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					                  Hydrologie Applications of Space Technology (Proceedings of the Cocoa Beach
                  Workshop, Florida, August 1985). IAHS Publ. no. 160, 1986.



                   Future applications of GOES satellite VAS
                   data to estimating and forecasting heavy
                   precipitation


                   LEROY E, SPAYD JR
                  Satellite      Applications      Laboratory,
                  National      Environmental   Satellite,     Data, S
                  Information       Service,  NOAA/Dept. of Commerce,
                  5200 Auth      Road, Room 601, Camp        Springs,
                  Maryland      20746, USA


Abstract
The purpose of t h i s paper is to report on some p r e l i m i n a r y a p p l i c a t i o n s of
VAS data f o r e s t i m a t i n g r a i n f a l l and short-range f o r e c a s t i n g of heavy con-
v e r t i ve r a i n f a l l events. As part of the NOAA Flash Flood Program of the
United States meteorologists in the Synoptic Analysis Branch (SAB) of the
National Environmental S a t e l l i t e , Data, and I n f o r m a t i o n Services (NESDIS)
r o u t i n e l y monitor on a h a l f hourly operational basis GOES v i s i b l e and
i n f r a r e d imagery on the I n t e r a c t i v e Flash Flood Analyzer (IFFA) and produce
estimates and short-range forecasts of maximum r a i n f a l l amounts by counties
f o r f l a s h f l o o d t h r e a t e n i n g storms (Clark and Borneman, 1984).          Satellite
P r e c i p i t a t i o n Estimate (SPE) messages are disseminated through the
Automated F i e l d Operational Services (AFOS) computer system to hydrome-
t e o r o l o g i s t s in the National Weather Service (NWS) f i e l d forecast o f f i c e s .
In 1986 products from the GOES VISSR Atmospheric Sounder (VAS) data (Smith,
1983) w i l l be o p e r a t i o n a l l y produced in the World Weather B u i l d i n g (WWB) i n
Washington, DC and a v a i l a b l e to SAB m e t e o r o l o g i s t s .


1985 VAS Data Assessment
                                 AS
"In the spring of 1984 V ~ data was i n i t i a l l y examined via the
Man-Computer I n t e r a c t i v e Data Access System (McIDAS) (Suomi, et a l . , 1983)
                          W
terminal in the W B f o r f u t u r e a p p l i c a t i o n s to NOAA's IFFA (Spayd, 1985).
I n 1985, meteorologists in the Advanced S a t e l l i t e Products Project of
NESDIS processed VAS data on a d a i l y basis beginning March 19 f o r r e a l - t i m e
e v a l u a t i o n at the National Severe Storms Forecast Center (NSSFC) (Anthony
and L e f t w i c h , 1984) and to support the i n i t i a l a c t i v i t i e s of the Program
f o r Regional Observing and Forecast Services (PROFS) i n preparation f o r the
National Stormscale Operational and Research Meteorology (STORM) Program.
S p e c i f i c sets of VAS-derived products are produced at 1100 GMT, 1400 GMT,
                             M
1700 GMT, and 2000 G T d a i l y with an a d d i t i o n a l data set processed at 2300
GMT at the request of PROFS.
        For each time period VAS derived f i e l d s of temperatures, dewpoints,
h e i g h t s , and gradient winds are produced at standard radiosonde r e p o r t i n g
l e v e l s as 80 km g r i d p o i n t data. Atmospheric elements that r e l a t e to layers
such as p r e c i p i t a b l e water, t o t a l t o t a l s and l i f t e d indices are produced
f o r each data set both as g r i d p o i n t data and as a high r e s o l u t i o n (7 km)
"image" (Smith, et a l . , 1985). The VAS 6.7 urn channel water vapor imagery
i s produced on a h a l f - h o u r l y or hourly basis. Regression equations pro-
ducing the K e l l e r p r o b a b i l i t i e s of severe weather ( K e l l e r and Smith, 1983)
are solved f o r each VAS data s e t .
       D a i l y evaluation of the VAS data in March and A p r i l 1985 revealed
d i f f e r e n c e s in the consistency of the VAS data sets from one time period to
                                                                                                  71
72   LeRoy E.Spaya       Jr

another and one day to another. According to Menzel and Schreiner (1985)
the f a i l u r e of the servo mechanism t h a t maintains the VAS f i l t e r wheel tem-
perature on GOES-6 at 40°C is allowing 10°C t o 15°C d a i l y temperature f l u c -
t u a t i o n s of the VAS instrument. These temperature f l u c t u a t i o n s are caused
e x t e r n a l l y by solar heating and i n t e r n a l l y by f r i c t i o n associated w i t h the
f i l t e r wheel movement. This means i t is very d i f f i c u l t , i f not impossible,
t o d i f f e r e n t i a t e between changes in the atmospheric s t a t e and changes in
the spectral response of the VAS instrument.
       With the VAS instrument on GOES-6 producing varying q u a l i t y of data
w i t h time i t would be premature to s t a t e any f i r m conclusions about the
c o n t r i b u t i o n of the VAS data to increased accuracy of s a t e l l i t e r a i n f a l l
estimates. I n s t e a d , t h i s paper w i l l concentrate on the evaluation of the
o p e r a t i o n a l l y produced VAS data sets in a pre-convective environment p r i o r
t o a s i g n i f i c a n t convective outbreak in the Midwest United S t a t e s .


Case Study
On the afternoon of A p r i l 23, 1985 a large convective outbreak occurred
from Texas northeastward through Wisconsin. Although no reports of f l a s h
f l o o d i n g were received, numerous areas received over three inches of rain
(Figure 1 ) . Meteorologists in SAB began e s t i m a t i n g r a i n f a l l from thun-
derstorms developing in Arkansas at 1800 GMT. Continuous r a i n f a l l estima-
                                       M             M
tes were computed from 1800 G T through 0600 G T A p r i l 24 w i t h over 3
inches estimated in Arkansas. Twelve separate SPE messages were sent to
  W
N S f i e l d forecast o f f i c e s .




                   FIG.l   Twenty        four hour rainfall          reports     ending    at   1200
                   GMT April 24,          1985.


Synoptic Overview
An atmospheric composite chart (Figure 2) at 1200 G T on A p r i l 23, 1985        M
shows the wind f i e l d s at 300, 700, and 850 mb, contours of 850 mb
dewpoints, 700 mb moisture areas, and trough axes as determined from the
standard upper a i r c h a r t s . Features of the 500 mb v o r t i c i t y analysis are
also shown: axes of high v o r t i c i t y c a l l e d v o r t i c i t y lobes and axes of low
v o r t i c i t y c a l l e d v o r t i c i t y r i d g e s . Note the v e r t i c a l l y deep trough over
Nebraska, with a large d i f f l u e n t area at 300 mb along the M i s s i s s i p p i
                                      VAS data         for forecasting                        heavy     precipitation   73




                                                                    • + 10 - Al—!-•

                  o o o o - 300MB TROUGH AXIS      .           ;    , - 700MB T-TD56 C
                  • • • - 500MB V0RTIC1TY RIDGE        _ { > - 300HB WIND (MPS)
                           500MI? VORTICITY LOBE       — j > - 700MB HIND (MPS)
                  • • • • - 700MB TROUGH               _   »       _ - 8 5 0 H B WIND ( M P S )

                           850MB DEHPOINTS

                FIG.2   Atmospheric  composite  chart at 1200 GMT April     23,
                1985. The wind barbs are in meters per second S       contours
                of 850 nib dewpoints at 5 degrees Celsius     intervals.



R i v e r . Also note the dry a i r at 700 mb o v e r l a y i n g the high 850 mb dewpoint
area from eastern Texas to M i s s o u r i . At 500 mb a strong v o r t i c i t y lobe
extended southwestward from the c u t o f f low i n Nebraska to the Texas
Panhandle.
     The McIDAS-analyzed surface wind f i e l d and dewpoint contours at 1100
GMT (Figure 3) show a cyclone i n eastern Nebraska which had a central
pressure of 999 mb. The convergence i n the streamline f i e l d depicts f r o n -
t a l zone l o c a t i o n s from Minnesota to Nebraska and southward to central




                 FIG.3    Surface streamlines                            & dewpoint               contours   at 1100
                 GMT April 23, 1985.
74   LeRoy E.Spayd        Jr

Texas. A sharp dewpoint gradient i s observed across c e n t r a l Texas and
Oklahoma with s i g n i f i c a n t dry a i r advection across western and central
Texas and moist a i r advection i n eastern Texas.


VAS Data Sets
VAS dwell sounding data was processed on A p r i l 23 at 1100.GMT, 1400 G T and    M
1700 GMT. The NSSFC requested rapid VISSR scans j u s t a f t e r 1800 G T on  M
A p r i l 23 t o monitor the convective development so VAS data c o l l e c t i o n was
                                    M
i n t e r r u p t e d and the 2000 G T VAS data set was not produced.


VAS Low-level Gradient. Hinds
The VAS gradient wind f i e l d s accurately depicted the magnitude and d i r e c -
t i o n of the axis of maximum winds at 850 mb in southwestern Missouri (see
f i g u r e 2 ) . Throughout the day the VAS data showed an eastward progression
of these maximum winds i n t o eastern Missouri and I l l i n o i s (Figure 4) where
the northern l i n e of convection developed. The l i n e of convection in
eastern Texas to central Arkansas developed along the axis of confluence
from the moist flow from the Gulf of Mexico and the dry flow from western
Texas as seen from the VAS 850 mb gradient winds (Figure 4 ) . The VAS data
allowed the meteorologist to c o n t i n u a l l y monitor the progression and e v o l u -
t i o n of the axis of maximum winds and the confluence zone. Accurately




                  1985.


m o n i t i o r i n g the changes in the low-level wind f i e l d i s important f o r f o r e -
c a s t i n g heavy r a i n f a l l events.


 VAS-Derived K e l l e r P r o b a b i l i t i e s of Severe Weather
 K e l l e r and Smith (1983) developed a s t a t i s t i c a l procedure that estimates
 p r o b a b i l i t i e s of f u t u r e severe local storm occurrence using polar o r b i t i n g
 atmospheric soundings and radiosonde data. This work was subsequently
 adapted to use GOES VAS soundings (Anthony and L e f t w i c h , 1984). Current
 research involves d e r i v i n g a s i m i l a r set of regression equations f o r f o r e -
                                    VAS data    for   forecasting       heavy   precipitation    75

c a s t i n g heavy r a i n f a l l events. These equations would be c a l l e d the Flash
Flood Index and would include some of the K e l l e r p r o b a b i l i t y p r e d i c t o r s .
The ten best p r e d i c t o r s of the K e l l e r p r o b a b i l i t i e s in order of importance
are l i f t e d index, 850 mb dewpoint, a v e r t i c a l wind shear term, p r e c i p i t a b l e
water, 850 mb three hourly temperature advection, 850 mb wind speed, 850 mb
moisture divergence, 300 mb wind speed, 500 mb three hourly temperature
advection and three hourly change in the Showalter s t a b i l i t y index.
According to K e l l e r and Smith (1983) p r o b a b i l i t i e s greater than 20% i n the
K e l l e r index i n d i c a t e severe weather may occur i n the next several hours.
     On A p r i l 23 the NSSFC issued the f i r s t severe thuderstorm watch area at
            M
1817 G T f o r central Arkansas to northeastern Texas and f i v e a d d i t i o n a l
severe weather watch areas were issued by NSSFC w i t h i n the next s i x hours.
There were u n o f f i c i a l l y 95 separate reports of severe weather i n c l u d i n g 23
                                  M                        M
tornadoes from 1810 G T A p r i l 23 t o 0215 G T on A p r i l 24 (Figure 5 ) . Many
of these reports were c o l l o c a t e d and p o s t - a n a l y s i s may reveal t h a t some of
these reports were produced by the same storm.




                  FIG.5     Unofficial severe storm           reports     from 1810 GMT
                  April   23 to 0215 GMT April 24,             1985.



                                                       M
    The K e l l e r p r o b a b i l i t i e s at 1130 G T are shown in Figure 6. Note t h a t
                                                                    M
the axis of the maximum 30% p r o b a b i l i t i e s at 1130 G T corresponds very
accurately to the severe weather reports from southeastern Missouri through
northeastern Texas.


VAS S t a b i l i t y Fields
L i f t e d indices and t o t a l t o t a l indices are r o u t i n e l y produced by the VAS
r e t r i e v a l a l g o r i t h m s . The l i f t e d indices appeared to more accurately
r e f l e c t the s t a b i l i t y differences across the b a r o c l i n i c zone in the
southern plains than the t o t a l t o t a l s index. The p l o t t e d l i f t e d indices at
                            M
1100 and 1700 G T are shown in f i g u r e 7. Due to solar i n s o l a t i o n the
l i f t e d indices decrease throughout the region during the day, however a
                                                         M
sharp gradient is evident at 1700 G T across eastern Texas with values of
-5 appearing from central Arkansas i n t o eastern Texas. Although low l i f t e d
76    LeRoy E.Spayd       Jr




                    FIG.6      VAS-derived Keller         probabilities        of    severe
                    weather    at 1130 GMT April          23, 1985.


indices e x i s t throughout southern Texas, L o u i s i a n a , and offshore i t is
along the gradient of l i f t e d index values t h a t the l i n e of convection in
Arkansas and Texas formed. I t is hoped t h a t VAS data w i l l allow short term
monitoring of s t a b i l i t y changes important in f o r e c a s t i n g heavy r a i n f a l l
producing convection.


VAS P r e c i p i t a b l e Water
Accurately monitoring the changes in low-level moisture is extremely impor-
t a n t in f o r e c a s t i n g heavy r a i n f a l l events and i t is hoped that the s h o r t -
term changes in the VAS p r e c i p i t a b l e water f i e l d s w i l l allow a continual
 " c a l i b r a t i o n " of the moisture c o r r e c t i o n f a c t o r s to r a i n f a l l estimation
techniques. The t o t a l p r e c i p i t a b l e water amounts can be displayed as g r i d -
p o i n t values and the g r i d p o i n t s can be contoured. Contours of the 1100 G T                    M
p r e c i p i t a b l e water values are shown in f i g u r e 8 along w i t h the p l o t t e d RAOB
surface to 500 mb p r e c i p i t a b l e water values. Because the contouring
program contours across the cloudy (sounding void) areas, the p l o t t e d g r i d -
                                                              M
p o i n t values were compared to the 1200 G T RAOB p r e c i p i t a b l e water amounts.
For t h i s time period sixteen comparisons were made and in 75% of these com-
parisons the VAS values were higher than the RAOB values. The average d i f -
                                                                             m
ference between the RAOB and VAS values was 6.3 m w i t h a R S d i f f e r e n c e of       M
7.0 mm. The l a r g e s t differences were i n central Texas at Stephensville and
Del R i o , Texas. The RAOB sounding at S t e p h e n s v i l l e (dotted l i n e ) and the
nearest VAS sounding ( s o l i d l i n e ) are p l o t t e d in f i g u r e 9. Note t h a t the
VAS sounding has much more moisture between 900 and 600 m i l l i b a r s even
though the surface dewpoints are very s i m i l a r . Also note t h a t the VAS tem-
peratures between 850 and 300 mb are higher than the RAOB temperatures.
The actual values of the RAOB and VAS soundings at the mandatory levels are
shown respectively in f i g u r e 10. The t h i r d column of f i g u r e 10 shows the
a l g e b r a i c d i f f e r e n c e s of (VAS - RAOB) f o r temperatures, dewpoints and
h e i g h t s . The VAS/RAÛB sounding differences at S t e p h e n s v i l l e were larger
than the other f i f t e e n VAS/RA0B comparisons f o r t h i s time p e r i o d . There
has been a consistent bias of the VAS sounding r e t r i e v a l s to overestimate
the t o t a l p r e c i p i t a b l e water in areas of high surface dewpoints o v e r l a i n by
a deep layer of dry a i r . This bias has been observed on many separate days
                                                   VAS data        for    forecasting       heavy     precipitation      11




                                                           -2 -3 -s -e -3 -s
                                                                     , ^ - 2 3 - 8 5 11 GHT LI




 13 12      10   9

 12    11    8   9

 13    12    9   91
 12    11    9   8

 11"        11   9

 10    10   V9   8

  9    8     7

  8     8    6   •\   3   2-1   0 ^    -S -H -7     <rj$''~1 " 8 " 8 ~ 8 ~ 8 ~ 8 " 8 ~* "*    ~5    "^ ~ 3
 10     9    8   5                                         _g -g -g ~e -7 - 7 - 7 - 6      -5 -6 -€

                                -2 -3 - i > s -7 -fc\-8 - e - 6 - 6 - 6       ^-23-85   17 GMT LI
                                       -3-6-6         \9    -9 -6 -5 -*• "8

                           FIG.7   VAS-derived plotted lifted                           index    values      at   1100
                           S 1700 GMT April 23, 1985.

and different synoptic conditions. This bias is revealed in figure 10.
Forecasters should be cautious in using the VAS p r e c i p i t a b l e water amounts
for moisture correction factors to rainfall estimation techniques in areas
with deep layer dry a i r and high shallow surface moisture until efforts in
improving the VAS r e t r i e v a l algorithms remove t h i s b i a s .
78   LeRoy E.Spayd       Jr




                     NV CMMl TIME     U;30,

                FIG.8   Contours of VAS-derived        total precipitable       water
                amounts at 1100 GMT and plotted        RAOB observed    surface    to
                500 mb precipitable       water amounts in millimeters      at 1200
                GMT April 23,       1985.




                       6Hr      ID    TOTAL   EOUIP   FMFIX   c33
                                                               vr   L.I.   «NX    PH
                  231200      72260      38     388     27             1     10    lb'
                 1231118       tas       11     ISO      0     33      S     20   2S

                FIG.9      Atmospheric  RAOB sounding at 1200 GMT at
                Stephensville,     Texas and nearest  VAS sounding at 1100 GMT
                April 23, 1985.



Conclusions and Outlook
This paper presented a case study showing the usefullness of GOES VAS data
in detecting, monitoring and tracking atmospheric features responsible for
i n i t i a t i n g , focusing and maintaining a line of heavy rainfall producing
convection. In an operational environment i t would be impossible to review
a l l the VAS data fields for each set of VAS r e t r i e v a l s . This paper has
presented a few of the most useful VAS data sets that may be used by opera-
                                         VAS data    for     forecasting        heavy   precipitation             79

                     RAOB                                       VAS                            VAS - RAOB
P (MB )
970
          "ï(p_r             l®. iM "MPf      T(C)
                                              Ï87?
                                                     TD(C)
                                                     T5TT
                                                                 Z{M)   ~DJi   M_PS~     T                  Z
982        15.0     14.8      399 310    1
850        16.2     -5.8      1461260    3    16.2    6.6        1454    239    11       -.0       12.4      -7
700        4.8      -9.2      3074245   21     6.7    -5.0       3079    235    17       1.9        4.2       5
500       -13.7    -26.7      5720255   27    -9.0   -27.7       5768    233    26       4.7       -1.0      48
400       -25.5    -55.5      7380245   33   -21.4   -42.2       7455                    4.1       13.3      75
300       -40.1               9400250   37   -38.6   -58.3       9504    248    34       1.5                104
250       -49.3              10620245   37   -49.1              10729                     .2                109
200       -60.3              12050240   45   -59.6              12159                     .7                109
150       -58.3              13830250   26   -60.8              13953                   -2.5                123
100       -60.5              16380230   17   -63.7              16457                   -3.2                 77
                           FIG.10   Printout of Stephensville,       Texas RAOB sounding
                           at 1200 GMT, the nearest  VAS sounding        at 1100 GMT April
                           23, 1985 and the VAS-RAOB        differences.



t i o n a l meteorologists responsible f o r e s t i m a t i n g and nowcasting heavy con-
v e c t i v e r a i n f a l l . Developmental work is underway to combine the VAS data
f i e l d s t o produce s t a t i s t i c a l p r o b a b i l i t i e s of heavy r a i n f a l l occurrence
c a l l e d the Flash Flood Index.


Acknowledgements
The author would l i k e to thank Ralph Anderson f o r reviewing t h i s                           manuscript
and John Shadid f o r producing the f i g u r e s .



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