Explicit Prediction of Supercooled Liquid WaterApplication to Aircraft and Ground Icing Problems

Gregory Thompson, Roy Rasmussen, Trude Eidhammer

WRF User’s Workshop 27 Jun 2012, Boulder, CO

MotivationAircraft

Icing

GroundIcing

Coming soon … aerosols & droplet number• activated fraction depends on:

1) aerosol concentration2) updraft velocity3) temperature4) hygroscopicity (kappa)5) aerosol mean radius

• implementation details:1) nearest neighbor T, k = 0.4, r =

0.022) interpolated Na, w3) grid-scale vertical velocity only4) negative w uses 1 cm s-1

5) added 2 components to WRF scalar array (Nc, Nccn)

• potential improvements:1) vertical velocity variance2) variable kappa and mean size depend

on ocean vs. land3) connect to radiation4) separate sulfates, sea salts, other

aerosol species5) couple with WRF-CHEM

Cloud droplets (size)

Continentalmore drops smaller mean size

Maritimefewer dropslarger mean size

Liquid water content = 0.25 g m–3

(1)

(2)

(3)

(4)

Affects “autoconversion”3 characteristic diameters considered when converting cloud water to rain

Affects accretiondue to changes in MVD

Affects droplet freezinglarger drops more likely to freeze than small drops

Aerosol testingTypical vs. polluted conditions

Typical marine aerosols result in low cloud droplet concentrations (about 10-100 drops per cc)

Polluted aerosols result in high cloud droplet concentrations (about 150-700 drops per cc)

Resulting LWC & MVD

Continentalmore dropsmuch smaller mean sizemore liquid water contentdelayed drizzle/rain onsetalters upper cloud

Maritimefewer dropsmuch largermean sizeless liquid water contentmore drizzle or light rain

CO Headwaters (WRF simulations)

References: Ikeda et al, 2010Rasmussen et al, 2011Liu et al, 2011

Advantages: 8 years same code high-resolution (4km) project leveraging excellent QPF (5-10%) prototype HRRR model

Disadvantages: low air traffic infrequent FZDZ/FZRA “reanalysis” system

Purdue-Lin

WSM5/WSM6/WDM6,GoddardThompson, MorrisonObservations

Data and analysis by Changhai Liu & Kyoko Ikeda

WRF’s LWC v. MVD

Plot by Nick Ledru

CO Headwaters: icing pilot report comparison

Year TRC LGT MOD SEV RIME CLEAR

MIXED

2001 113 793 541 20 994 91 2932002 63 609 361 10 720 67 2052003 109 717 400 25 853 76 2602004 78 741 445 19 907 72 2562005 105 765 457 11 956 83 2492006 81 688 438 11 858 78 2292007 78 709 446 9 856 74 2532008 48 784 466 21 944 86 254Totals 650 6113 3644 136 7445 654 2042FINAL 422 4303 2611 94

7,430 with yes icing (excluding Jun, Jul, Aug & 36km WRF perimeter)26,269 with clear-sky assumed “No” icing, 65 with explicit “No” icing

(20110201) 1222 MCI UA /OV MCI360003 /TM 1222 /FLUNKN /TP MD82 /SK OVCUNKN-TOP030 /IC MOD RIME /RM /TA UNKN

Results: Can the model capture icing?

TRC LGT MOD SEV ALL

237 / 422 2211 / 4303

1491 / 2611

64 / 94 3998 / 7430

55% 51% 57% 68% 54%

Probability of Detection (PoD)

Reference: Wolff and McDonough, 2010

Ice accretion application

where:M = massa1 = collision efficiencyn = velocity (89.4m/s =

200mph)A = cross-section areaD = diameter (3-inch cylinder)

and:K is “Stokes number”Re is “Reynolds number”f is “Langmuir’s parameter”ma is dynamic viscosityrw is density of waterra is density of air

References: Finstad et al, 1988; Makkonen, 2000

Final application of predicted ice load on TV tower

Provided by Bjorn Egil Nygaard

Summary and future plans Microphysics scheme produces physically reasonable

characteristic water contents and droplet size. Initial tests with variable aerosols produce well correlated cloud

droplet concentrations and resulting precip stays within bounds of constant/imposed droplet number tests.

Explicit forecasts of supercooled water capture significant portion of pilot-reported icing.

Continue testing and advancing the “aerosol-aware” scheme. Directly utilize explicit SLW forecasts into future FAA icing end-

user applications (eventual replacement of CIP/FIP). Incorporate Thompson et al (2008) scheme into H-WRF and

NMM-B

Thank you

•This research is in response to requirements and funding of the Federal Aviation Administration. The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.

especially Dave Gill, Jimy Dudhia and the entire WRF model development community.