Table of ContentsAdapted from Material Developed By INTELLIGENT USE OF THE MODEL REQUIRES THAT THE FORECASTER Why models have forecast problems 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 convection occurs it Neither the ETA model or MRF/AVN model handles convection realistically. THE ETA OFTEN FORECAST TOO MUCH RAINFALL NEAR THE GULF AND SOUTHEAST COASTS BECAUSE OF THE PROBLEMS WITH THE WAY THE ETA HANDLES THE LAND-SEA INTERFACE FOR ANY MODEL, ALWAYS BEWARE OF THE 1ST GUESS A POOR INITIAL ANALYSIS CAN PRODUCE HUGE FORECAST ERRORS 32-km terrain DESPITE ITS RECENT PROBLEMS, THE ETA IS STILL USUALLY BETTER THAN THE AVN OR NGM In FORECASTING PRECIPITATION OVER COMPLEX TERRAIN DURING WINTER IN A ZONAL PATTERN. The T170 version of the Avn does a better job depicting in the West than the T126 but the model still misses much of the detail in the West. NOTE THAT NEITHER THE AVN OR ETA PREDICT A MAX IN THE SISKIYOU MOUNTAINS OF NORTHERN CA. A FORECASTER NEEDS TO KNOWHOW THE MODEL TERRAIN COMPARES TO THE ACTUAL TERRAIN. HE OR SHE ALSO NEEDS TO REALIZE THAT THE MODEL DOES NOT ADVECT PRECIPITATION AWAY FROM WHERE IT IS FORMED THE MODELíS TERRAIN IS AVERAGED OVER THE GRID BOX SO THE SLOPE OF THE TERRAIN IS USUALLY NOT STEEP ENOUGH THINGS TO REMEMBER ABOUT MODEL QPFS IN COMPLEX TERRAIN DURING WINTER WITH STRONG VERY MOIST SOUTHWESTERLY FLOW AT 850 AND 700 MB NOTE HOW CLOSELY THE PRECIPITATION CONFORMS TO THE TERRAIN Evapotranspiration is parameterized IN THE PAST, ETA MODEL HAS HAD PROBLEMS PREDICTING THE STABILITY. PROBLEM IS OFTEN TIED TO THE 1ST GUESS WHEN HIGH SOIL MOISTURE IS PRESENT, OR WHEN THE MODEL FIRST GUESS THINKS THE SOIL MOISTURE IS HIGH, WHEN LOW SOIL MOISURE IS PRESENT DURING SUMMER OVER THE HIGH PLAINS, ESPECIALLY WEST TX, THE FORECAST CAPE IS TOO LOW WHEN SOIL MOISTURE IS LOW IN SUMMER IN THE PLAINS, THE SURFACE DEWPOINT IS TOO LOW AND THE TEMPERATURE IS TOO HIGH THE MODEL HANDLING OF VEGATATION IS VERY SIMPLE AND CAN LEAD TO PROBLEMS. IN EARLY SPRING, BEFORE MOST OF THE VEGETATION IS OUT , THE MODEL OFTEN FORECASTS TOO MUCH EVAPOTRANSPIRATION Eta Forecast -Vs- Observed Best Cape Spring 96 MORE ON ETA PERFORMANCE ETA AND STORM TRACKS COMMON ETA ERROR ALONG EAST COAST 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 LOW VERIFIES A LITTLE NORTH AND EAST OF THE NGM. REMEMBER, THE NGM IS TYPICALLY TOO SLOW WITH LOWS ALONG THE COAST. 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: The Eta predicted a major east coast snowstorm. The NGM and AVN predicted light snow at best HOW THE MODEL VERIFIED. NO MAJOR SNOWSTORM DEVELOPED. LOWS TO THE LEE OF THE ROCKIES 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. 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. The eta underplays the second shortwave diving into the mean trough and overplays the first one. 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. 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 Why models have problems with arctic airmasses THE NGM AND AVN/MRF HAVE SERIOUS PROBLEMS WITH ARCTIC AIRMASSES. How about using an ensemble approach to estimate uncertainty WHAT HAPPENED? Ensemble spread of 700 mb of rh in %. Shows uncertainty over NC, VA and MD 3 of 10 ensemble runs predict almost .25î over DCA. Did ensemble approach predict a high probability of a major snowstorm? What happened? VERIFYING PRECIPITATION MODEL BIAS AND THREAT SCORE Study of heavy rainfall events in middle of country. Note low bias for mesohigh events and general low bias for heavier thresholds Accuracy decreases rapidly as threshold increases PPT Slide 12-36 hr Eta 1.00î model verification during Feb 2000 Regional ETA verification using model grid (80 km) Regional ETA verification using model grid (80 km) PPT Slide Regional ETA verification using model grid (80 km) Regional ETA verification using model grid (80 km) When you see a precipitation bulleye. How can you tell when latent heat feedback may be causing problems More reasons to suspect a feedback What happened? A 3 inch max near Shreveport, LA. A test. From the following charts try to decide where you would predict the precipitation max Do you think, the 3 inch area is correct in NM? Why or Why not? Would you modify the axis of heavy rain in TX. IF so how? How did you do? The axis was east of the modelís forecast. Note that the surface low was weaker. Did the possible feedback cause the lower pressure? Doesnít this case look like the one in 1998? HPC IS NOW TRYING VERIFY 500 HEIGHT ERRORS BASED ON REGIME, THIS IS A VERFICATION OF 72 HR FORECASTS ANOTHER PATTERN. NOTE SIMILARITIES IN WHERE THE BIGGEST ERRORS OCCUR IN CONCLUSION PRIOR TO THE CHANGES LAST SUMMER, THE ETA MODEL WAS BEST IN CONCLUSION |
Author: Dr. Ken Crawford Email: kcrawford@ou.edu Home Page: http://rossby.ou.edu/~metr4424 |