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QPF ISSUES IN NWP

QPF ISSUES IN NWP. William A. Gallus, Jr. Dept. of Geological & Atmospheric Science Iowa State University. ETA. MM5. OBS PREC:3/12-4/12. OBS PREC:4/12-5/12. 24HR PRECIPITATION 4/00-5/00. MM5. ETA. OBS:5/12-6/12. OBS:3/12-4/12. OBS:4/12-5/12. 48 HR PRECIPITATION (4/00-6/00).

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QPF ISSUES IN NWP

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  1. QPF ISSUES IN NWP William A. Gallus, Jr. Dept. of Geological & Atmospheric Science Iowa State University

  2. ETA MM5 OBS PREC:3/12-4/12 OBS PREC:4/12-5/12 24HR PRECIPITATION 4/00-5/00

  3. MM5 ETA OBS:5/12-6/12 OBS:3/12-4/12 OBS:4/12-5/12 48 HR PRECIPITATION (4/00-6/00)

  4. What is “TRUTH” for QPF verification?

  5. GOOD NEWS: QPF is improving!! Increased computer resources have allowed better parameterization schemes and model resolution 2-day precipitation forecast today is now as accurate as 1-day forecast in 1974 Each resolution improvement in NCEP Eta model improves skill scores

  6. MRF has some skill compared to persistence, even out to 7-8 days: Roads et al. 1991 (WAF)

  7. This skill is even more apparent for heavy rainfall cases

  8. BAD NEWS: Problems abound Most improvement in QPF scores occurs during cold season - little improvement in warm season Flash flooding kills more people than any other convective-related event QPF problems have several potential sources Skill scores themselves may be misleading or of little “real” value

  9. Roads and Maisel 1991 WAF: MRF has regional biases in precipitation over long periods

  10. Example of human improvements on numerical QPF (Olson et al. 1995, WAF) Manual NGM OBS

  11. Slow improvement in skill for human forecasters, but less skill for heavier amounts (Olson et al. 1995, WAF)

  12. Annual bias has also improved slowly, but interestingly, is better for Day 2 than Day 1 (Olson et al. 1995, WAF)

  13. QPF skill is better is winter than in summer, even when forecasters adjust the NWP guidance

  14. What are sources of QPF error? • Resolution inadequacies • Parameterization errors • Initialization deficiencies • Observational errors in verification

  15. If vertical motion is directly constrained by horizontal resolution….. Shouldn’t forecasts for heavy rain events be greatly improved with finer resolution? Is there a “magic” resolution where model QPF will approach observed peak values

  16. Gallus 1999 found QPF-horizontal resolution dependence is case-dependent and varies with convective parameterization 6/16/96 6/14/98 7/28/97 Mx obs: 225 Mx obs: 330 Mx obs: 250 5/27/97 7/17/96 Mx obs: 102 Mx obs: 300 BMJ -shaded KF - clear

  17. Extreme example of unexpected results and Conv. Param. Impacts: 7/17/96 00UTC surface conditions

  18. 00 UTC 17 JUL 1996 - OMAHA Betts-Miller-JanjicReference T, Td profiles shown

  19. Large MCS drops up to 300 mm of rain, causing record river crests and severe flash flooding in far eastern NE and western IA.

  20. 7/17/96 BMJ simulations with 78,39,22 and 12 km horizontal resolution NOTE: actual reduction in peak QPF amounts as resolution improves MX: 46 MX: 45 MX: 32 MX: 32

  21. 7/17/96 KF simulations: NOTE: very strong QPF sensitivity to horizontal resolution. Precipitation area shifted much farther north than in BMJ runs, or observations MX: 70 MX: 11 MX: 135 MX: 186

  22. Daytime precipitation (12-00 UTC 7/16-17/96) BMJ produces much larger area and amounts

  23. KF BMJ Convective scheme influences cold pool strength, which in turn, affects evolution of events outside initial rain region

  24. Impacts of convective schemes may be felt outside region of precipitation. Here, stronger downdrafts in KF scheme result in greater northward transport of instability into Minnesota - leading to more intense subsequent development. KF BMJ

  25. Another case: Iowa flood of June 1996 Large-scale region looked favorable for excessive rains Heaviest rains (225 mm) fell in small area in warm sector Impacts of horizontal resolution changes strongly depend on convective scheme used

  26. Tropical-like soundings with very deep moisture Td at 850 mb = 18 C Td at 700 mb = 8 C

  27. BMJ simulations: Almost no horizontal resolution-QPF dependence No hint of C IA maximum

  28. 12 UTC 6/16 cold pool affecting Iowa

  29. 12 UTC 6/16 Eta model 00 hr - initialization NOTE: cold pool is missing: winds are southerly, without E component

  30. 21 UTC 6/16 Observed Surface Moisture Convergence Flood-producing storms would form on C IA enhancement

  31. Simulated Moisture Convergence -21 UTC - BMJ run with 12 km resolution Despite poor initial wind field, model does show enhancement in W IA

  32. BMJ simulation: No general clearing into Iowa by 1 pm - Less destabilization than actually occurred

  33. KF simulations: Strong horizontal resolution-QPF dependence Some evidence of C IA enhancement with 22 and 12 km resolution

  34. KF 6 hr forecast: Some clearing into SW Iowa more agreement with obs.

  35. June case shows: • Moist low-mid troposphere allows BMJ scheme to be aggressive • Even high resolution may not improve simulation of small QPF maxima if other simulated parameters are incorrect • Generation of QPF upstream due to resolution changes may affect QPF downstream

  36. Changes within a specific convective parameterization can also have a very pronounced effect on QPF Spencer and Stensrud (1998) show this using MM4 with KF scheme

  37. Spencer & Stensrud variations in KF scheme Permit Precipitation Efficiency to remain at maximum (90%) instead of varying from 10-90% Neglect convective downdrafts Delay convective downdrafts

  38. Max. Prec for 4 tests Maximum QPF in 4 KF MM4 runs From Spencer and Stensrud 1998 - MWR

  39. Microphysicalschemes may be the next challenge - once resolution improves so that convective parameterization is no longer necessary • Colle and Mass examine resolution-orographic precipitation (1999) dependence • Microphysical schemes influence results

  40. OBS PRECIP IN PACIFIC NORTHWEST FLOOD EVENT (1996) from Colle and Mass (1999; MWR) Pronounced orographic effects

  41. 4 km MM5 run does well at crest but underestimates lee precipitation

  42. Horizontal resolution affects precipitation patterns near mountain due to resolution of mountain wave effects. Model QPF performance in lee of mountain fluctuates - low bias is best in coarsest run, but heaviest precipitation just to lee of crest occurs with highest resolution 1.33 4 12 36

  43. Although precipitation forecasts generally improved as resolution was refined from 36 to 4km, little additional improvement occurred with 1.3 km resolution (Colle & Mass)

  44. Model QPF in relation to resolution of topography

  45. Microphysical schemes may have significant influences at high resolution. Colle and Mass (1999; MWR) found that lee-side precipitation was too small in high-res MM5 simulations, partly because snow fallspeeds were too large.

  46. Best results may not occur with most sophisticated microphysical scheme

  47. Microphysical scheme differences affect QPF in different areas

  48. Mesoscale initialization may be poor and affect QPF Stensrud and Fritsch (1994) have shown the impacts of improved cold pool initialization

  49. Typical initialization deficiencies • Low-level jet characteristics • Cloud boundaries • Fronts and drylines • Convective outflows • Surface characteristics

  50. Stensrud and Fritsch 1994 MWR: Initialization of NE KS mesoscale boundary has important impact on QPF MM4 -25KM

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