Vol. 3, No.4, November 1918

ON THE USE OF LFM PREDICTORS IN PE-BASED PoPA EQUATIONS·

Robert J. Bermowitz and Edward A. Zurndorfer

NOAA, National Weather Service, Techniques Development Laboratory Washington, D.C. 20910

*Previous!y published as TDC office Note 78-4, March 1978.

1. INTRODUCTION

Recently, significant model changes have occurred at the National Meteorological Center. A limited area fine mesh model with a smaller gridlength (LFM-II) (Brown, 1977a) has replaced the original model (LFM-I) (Gerrity, 1977); the former's gridlength is about 2/3 that of the latter's. In addition, a 7-level primitive equation model (7LPE) (Brown, 1977b) has replaced the 6-level primitive equation model (6LPE) (Shuman and Hovermale, 1968). The 7LPE has a gridlength 1/2 that of the 6LPE or the same as that of the LFM-I.

To accommodate these changes, PoP A equations (Bermowitz and Zurndorfer, 1978), developed with the MOS technique (Glahn and Lowry, 1972) and based on forecasts from the LFM­II and 7LPE models, should be derived to replace those based on models that no longer exist. At present, however, it is not possible to do this since an adequate sample of output from these new models has not been archived. Until such time that a large enough sample exists, we must use either LFM-I or 6LPE-based equations (from now on in this paper referred to as LFM­I and 6LPE equations, respectively) with LFM­II forecasts as input for our early guidance product and 6LPE equations with 7LPE fields as input for our final guidance product. An alternative for the final guidance is to use LFM-I equations with 7LPE input where LFM-I equations can be or already have been derived.

Since we must now use predictors from a model that is different for the one upon which the equations were developed, it would be desirable to know the effect this has on the quality of the resulting PaPA forecasts. Dallavalle and Hammons (1976) have shown that there is little deterioration in maximum and minimum tempera­ture forecasts when LFM-I fields are substituted into 6LPE equations. To determine the effect here, we performed a comparative verification of probability and categorical forecasts of precipita­tion amount generated from operational, 6LPE equations with 6LPE forecasts as input against those produced with the same equations but with LFM-I forecasts as input. The results of this verification should indicate the effect of substi-

tuting 7LPE forecasts into 6LPE equations and perhaps the effect of substituting LFM-II fields into either 6LPE or LFM-I equations.

2. VERIFICATION PROCEDURE

To perform the verification, forecasts for the periods 12-36, 36-60, 12-24, 24-36, and 36-48 h "Il after 0000 GMT were compared for the 1976-77 0 cool season (October-March) and the 1977 warm ;:! season (April-August). September 1977 was I

excluded from the warm season verification since gJ the LFM-l was replaced with the LFM-II as of the ::c 1200 GMT run on August 31, 1977. Brier P- ~ scores, expressed as improvement over climatic :::!l forecasts " threat scores, and categorical biases ::1 were computed at 233 cities over the conter- ,~ minous U.S. R'

~ Results for 24-h period forecasts were also broken ::c down by National Weather Service (NWS) region o~ since these forecasts are used operationally by the field forecasters. Verification of the fore- ::c casts for 12-h periods was performed to better ;:; define when deterioration, if any, would begin ::c when LFM-I predictors are used in 6LPE equa­tions.

3. RESULTS AND CONCLUSIONS

Results for the cool season are shown in Tables 1-5. For both warm and cool seasons the improve­ment over climatic forecasts was not computed for> 2.0 inches since a relative frequency for this category was not available. In addition, we did not compute threat scores and biases for> 2.0 inches for 12-h periods because too few cases frequently precluded development of regionalized equations for this category.

Cool season results for the conterminous U.S. (Table 1) indicate no deterioration of the fore­casts dUring the 12-24 and 12-36 h periods when

1Climatic forecasts are seasonal 6-monthly rela­tive frequencies at each of 233 cities of > .25, > .50, and> 1.0 inch previously developed on five years or data for use in developing PoP A equations.

45

National Weather Digest LFM-I predictors were used. However, beyond these projections all the scores indicate some deterioration.

A breakdown by NWS region for the cool season is shown in Tables 2-5. In the Eastern Region the results indicate that there was some improvement in the forecasts during the 12-36 h period, but some deterioration during the 36-60 h period with use of LFM-I predictors. Results in the Southern and particularly the Central Region are similar to those for the conterminous U.S.; that is, no dete­rioration of the forecasts during the 12-36 h period but some during the 36-60 h period. In the Western Region, results indicate deterioration of the forecasts during both 24-h periods when LFM­I predictors were used. The negative improve­ment over climatic forecasts for the category > 1.0 inch in the Western Region could be partially a ttributed to the abnormally dry cool season 1976-77.

gj Results for the warm season are shown in Tables ~ 6-10. For the conterminous U.S. (Table 6), the ~ results indicate no deterioration of the forecasts ~ for all periods except 36-48 h when a slight It: deterioration occurred when LFM-I predictors :J were used. In fact, some improvement is evident N for the periods 12-36, 12-24, and 24-36 h. These ~ results differ somewhat from those of the cool ~ season where some deterioration of all forecasts ~ beyond 24 h made with LFM-I predictors was o evident. A possible explanation is that smaller ~ scale systems, which are more likely to be signifi­_ cant preCipitation producers in the warm than the al cool season, are maintained longer and predicted

I better by the LFM-I than by the 6LPE. :: ~ A regional breakdown shown in Tables 7-10 indi­

cates that in the Eastern, Southern, and Central Regions results are similar to those over the conterminous U.S.; that is, some improvement of the forecasts for the 12-36 h period and no deterioration for the 36-60 h period when LFM-I predictors were used. In fact, some improvement is noted in the Southern and Central Regions for 36-60 h forecasts. In the Western Region, no deterioration is apparent for the 12-36 h fore­casts, but some is apparent for the 36-60 h forecasts. As in the cool season, the negative improvements over climatic forecasts for the lower categories and the relatively large improve­ment for the category > 2.0 inches could be attributed to the abnormal dryness during the 1977 warm season.

In summary, during the cool season, the results indicate that use of 7LPE forecasts in 6LPE equations will not deteriorate the final guidance PoPA forecasts during the 12-36 h period except in the Western Region. However, during the 36-60 h period, indications are that some deteriora­tion in the forecasts will occur in all regions.

46

During the warm season, it appears that the final guidance forecasts will hold up longer than during the cool season when 7LPE forecasts are used in 6LPE equations. Indications are that there will be no deterioration of the forecasts for all projec­tions - some improvement may even occur -except jn Western Region during the 36-60 h period.

It should be pointed out that the operational forecasts in the Western Region could be better than indicated here in both seasons. It is possible that the poorer results in the West are caused by the LFM-I's western boundary. If this is so, use of the 7LPE, which does not have this boundary, should improve the forecasts.

A more general conclusion is that a change in model .does not completely invalidate the MOS equations. Therefore, for the early guidance, it is likely that LFM-II forecasts can be substituted in 6LPE or LFM-I equations without significant deterioration of the foreca~' "

REFERENCES

Bermowitz . R. J .• and E. A. Zurndorfer. 1978: Automated guidance for predicting quantitative precipitation. Mon. Wea. Rev. , (To be published).

Brown. J. A .• 1977a: High resolution LFM (LFM­II). NWS Technical Procedures Bulletin No. 206. National Oceanic and Atmospheric Administra­tion. U.S. Department of Commerce. 6 pp.

,1977b: The 7LPE model. NWS Technical Procedures Bulletin No. 218. National Oceanic and Atmospheric Administration, U.S. Depart­ment of Commerce. 14 pp .

Dallavalle. J. P .. and G. A. Hammons. 1976: Use of LFM data in PE-based max/ min forecast equa­tions. TDL Office Note 76-14. National Weather Service. NOAA, U.S. Department of Commerce. 10 pp.

Gerrity. J. F .• 1977: The LFM model - 1976: A documentation . NOAA Te chnical Memorandum NWS NMC-60. National Oceanic and Atmospheric Administration, U.S. Department of Commerce. 68 pp.

Glahn. H. R .• and D. A. Lowry. 1972: The use of model output statistics (MOS) in objective weather forecasting. J. Appl. Meteor .. 11. 1203-1211.

Shuman. F. G., and J. B. Hovermale. 1968: An operational six-layer primitive equation model. J. Appl. Meteor .• 7, 525-547.

T.1bl e I. Co"p."at\v~ .ol·\fl<",lon o f 24- .1nd 12- h prob,b ll1ty .H,d c .• ,et0rl • .• , r"ce-0'10.1 af ~r~cl~I'HI~n .", ,,,,, 1 .. ~dH<~d wJ." "'. of 61.1' '' prodleto .. I" 6T.P': pq",, ' ions (Yr.) ",,<1 LHI- l l'ee<1ldo, . I" b U' P. " q". ' Llu,," ( l,t·M). r-. , , ~rc . .. >t e "' I'" .,." .. ! ~. 1>0-I"~w'."'" ov~< ell",,(lc r~I·<"C .'~'" (I<>j" .) in ~ . ~ ,'~r ' " eO""'s l"d " I r", .. <~ " to r • .>t

I II "Hlou oyer th. U.S. (". lH' .tod (\cLob~< 1976 - M" on 1~1J.

12-36 i mpr. Threat Scou ~ Ia.

36.91 36 . 13 .1,13 .1,01,

I.U 1. 31,

21.~9 27.40 . H9 .312

I . n \.63 ,~.

n.9l 21 . 91 . 216 . 208

1.9l 1.56

'" . 045 .on

l.a) 2.U

" --~----~---+----~---~----No . ab • • 2988

~6-60

IlII'r. Tl>rut ScOT_ II .. No. 01> ••

il!pr. I1" u t S~o.e 51 .. No. Ob • •

18.36 15.46 .211 .150

1.4' 2.00 2S65

lO .)(l )(l . ll .340 .3J3

1.6S 1. 5. 1565

ll . ll 10 .01 . 204 .19'

I.~' 2.0' IS41

22 . 37 21.13 .241 .250

2.10 1. 78

'"

16.02 14.2'1 .119 . 09 6

1.33 \.8 '

'M

21,.1l 25.02 . 149 . I !><I

2.1 9 2 . 10 m

. 0 16 .01 8 1.69 1. 49

"

---~--------_4----~-----~-----2&-36

'''1' < ' Thr ... t S<o r ft ",U No. 01> • •

I"",r. Tt.rea< Seorft II .. No. abo .

n.ll 20.63 .21 1 .249

I.U 1.61 m,

14.42 12.)9 .210 . 191

1.12 2 . 1" 1502

U.69 U.2 ' .190 .H'

I. 78 1 10

'" 11.11 9.61

.131 .lIS 1.12 2.32

'"

n .)9 25.05 . \00 .0.'11

1.10 1.91 m

18.59 .0~ 9

\.l.9 m

18.32 .039

2 .11

r .• b l o 2. S.~"", as Table "xee"t faT only 24 - h pe riods for 56 stations in the

..!"~",,~.l .• ·r.!., .Rca;.l,.on.

Forec.ast ProJ,'ctlon

(0'

12-36

36-60

Verific" tion Score

I .. pr. Threat Score Bias No. Obs.

l .. pr. Threat Score Bias No. Obi.

_25

PE LFM

43 . 85 45.77 .488 .493

1.35 1 23 1080

21. 17 .310

18.17 . 28)

1.99 1. 52 1050

Category (In"O'oL'.-_____ ~

!.. .50

I'~ LFM

31.69 .359

L 70

'" 13.33

.202 1.68

'"

34.70 .380

1.58

10.18 . 201

1.97

PE: l..FK

17.76 19.04 .244 . 265

1.71 1. 39 m

7 .54 85 .1 33 .070 10 1.35

Z27

Tllbl,,:. 3. SII .... as Table 2 exc~Pt for 57 stat Ion" 1£1 the South"rn R'Sion.

Forec ast P r oJ ec c:lon

(0'

12- 36

36-60

Verification Seore

l tOpr. Threat Score

"" No. Obs.

Impr . Threllt Score Bias No. Oba .

." PE LFM

35 . 59 33.63 .407 . 409

1.38 1. 1';

"''' 19 . 26

.307 1.41

18 . 74 .293

1.49

'"

Catellory (inch)

> .50

PE LFM

26.91 25.82 . 328

1.6 5 . 331

1.34

12.83 .225

1 . 44

'" 12.89

. 217 1.86

m

>1.0

9.59 .108

1. 45

17 . 75 .208

1. 39

'" 9 . 40

.120 2.13

242

.!~-=.;..:...... ~~!",,~~!-~T~b_le ~.-~~o:.:;"..c....~~.~: "!.:~ ~ .. !,.' :"~ i~"H~". _~,:: '_'~ r.'~} Rel:..!!!..".:..,.,...... __

For~e"st Projection

(0 '

12-36

36- 60

Veriflc3tion Score

I .. pr. Th r eat Sco Te Bias No . Obs.

L _.. r.·""II<>'·Y (1I",h)

> . 25' .50 - ~O ._ ---=-

l3.94 .391

1. 38

34 70 . 397

'18

'"

I'E ,-'" 2 ~ 92 2~.4J IS.B3 19 . 07

. 3 16 . 298 . 20S . 205 l. 50 1.45 2.33 l.81

m " {ropr. 16 20 10 • .59 11.75 10 . 38 13 . 15 13 . 48 Threa t Score . 240 .1 96 .190 .204 .140 .1 1,8 Bi3S 1. 33 2.61 1.56 2.17 1. 55 1.30 No. Oba. 489 247 64

Table 5. Sa .... as Table 2 e xcept fo r 51 sCaclona In the \lestern Rllglon.

Cote o ry (inch) Forec.at Verification -" " Projection Score

I __ C(OC''--_ + _ _ _ _ _ -j--''C'''--_ _ '','c~'__f_-''~E . LH1

12-)6

36-60

Impr. ThreAt Score Bias No. Obs.

hpr. Threa t Sc.ore Bias No . Obs.

9.77 .299

1.96

-6.54 .201

1.71

m

9.66 . 260

2.36

~6.97

.116 2.55

30'

18.71 .202

2.48

." 13.93

.167 I. 75

"0

14 .4 9 .164

3. 2~

6 . 09 .105

2.67

?.1.<!.._

PE U' M

34.16 .Og9

1.94

37.16 .079

1.65

"

31

26.99 .047

3.62

28 59 .028

3 . 74

VoL 3, No.4, November 1978

=~ .-- .. -~ ... -. 'o.~e .. t ProJac<lo

'"' 12-36

Vulfleulon

S<orc

1.1'" Throat 5<:or_ Ua. No. Ob i .

---+---·-f_- ---I------+---I-----S. I, ) ~ . 98 1.3& J . ld

.191 . 202 . 114 .IJ6 Z.IO 2.11 2. 10 Z.J~

,,~ Ina

1.12 5.2. .<"161 . tl68

2.1. 2.16

". .021

'-00

-- ~ - ----I-----+- - f_ 1111,, ' . Tt.fU < S<o r _ ~Ia.

No. Ob i .

8 . 16 10. Ol . i91

1.15 1.46 1816

,. ~2 S.61 11.89 11.'8 . III .11 1 . 11'.0 .019

1.'16 I '.21 1.}0

'" '"

4 . 81

".

--------- -.-~----+----+----2~ "36

)6-48

I~pc.

Thn .. Seor. !1U 11<> . 01> • •

J.apc. "nou • • S<ou

110. O~ • •

6.29 1. 56 .1 14 . IM

L W 1101

4 .36 ].I~ .1:.0 .1l9

2.13 2 . 24 ,~,

I •. "IS 1.0. . 106 .119

1.19 1.8, ... 2 . 86 1.]9

.09) .081 2.45 2.<8

".

11 .45 .OH

1.55

'"

T,1hlc 7 . S.l .... 38 T"ble 6 " xC"pt r"T 011 1)" 2~-h periods for 56 StH Ions in th .. r~~ s le Tn Reg ion.

~~~ --~ ~;--~-~~~~-7--------=~----=;-- '--~:;=:=I 1- C"t l! ur (I.,o;h)

Fo r e cast V~rl.Cj c"tlon ~ .25 ~.50 Proj ec tion Sc" re.

(h) PE U"M PE U"M

12-36

36-60

impr. Threat Scor .. Bias No. Obi.

Impr. Threat Score Bin No . Ob , .

17. 53 10.82 .302 .295

1.66 1.45 1041

7. 75 7.97 . 22) .231 97 1.99

1066

10.92 .201

1. 92

6 .44 . 148 n

12.45 . 213

1.58 m

'"

3.09 . 150

"

?. 1.0

PE LFIi

11.68 .103 89

212

1.51 .058

2.20

'"

6. 19 .118

1.72

1. 35 .071

3 . 75

Table 8 . Same as Table 1 """ept for 57 "tations in the South .. rn Reg ion .

Forec3s t Proj~ctien

V~rHlca~ion Scors FE LFM

Categor y (inch) _">0

PE LFM PE LFM (0' -----~----~----~------~----~

12-36

36 - 60

12-36

impr. Threlt Sco r e B11S No. Obs .

lllp r . Thr ..... t SCQr~ ,,~

No. Obi .

Impr. Threat Score 81as No. Obi .

8.86 11 . 14 . 221

2 . 45

4.89 .199

OS

""

"8)

. 229 1.93

" .198 1.88

10 .71 12.92 .21.5 .249

1.63 1. 71 1251

6.66 .167

2.44

3.65 .148

3. 14

6 . ~8

. 175 \. 50

".

547

'"

8.0'1 .181

1.89

5.23 .153

1.87

8.04 . 1113

1. ~1

4.3~ 33 .112 .119

1.99 1.10 m

2.40 .087

2.79

'"

2 . 14 .067

I. 58

2.98 . 098

2.1a

2 . 97 . 076

1. 2)

~--4-----+---~-----+----~ 36-60

I .. pr . Threat Score B1a9 No. Obi.

4 . 88 5 .89 . 193 . 204

1.87 21 1246

2 .84 3.54 1.11 l.16 .In . 134 . 054 . 01,9

1.81 2 48 1.69 2 M

'" 26 1

Tlbl e 10. Same as Table 7 e ""ept fO r" 51 stat ions in the lIeste r n Regl .... n.

Forecaa t Pr ojection

(0'

12- 36

36-60

Veri! i cat 10 ... Scor.

I"pr . Threat Score lIi&l No. Oba .

I llpr. Threat Score BbB No. ObI!.

2:.25 FE LFM

10 .~6 . 204

2.03

3.21 .10 7

1.69

12 . 11 .162

2.56

".

'"

-0.34 _,00 2.52

PE LFH.

4 .62 5 . 11 .083 .110

1. 79 2.18

1.56 .035

1. 68

>0,

- 2 .01 . 034

3.14 no

PE LFM

18 71 .047

L03

18.64 0_0 0.34

"

lao 2~ .024

1.61

17 . 69 0 _0 1. 71

47