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Improving Upper-Level Performance in AMPS: Longwave Radiation

Improving Upper-Level Performance in AMPS: Longwave Radiation. Jordan G. Powers, Steven M. Cavallo, and Kevin W. Manning. • Motivation for AMPS Investigation – Examination of WRF simulations of Atlantic basin hurricanes: T biases at upper levels found

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Improving Upper-Level Performance in AMPS: Longwave Radiation

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  1. Improving Upper-Level Performance in AMPS: Longwave Radiation Jordan G. Powers, Steven M. Cavallo, and Kevin W. Manning

  2. • Motivation for AMPS Investigation – Examination of WRF simulations of Atlantic basin hurricanes: T biases at upper levels found – Model top cooling from longwave (LW) processes (RRTM LW scheme) significantly higher than observation • AMPS Testing – Analysis of summer and winter periods to assess extent of problem – Test simulations with RRTM LW scheme modifications performed

  3. Upper-level T Biases: WRF 2009 Atlantic Basin Hurricane Forecasts  (WRF) (v. Time) WRF–GFS Analysis (v. Time) -10K max Upper-level cooling over time Output from fcst hr 6

  4. RRTM LW Scheme Modification— Atlantic Basin Experiments Heating Rates Heating Rate Differences(Modified – Unmodified RRTM) Bias reductions from mods W/o H2O adj: Refined buffer layer and T profile Full mods: H2O adjustment (std profile) in buffer layer (to avoid excessive MT moisture) – 1 week period / Fcsts every 12 hrs / 6-hr fcsts – MLS= Mid-Latitude Summer / TROP= Tropical Note: SLP RMSEs also decrease with modified scheme.

  5. Configuration of AMPS for Investigation / Testing Domains: 45-km / 15-km Test fcsts: 6-hr IC/BCs: GFS Test periods: Summer January 1-7, 2010 Winter July 1-7, 2009 45 km 15 km

  6. RRTM LW Scheme: Original Model Top Treatment • Buffer layer from model top (MT) to top of atmosphere (TOA) – Extra computational level in LW scheme only: No new model η-level • Layer properties – T isothermal: MT value – qv constant: MT value – O3 set to .6O3 MT value

  7. RRTM LW Modifications • Computational layer refined: Multiple levels to TOA added – p= 2.5 mb – Extra levels in scheme, not η-levels (no significant extra run time) • Improved T representation – Temps at new levels related to average T profile (using T at MT) • Excessive moisture prevented: Layer H2O= 5 ppmv • O3 interpolated from table

  8. WRF Water Vapor Issue • Potential for Excessive Moisture at High Levels: Affects LW Flux Calculations – <Jan 2010: No H2O vapor fields above 100 mb in GFS files – WPS assumption (where nec’y): 5%≥ RH ≥1% for 300–50 mb – Problem: Too moist in stratosphere • Standard profile value: 5 ppmv • WRF-Var minimum qv: qv= 1e-6 kg/kg (o(5 ppmv)) (if qv < 1e-6 kg/kg) WRF: Atlantic Basin Tests

  9. AMPS Upper-Level Water Vapor Winter Testing Summer Testing Domain avg qv Top η1/2 Level (12 mb) Sounding maxima WRF-Var min qv 1e-6 SAW= Sub-Arctic Winter SAS= Sub-Arctic Summer

  10. Analysis of AMPS Heating Rates: Original RRTM LW Winter Summer Excessive LW Heating Rates SAS LW SAS SW Net= ∂/∂t LW + ∂/∂t SW AMPS–SAS AMPS–SAW Cooling bias Heating Rate Bias SAW= Sub-Arctic Winter SAS= Sub-Arctic Summer MLW= Mid-Latitude Winter MLS= Mid-Latitude Summer

  11. AMPS Differences from Standard Profiles and Single-Column (SC) Tests Winter Summer SC top value: Artifact of extra level : Extrapolated SC : Extrapolated SC AMPS: Cooling bias SC SAS: Projected cooling bias at MT (excl. artifact) AMPS’s lesser cooling rate may reflect colder Antarctic stratosphere for SC model for SC model Summer SC SAS test: Problem in RRTM LW scheme SAW Temps/SAS Temps: SC model run w/given temp profiles Single column: SC version of RRTM (run from domain-avgd profile of T)

  12. Analysis of AMPS Heating Rates: Modified RRTM LW Winter Summer Heating Rates Max  ~1.8K/d MT T 5 days: ~9 K Modified – Control Control= Original RRTM LW Experiment= Modified RRTM LW

  13. Model Top Improvement: Summer ∂/∂t (LW) Control ∂/∂t (LW) New Mods reduce cooling and eliminate excess qv impacts ∂/∂t (LW) New–Control _6h (Total) New–Control hr6 – hr0 Mods reduce cooling bias ∂/∂t (LW)= Instantaneous heating rates avg’d/fcst hr 6 ∂/∂t (Total)= hr6 – hr0 Level = η1/2

  14. Summary • WRF MT cooling bias seen in Antarctic/AMPS application • – Summer signal • – Moderate compared to non-polar WRF applications • AMPS upper-level H2O vapor • – Localized high qv biases near MT from soundings • – Large vapor amounts can influence LW calculations • RRTM LW Mods: Decreased MT cooling & T errors in AMPS • – Mods reduce LW flux errors and excessive cooling • – Mods avoid LW errors due to areas of excessive qv at MT

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