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Wes Junker e-mail: norman.junker@noaa

SEMI-INTELLEGENT USE OF THE NCEP MODELS (FALL 1999). Wes Junker e-mail: norman.junker@noaa.gov. Presented at COMET for Hydromet 00-1 Friday, 15 October 1999. Why models have forecast problems.

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Wes Junker e-mail: norman.junker@noaa

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  1. SEMI-INTELLEGENT USE OF THE NCEP MODELS (FALL 1999) Wes Junker e-mail: norman.junker@noaa.gov Presented at COMET for Hydromet 00-1 Friday, 15 October 1999

  2. Why models have forecast problems • Initialization and quality control smooth data fields, but some of the lost detail may be important. • May have poor first guess • Lack of data over the oceans and Mexico. • Atmospheric processes are non-linear; small changes in initial conditions can lead to large forecast variations (this is the basis for ensemble forecasting). • Model physics are approximations (radiation, cloud physics, convection, boundary layer, etc.) • for lower resolution models (the current operational models), convection is parameterized • for higher resolution models (models with a resolution below 5 km) the micro-physical processes are parameterized

  3. INTELLIGENT USE OF THE MODEL REQUIRES THAT THE FORECASTER • COMPARE THE INITIAL 00HR FORECAST WITH DATA • MAY BECOME HARDER TO DO AS NEW DATA STREAMS ARE USED • BE FAMILIAR WITH CHARACTERTIC MODEL ERRORS AND BIASES. • THESE MAY VARY BY SEASON AND REGIME • HAVE A ROUGH UNDERSTANDING OF HOW APPROXIMATIONS OF THE PHYSICS MAY NEGATIVELY IMPACT A FORECAST.

  4. Understanding how the physics may impact a forecast is tough because the atmosphere is complicated and acts in a non-linear fashion. For example, whenever the parameterization for convection kicks in, it • Changes the vertical stability • redistributes and generates heat • redistributes and removes moisture • redistributes momentum • makes clouds Adapted from notes of Bernard Meisner

  5. Eta Model Physics • Eta model calculates grid-scale precipitation using a simplified explicit cloud water scheme • includes super-cooled water, simplified snow processes and the advection of cloud water and cloud ice • but does not include horizontal advection of snow and rain. • In fast flow snow can advect 50 to 100 km downwind of its source region (Rauber, 1992))

  6. EXPLICIT CLOUD PREDICTION SCHEME (large scale) • Cloud condensation is allowed to occur when the RH reaches a critical value • Cloud evaporation is allowed to take place only when the RH falls below the critical value • 70% over land, 80% over water • the difference in the critical value between land and water can produce discontinuities along the coast • this may be one of the reasons the Eta over predicts cold season precipitation along the Gulf and Atlantic Coasts.

  7. The BMJ Convective Scheme • 1st looks for deep convection • step 1 is to look for most unstable layer within the lowest 130 mb • Next calculates LCL to get cloud base • then lifts parcel to Equilibrium Level to get cloud top • then looks to see if the cloud layer is at least 290 mb in depth • If the cloud is not 290 mb then it searches for shallow convection • does not need low level convergence to develop convection

  8. The BMJ scheme • Was developed for tropical systems • does not handle elevated convection well • the convection may not extend through a deep enough layer • does not develop realistic downdrafts/outflow boundaries • therefore, during summer it sometimes predicts the convective development too far north • the saturation pressure deficits that allow condensation in the scheme are different over land and water

  9. THE ETA OFTEN FORECASTSTOO MUCH RAINFALL NEAR THE GULF AND SOUTHEAST COASTS BECAUSE OF THE PROBLEMS WITH THE WAY THE ETA HANDLES THE LAND-SEA INTERFACE 24 HR PRECIPITATION ANALYSIS V. T. 12Z 1 APR 12-36 HR PRECIPITATION FORECAST V. T. 12Z 1 APR

  10. A forecaster needs to know how the model terrain compares to the actual terrain

  11. THE MODEL’S TERRAIN IS AVERAGED OVER THE GRID BOX SO THE SLOPE OF THE TERRAIN IS USUALLY NOT STEEP ENOUGH THIS CAUSES THE VERTICAL MOTION FIELD TO BE SHIFTED AWAY FROM THE MOUNTAINS

  12. THINGS TO REMEMBER ABOUT MODEL QPFS IN COMPLEX TERRAIN DURING WINTER BECAUSE OF THE SIMPLIFIED MICROPYSICS AND INADEQUATE RESOLUTION OF MOUNTAINS. MODELS USUALLY: 1) PREDICT PRECIPITATION TOO FAR WEST AWAY FROM MOUNTAIN PEAKS 2) DO NOT ALLOW ENOUGH PRECIPITATION ON THE IMMEDIATE DOWNWIND SIDE OF MOUNTAIN RANGES THIS IS ESPECIALLY TRUE IN WINTER

  13. DESPITE ITS RECENT PROBLEMS, THE ETA IS STILL USUALLY BETTER THAN THE AVN OR NGM FORECASTING PRECIPITATION OVER COMPLEX TERRAIN DURING WINTER IN A ZONAL PATTERN. 12-36 H ETA V.T. 12Z 3 JAN 97 ANALYSIS V.T. 12Z 3 JAN 97 12-36 H NGM V.T. 12Z 3 JAN 97 Note that the Eta max in California is a little too far west. It also often under predicts precipitation over the Siskiyou Mountains of northern California.

  14. FOR ANY MODEL, ALWAYS BEWARE OF THE 1ST GUESS 00 HR ETA SURFACE AND 1000-500 MB THICKNESS V.T. 00Z 3 SEP 1998 00 HR ETA 500 MB HEIGHT AND VORTICITY V.T. 00Z 3 SEP 1998 SURFACE ANALYSIS V.T. 00Z 3 SEP 1998 TROPICAL STORM EARL WAS LOCATED JUST SOUTHWEST OF THE FLORIDA PENINSULA. THE 1ST GUESS WILL SOMETIMES OVERRIDE DATA WHEN INTENSE SMALL SCALE FEATURES ARE PRESENT.

  15. WHAT HAPPENED? THE MRF 1ST GUESS FIELD TRIED TO DRAW TO THE DATA BUT THE FIRST GUESS FIELD OVERWHELMED IT. IF NCEP HAS A COMPUTER FAILURE, THE ETA DATA ASSIMILATION MAY BE CANCELLED AND THE ETA MAY RUN ON AN EARLIER GLOBAL MODEL 1ST GUESS

  16. A POOR INITIAL ANALYSIS CAN PRODUCE HUGE FORECAST ERRORS ETA 48 HR V.T. 00Z SEP 5 ETA 48 HR V.T. 00Z SEP 5 EARL EARL EARL EARL ETA 00 HR V.T. 00Z SEP 5 ETA 00 HR V.T. 00Z SEP 5

  17. WHEN THE MODEL FIRST GUESS THINKS THE SOIL MOISTURE IS HIGH, THEN, THE MODEL FORECASTS SURFACE DEWPOINTS TOO HIGH AND SURFACE TEMPS TOO LOW. FORECAST CAPES WILL BE TOO HIGH 32 SURFACE TEMPERATURE 28 24 OBSERVED 20 ETA FORECAST 24 DEWPOINT TEMPERATURE 20 16 12 1024 1020 SURFACE PRESSURE 1016 1012 THE MODEL UNDERPREDICTS THE BOUNDARY LAYER WINDS. HOWEVER, MODEL FORECASTS OF 850 MB WINDS ARE OFTEN TOO STRONG 18/00 18/06 18/12 18/18 19/00 19/06 19/12 19/18 20/00

  18. WHEN THE FIRST GUESS THINKS THE SOIL MOISTURE IS LOW IN SUMMER IN THE PLAINS, THE SURFACE DEWPOINT WILL BE LOW AND THE TEMPERATURE WILL BE TOO HIGH. FORECAST CAPES WILL BE TOO LOW OKLAHOMA CITY 36 SURFACE TEMPERATURE 32 OBSERVED 28 24 ETA FORECAST 20 20 DEWPOINT 16 TEMPERATURE 12 1016 SURFACE PRESSURE 1008 ETA SURFACE WINDS WERE TOO WESTERLY, WAS THERE TOO MUCH DOWN-SLOPE? 19/00 19/06 19/12 19/18 20/00 20/06 20/12 20/180 21/00 MAY 1998

  19. Forecast -Vs- Observed Best CapeSpring 96 Line x=y Note the large spread. The model stability forecasts are worst when precipitation is forecast Line x=y Forecast precipitation 1 - less than .25” 2 - more than .25”

  20. The performance characteristics of the eta have changed dramatically during the past year. • QPF forecasts during the past winter deteriorated when compared to the AVN or NGM. • ETA surface and 500 mb forecasts were also worse compared to the other models. • April 1999 ETA 500 h and 250 mb forecasts usually verified worse than the AVN. • June-August 1999, The eta model QPF usually verified better than the AVN or NGM.

  21. ETA AND STORM TRACKS DURING 1999 TENDED TO BE TOO FAR SOUTH WITH LOWS AS THEY REFORMED EAST OF ROCKIES. THIS ERROR USUALLY CONTINUED UNTIL THE LOW MOVED EAST OF THE MISSISSIPPI RIVER. BY CONTRAST, THE AVN IS SOMETIMES TOO FAR NORTHANDTENDS TO SOMETIMES TRACK LOWS TOO FAR NORTH AND WEST WITH LOWSALONG THE EAST COAST. ESPECIALLY DURING MAJOR CYCLOGENESIS WHEN A COASTAL TROUGH IS PRESENT

  22. LOWS TO THE LEE OF THE ROCKIES • THE AVN AND NGM USUALLY PREDICT THEM TO FORM TOO FAR NORTH • THE ETA IS SOMETIMES A LITTLE TOO FAR SOUTH • USE THE 300 MB UPPER LEVEL JET. THE SURFACE LOW IS USUALLY FOUND IN THE LEFT EXIT REGION OF THE JET, USUALLY JUST TO THE NORTH

  23. 28 ETA model runs were evaluated during the period from 00Z March 30-12 Z April 13. During the entire period the mean 500h pattern was similar to the one shown below. A RIDGE AND POSITIVE ANOMALY NEAR 160W, BELOW NORMAL HEIGHTS OVER ALASKA AND A TROUGH NEAR OF JUST INLAND FROM THE WEST COAST WITH BELOW NORMAL HEIGHTS EXTENDING EASTWARD INTO THE SOUTHWESTERN U.S. THE ETA SHOWED A CONSISENT CHARACTERISTIC ERROR DURING THE PERIOD. THE NEXT FEW SLIDES WILL DESCRIBE THE ERROR

  24. As the upper trough digs into the west the ETA did not dig the shortwaves strongly enough once the trough reached the ca coast. Note how much lower the heights are across NV and CA. 48 hr ETA valid 00Z 1 April 00 hr eta valid 00Z 1 April

  25. The eta underplays the second shortwave diving into the mean trough and overplays the first one. 48 hr ETA 500 h and vorticity v.t. 12Z 4 April 00 hr ETA 500 h and vorticity v.t. 12Z 4 April THE ETA PREDICTED THE UPPER LOW ASSOCIATED WITH THE FIRST SHORTWAVE TOO FAR SOUTH AND EAST IN THE PLAINS. INSTEAD THE INITIAL SHORTWAVE LIFTED MORE TO THE NORTH BEFORE BEING FORCED EASTWARD. THIS HAPPENED SEVERAL TIMES DURING THE STUDY.

  26. The eta was generally too fast and far southeast with the 500h low over the Plains with 120 meter errors over MO and IA. This can have a very serious impact on frontal speed and on the position of the low level convergence and resulting convection. 546 558 00 hr Eta v.t. 12Z 10 Apr 48 hr Eta v.t. 12Z 10 Apr

  27. The Eta surface low and associated fronts can also be affected. The slower eastward movement of the ridge axis may allowed for the flow along the east to be more northwesterly which allowed the surface boundary to sink farther to the south 00 hr Eta v.t. 12Z 10 Apr 48 hr Eta v.t. 12Z 10 Apr

  28. COMMON ETA ERROR ALONG EAST COAST WHEN A CLOSED UPPER LOW APPROACHES THE COAST THE ETA SOMETIMES HAS PROBLEMS FORECASTING THE LOCATION OF THE SURFACE LOW. NOTE WHERE THE UPPER LOW IS CENTERED AND WHERE THE STRONGEST UPPER-LEVEL DIVERGENCE IS IMPLIED. 48 H ETA 500H V.T. 12Z 23 APR 98 48 H NGM 500H V.T. 12Z 23 APR 98 Based on the 500 h and vorticity pattern, where would you predict the surface low?

  29. NOTE THAT THE ETA SURFACE LOW IS A LITTLE WEST OF ITS 500 MB CENTER. THE NGM HAS A MUCH BETTER FIT TO THE 500 MB PATTERN. THE STRONG EASTERLY COMPONENT TO THE WINDS NORTH OF THE ETA MODEL LOW ALLOWS IT TO WRAP MOISTURE AND PRECIPITATION TOO FAR WEST 48 H ETA SURFACE V.T. 12Z 23 APR 98 48 H NGM SURFACE V.T. 12Z 23 APR 98

  30. THE LOW VERIFIES A LITTLE NORTH AND EAST OF THE NGM. REMEMBER, THE NGM IS TYPICALLY TOO SLOW WITH LOWS ALONG THE COAST. VERIFYING SURFACE V.T. 12Z 23 APR 98 VERIFYING 500H V.T. 12Z 23 APR 98 L ETA SURFACE LOW

  31. When the NGM and AVN sheared 500 troughs approaching the east coast in 1999, the eta often amplified the trough and overdeepened the surface low. An example: 48 HR ETA 500 48 HR ETA SFC 48 HR NGM 500 48 HR NGM SFC

  32. The Eta predicted a major east coast snowstorm. The NGM and AVN predicted light snow at best 36-48 hr ETA precipitation 36-48 hr NGM precipitation

  33. HOW THE MODEL VERIFIED. NO MAJOR SNOWSTORM DEVELOPED. 48 HR ETA 500 VERIFYING 500 MB 48 HR ETA SFC VERIFYING SFC

  34. MORE ON ETA PERFORMANCE • TOO WET IN FLORIDA • SOMETIMES OVERDEVELOPS LOW-LEVEL JET • DURING WINTER HAS BEEN TOO FAST BRINGING SHORTWAVES THROUGH THE ROCKIES INTO THE PLAINS. • HAS BEEN TOO FAR SOUTH WITH CLOSED LOWS COMING EASTWARD INTO THE PLAINS • OVERFORECASTS THE STRENGTH OF ANTICYCLONES • HAS PROBLEMS INITIALIZING TROPICAL SYSTEMS

  35. Models have problems with arctic airmasses.The reasons why are listed below • Terrain is averaged • Initialization process sometimes robs shallow airmass of its coldness • Models have problems handling strong inversions • Models have problems handling cold air damming • The sigma coordinate system, the Eta coordinate system does better • The leading edge of the ETA LI gradient is often the best indicator of the frontal position

  36. 36 HR AVN V.T. 00Z APR 09, 1995 THE NGM AND AVN/MRF HAVE SERIOUS PROBLEMS WITH ARCTIC AIRMASSES. L AVN ANALYSIS V.T. 00Z APR 09, 1995 36 HR NGM V.T. 00Z APR 09, 1995 TEMPERATURES ACROSS KANSAS WERE IN THE LOW TO MID 50s WITH STRONG NORTH WINDS. SOUTH OF THE FRONT TEMPERATURES WERE IN THE UPPER 70s TO LOW 90s. WHEN THE ETA 500 H FORECAST IS COMPARABLE TO THE OTHER MODELS IT WILL DO A BETTER JOB IN HANDLING THE COLD AIR SURGE

  37. VERIFYING PRECIPITATION • BIAS=FORECAST/OBSERVED • EQUITABLE THREAT=(H-E)/(F+O-H-E) • THREAT SCORE=H/(F+O-H) • N=NUMBER OF HITS, F=NUMBER OF GRID POINTS FORECAST, O=GRID POINTS OBSERVED, E=(F*O)/N

  38. MODEL BIAS AND THREAT SCORE • IS DEPENDENT ON RESOLUTION OF MODEL • HOW THE MODEL IS DISPLAYED. THE FAX VERSION OF ETA IS NOT DISPLAYED WITH FULL MODEL RESOLUTION! • HOW THE MODEL IS VERIFIED • WHETHER VERIFIED AT A POINT, OR AVERAGED OVER A GRID BOX

  39. ETA THREAT SCORES WERE LOWER THAN THOSE FROM THE SUBJECTIVE AND AVN GUIDANCE DURING WINTER, AVN LAGS ETA IN SUMMER

  40. Regional ETA verification using model grid (80 km) WARM SEASON 1.00” OR MORE VERIFICATION VERIFIED TO AN 80 KM GRID .64 .15 .97 .18 .98 .15 .93 .17 .65 .14 .59 .19 .35 .09 .47 .08 .83 .12 BIAS TOP NUMBER, EQUITABLE THREAT BOTTOM

  41. From 1998 data Regional ETA verification using model grid (80 km) COLD SEASON 1.00” OR MORE VERIFICATION VERIFIED TO AN 80 KM GRID .69 .17 .94 .18 1.07 .23 1.36 .22 .74 .09 .71 .27 .58 .10 .71 .15 1.04 .19 BIAS ETS AGAIN NOTE HIGH BIAS ALONG EAST COAST AND LOW BIAS OVER WEST

  42. Regional ETA verification using model grid (80 km) .01” OR GREATER AMOUNTS DURING COLD SEASON VERIFIED TO AN 80 KM GRID 1.43 .25 1.05 .35 1.07 .35 .81 .37 1.23 .23 .79 .32 .95 .26 1.11 .34 1.07 .35 HIGHEST THREATS ALONG WEST COAST. HIGH BIAS OVER UPSLOPE AREAS EAST OF ROCKIES AND OVER PLAINS

  43. Regional ETA verification using model grid (80 km) .01” OR GREATER AMOUNTS DURING WARM SEASON VERIFIED TO AN 80 KM GRID 1.11 .28 .96 .39 .92 .37 .81 .34 1.21 .19 1.00 .37 .82 .23 1.01 .32 .99 .38 BIG DIFFERENCES WITH POINT VERIFICATION. USING A POINT VERIFICATION, YOU SEE THE HUGE BIASES OVER THE SOUTH

  44. ETA .50” OR MORE PERFORMANCE DURING WARM SEASON VERIFIED TO AN 80 KM GRID .77 .21 1.10 .23 1.09 .25 1.07 .24 .88 .12 .82 .17 .82 .28 .62 .14 .86 .20 BIAS ETS DURING SUMMER ETA UNDERPREDICTS .50” OR GREATER AMOUNTS IN PLAINS.

  45. ETA PERFORMANCE FOR .50 OR GREATER AMOUNTS APR 96-NOV 97 VERIFIED TO AN 80 KM GRID .89 .23 1.32 .31 1.00 .15 1.10 .23 1.13 .31 .83 .35 .97 .13 1.10 .26 .90 .23 BIAS THREAT ETA OVERPREDICTS .50 OR GREATER ACROSS SOUTH AND ALONG EAST COAST. MESO-ETA HAS SAME BIAS

  46. AVN/MRF APPROXIMATED PHYSICS • THE AVN/MRF USE A MODIFIED ARAKAWA- SHUBERT SCHEME • THIS USES THE CHANGE IN STABILITY TO DETERMINTE WHEN TO RELEASE ENERGY AS CONVECTION. • NO DIRECT MIXING BETWEEN THE CLOUDY AIR AND ENVIRONMENTAL AIR. (except at the cloud top and bottom) • NO CLOUD WATER EXISTS, THEREFORE ALL WATER IS CONVERTED TO RAIN.

  47. A NUMBER OF AVN/MRF PERFORMANCE CHARACTERISTICS HAVE CHANGED IN THE PAST YEAR. • SINCE JUNE THE AVN/MRF AGAIN UNDERPREDICTS HEAVIER PRECIPITATION THRESHOLDS. • THE AVN/MRF NO LONGER “OFTEN” UNDERPREDICTS SURFACE LOWS OVER OCEANS EXCEPT IN THE SUBTROPICS AT LONGER TIME RANGES. • ISOLATED PRECIPITATION “BULLSEYES” ARE STILL SOMETIMES A PROBLEM, ESPECIALLY DURING THE WARM SEASON. • THE PROBLEM IS MORE LIKELY WHEN THERE IS SLOW SYSTEM MOVEMENT • THE AVN CONTINUES TO HAVE A WARM BIAS AT MID LEVELS OVER THE PLAINS.

  48. LATEST AVN/MRF CHANGES • June 15, 1998: INCREASED HORIZONTAL RESOLUTION TO 170 AND LAYERS TO 42 • THIS LED TO A WARM BIAS AND THE DEVELOPMENT OF SPURIOUS PRECIPIATION BULLSEYES/TROPICAL SYSTEMS • July 21, 1998: EMERGENCY MODEL IMPLEMENTATION TO REDUCE ERRORS IN THE JUNE 15TH CHANGE

  49. The MRF has spin-up precipitation bombs and develops tropical systems erroneously, especially at longer time ranges. 24-h MRFX v.t. 12Z 27 May 1998 36-h MRFX v.t. 00Z 28 May 1998 10”+ bullseye 24-36-h MRFX v.t. 00Z 28 May 1998 SFC ANALYSIS v.t. 00Z 28 May 1998

  50. MRF PRECIPITATIONConvective - dashedGridscale - solid green(inches -Vs- time)BEFORE 7/21AFTER 7/21

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