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

– 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 schememodifications performed

Upper-level T Biases: WRF 2009 Atlantic Basin Hurricane Forecasts

(WRF) (v. Time) WRF–GFS Analysis (v. Time)

Upper-level cooling over time

Output from fcst hr 6

-10K max

Note: SLP RMSEs also decrease with modified scheme.

– 1 week period / Fcsts every 12 hrs / 6-hr fcsts – MLS= Mid-Latitude Summer / TROP= Tropical

W/o H2O adj: Refined buffer layer and T profile

Full mods: H2O adjustment (std profile) in buffer layer (to avoid excessive

MT moisture)

Bias reductionsfrom mods

RRTM LW Scheme Modification— Atlantic Basin Experiments

Heating Rates Heating Rate Differences (Modified – Unmodified RRTM)

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

15 km

45 km

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 .6O3 MT value

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

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

AMPS Upper-Level Water Vapor

Summer Testing

Top η1/2

Level(12 mb)

Domainavg qv

Winter Testing

SAW= Sub-Arctic Winter SAS= Sub-Arctic Summer

WRF-Var min qv 1e-6

Sounding maxima

Analysis of AMPS Heating Rates: Original RRTM LW

Winter Summer

Heating Rates

HeatingRateBias

SAW= Sub-Arctic Winter SAS= Sub-Arctic SummerMLW= Mid-Latitude Winter MLS= Mid-Latitude Summer

Net= ∂/∂t LW + ∂/∂t SW

SAS LW

AMPS–SAW AMPS–SAS

SAS SW

Cooling bias

Excessive LW

AMPS Differences from Standard Profiles and Single-Column (SC) Tests

Winter Summer

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)

AMPS: Cooling biasSC SAS: Projected cooling bias at MT(excl. artifact)

for SC model for SC model

AMPS’s lesser cooling rate may reflect colder Antarctic stratosphere

SC top value: Artifact of extra level

: Extrapolated SC: Extrapolated SC

Analysis of AMPS Heating Rates: Modified RRTM LWWinter Summer

HeatingRates

Modified –Control

Control= Original RRTM LW Experiment= Modified RRTM LW

MT T 5 days: ~9 K

Max ~1.8K/d

Model Top Improvement: Summer ∂/∂t (LW) Control

∂/∂t (LW) New–Control

∂/∂t (LW) New

_6h (Total) New–Control

Mods reduce cooling and eliminateexcess qv

impacts

hr6 – hr0

∂/∂t (LW)= Instantaneous heating rates avg’d/fcst hr 6 ∂/∂t (Total)= hr6 – hr0 Level = η1/2

Mods reduce cooling bias

SummarySummary

• 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 AMPSRRTM 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