Argentina) and Uruguay River subcatchment (Argentina, Brazil and
Uruguay.
• Conclusions.
• Some results and research activities in progress.
      ALGORITHM DESCRIPTION

• This is a fully automated method using an empirical power-law function that
generates rainfall rates (mm/h) based on GOES-8 channel 4 brightness
temperature
• Moisture correction factor (PWRH) defined as the product of precipitable water
(PW) (integrated over the layer from surface to 500 hPa) times the relative
humidity (RH) (mean value between surface and 500 hPa., in percentage) is
applied to decrease rainfall rates in dry environments and increases them in the
moist ones.
• New screening method: This technique assumes that raining pixels are colder
than the mean of the surrounding pixels.
• Standardized temperature is defined as:

                                      T - Tave
                               Ts =
                                         s
                     ALGORITHM DESCRIPTION

       • Tơ < -1.5                                               • Tơ = 0
                                                                 Stratiform precipitation: whose
             Convective precipitation:
                                                                 maximum value cannot exceed 12mmh-1
             defined essentially by the
                                                                 and must be less than the fifth part of the
             empirical power-law function
                                                                 convective rainfall for a given pixel
             corrected by PWRH.



       • –1.5 < Tơ < 0

Entirely
Convective




                                                                 • Tơ > 0  pp = 0



Entirely
Stratiform
                                                           0.0
              -1.5




                         -1.0




                                Standarized Tem perature
   CORRECTION METHODOLOGIES




GOES 8 - Ch4 - Image availability for southern hemisphere sector from 20 May -
12 Z to 22 June 12 Z (open circles). The time difference (in hours) between
consecutive images are plotted in blue (left axis).
CORRECTION METHODOLOGIES
· CONSTANT RATE INTEGRATION




 • Rain rate remains constant between images …
· CONSTANT RATE INTEGRATION




 • but something better may be made …
 · LOCAL BIAS CORRECTION

• This algorithm takes into account the difference between rain
gauges and the HE estimation for a given rain gauge network

                      Satellite Estimation




                                     Nine pixel compar ison kernel
                                                                     Schematic procedure of the
                                                                     best adjusted value (MVE).
                                                                     Rainfall data is compared
                                                                     with a nine pixels kernel
                                                                     centered in the rain gauge
                                                                     location


    Rain Data Gauge
     (ground truth)
    · LOCAL BIAS CORRECTION


                           BASIN
                           LIMITS



ARGENTINA


            BRAZIL


     URUGUAY

                 ATLANTIC OCEAN




                     5    50      100   150   200   250




            24-hour estimated rainfall: 21 – Aug -2002
                           · CASE STUDY : SALADO RIVER
                                                         • LOCAL BIAS CORRECTION
           -30                                                                                   -34


                                                                                                          Buenos Aires Province,
                                                                                                          Argentina                              R ío
                                                                                                                                                        de
                                                                                                                                                             la
           -32                                                                                   -35                                                            Pla
                                                                                                                                                                   t   a


                                                                  URUGUAY
           -34                                                                                   -36
                               ARGENTINA
LATITUDE




                                                        Ri



                                                                                       LATITUD
                                                           o   de
                                                                  la
                                                                     P   lat
                                                                            a

           -36                                                                                   -37




                                                                                                                                                                                    o
                                                                          tico                                                                                                  tic
                                                                                                                                                                              n
           -38
                                                                     tlan                        -38
                                                                                                                                                                        tla
                                                                   A                                                                                                   A
                                                             o                                                                                                     o
                                                        é an                                                                                                éa
                                                                                                                                                                 n
                                                      Oc                                                                                                  c
                                                                                                                                                         O
           -40                                                                                   -39
              -65    -63      -61               -59               -57            -55                -61          -60      -59              -58                   -57                    -56
                                    LONGITUDE                                                                                   LONGITUD

           The 10º x 10º box used to evaluate the technique. Dashed area belongs to the Salado River catchment. Solid
           triangles show the location of rain gauges used for the local bias correction. Right: Geographical distribution
           of rain gauges used to validate the technique.
 · CASE STUDY : SALADO RIVER




Observed vs. estimated values for the 23-24 September 2001 event.
Straight line represents the ideal estimation
  · CASE STUDY : SALADO RIVER

          VALIDATION STATISTICAL PARAMETERS
                                 Table I
        THRESH     NUM    BIAS     CORR    RMSE    POD    FAR    SKILL
           0.0      216   3.98     0.72    15.76   0.98   0.01   0.61
           7.0      186   4.63     0.67    16.93   0.87   0.12   0.67
          26.0      104   7.97     0.46    21.21   0.78   0.63   0.72
          52.0      34    5.65     0.10    24.28   0.65   0.70   0.79
         WTAVG     1276   5.82     0.49    19.85   0.81   0.39   0.70




• Overestimation are present in all intervals.
• Weighted averaged bias of 5.8 mm represents a positive difference of
around 27% between estimated and observed values.
• While for the first rows POD and FAR appear close to ideal, for the higher
intervals (26 and 52 mm) high values of FAR and lower of POD are present
                                      · CASE STUDY : URUGUAY RIVER
                                                                                         • CONSTANT RAIN RATE
          -28



                                          Sub-Catchment limit




                  ARGENTINA
          -29
                                                                  r
                                                                  ve
                                                               Ri
                                                              y
                                                         ua
                                                         ug




                                                                              BRAZIL
                                                       Ur
LATITUD




          -30                                            Qu
                                                              ar
                                                                eim
                                                                      Ri
                                                                        ve
                                                                          r




          -31


                  Salto Grande Dam




                                      URUGUAY
                                                                                       Country borders
          -32
            -59                 -58              -57                            -56                      -55
                                             LONGITUDE




                Geographical position of rain gauges                                                           Satellite rainfall estimation for Salto
                used for evaluation purposes. Dashed                                                           Grande Dam region - 31 May/ 1 June
                area belongs to the Salto Grande Dam                                                           2001
                Immediate catchment
 · CASE STUDY : SALADO RIVER




Observed vs. estimated values for the 31 May –1 June, 2001 event.
Straight line represents the ideal estimation
  · CASE STUDY : URUGUAY RIVER

          VALIDATION STATISTICAL PARAMETERS
                                   TABLE II
        THRESH     NUM    BIAS       CORR     RMSE    POD    FAR    SKILL
           0.0      321   -9.41       0.93    24.10   1.00   0.03   0.84
           7.0      313   -9.49       0.92    24.40   0.92   0.06   0.86
          26.0      210   -14.38      0.87    29.64   0.89   0.06   0.86
          52.0      145   -17.25      0.76    34.47   0.93   0.08   0.88
         WTAVG     2947   -13.91      0.85    29.82   0.93   0.07   0.86




• Underestimation are present in all intervals.
• Weighted bias represents only 15% of underestimation and the RMSE is
around 30%.
• The probability of detection (POD) and False alarm ratio (FAR) exhibit
very good values near 1 and 0 respectively.
· CONCLUSIONS

• The main purpose of this work is to present the recent
improvements of the Auto-Estimator Algorithm and the
application of this technique in two flash flood events in Del Plata
basin in South America.
• The main difference between the South American model and
the one for North America is the image availability. Gaps up to
three hours in South America imagery may be a very important
factor in the accuracy of the estimations.
• The errors involved in these kind of techniques were evaluated
in the cases study presented.
• Future efforts should include a detailed validation and
statistical analysis of a reasonable number of cases
              · OPERATIVE RESEARCH


-27

                                           gu ay                                      Areal rainfall estimation
                                    U ru
                              Rio
-28
                                                                                      • 15-Feb / 15 Mar2002 – 24
                                                                                      hours rainfall estimation and mean
-29                                                                                   river level at Paso Mariano Pinto.

-30
                                                                                      • Local bias correction applied


-31
                                               Rio Ibicuy Basin
                                               Aproximated area: 46.500 km2


      -58   -57   -56   -55            -54         -53     -52    -51     -50   -49
· OPERATIVE RESEARCH


          70                                                                                300

               Paso Mariano Pinto (Brazil)
          60
                                                 10 days                                    250



          50
                                                                                            200


          40




                                                                                                  height (cm)
pp (mm)




                                                                                            150

          30


                                                                                            100
          20



                                                                                            50
          10



          0                                                                                 0
          20/02/2002   25/02/2002   02/03/2002       07/03/2002   12/03/2002   17/03/2002
                                                    date
· OPERATIVE RESEARCH

70                                                                                                         40

      IBICUY RIVER BASIN: ESTIMATED
      & CORRECTED RAINFALL
                                                                                                           35
60



                                                                                                           30
50


                                                                                                           25

40

                                                                                                           20

30
      Mean Estimated rainfall: 9,6 mm
                                                                                                           15
      Mean Corrected rainfall: 10,4 mm
20
      Number of images (ave): 29 /48 (ideal)
                                                                                                           10



10
                                                                                                           5



0                                                                                                          0
20/02/2002               25/02/2002            02/03/2002               07/03/2002            12/03/2002

                                  Corrected                 Estimated                Nr of Images

				
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