Explicit Forecasts of Winter Precipitation using an improved bulk microphysics scheme

Explicit Forecasts of Winter Precipitationusing an improved bulk microphysics scheme

Collaborators: Bill Hall, Matt Garvert, Brian Colle, Axel Seifert, Istvan Geresdi,…

WMO Cloud Modeling WorkshopHamburg, Germany 12-16 July 2004

Funded by FAA to support aircraft icing research

Mount Washington, Oregon

Gregory Thompson and Roy Rasmussen

WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany

Outline

• 2D simulations

• idealized flow simulations using 2-d version of MM5 (PSU/NCAR)

• 18 sensitivity experiments plus 4 different cloud depths

• bulk scheme tuned to Geresdi’s bin/explicit model

• key results from sensitivity experiments

• 3D case studies

• one of numerous cases from variety of field projects

• different precip formation mechanisms

• different geographic regions (continental/maritime)

• Future work

• verification data

• other bulk species (aggregates) or 2-moments?

• WRF model simulations


Idealized 2-D simulations

original R-Gsounding

increase moistureto create deeperand colder clouds modified

sounding


MM5 microphysics testing in 2-d


2-D sensitivity experiments

• primary ice initiation• autoconversion (cloud to rain)• CCN spectra (maritime v. continental)• graupel

gamma vs. exp size distribution rimed snow to graupel conversion

• snow size distribution y-intercept constant y-intercept depends on mass y-intercept depends on temp

• rain size distribution• cloud depth increased to lower temps


Bin versus Control and Final bulk schemes


Key results of 2D bulk micro experiments

1. The choice of ice initiation schemes is relatively unimportant

for deep, precipitating snow clouds but more important for

shallow, warm clouds having cloud-top temperatures

greater than –13°C.



2. The assumed snow size

distribution and associated

snow diffusional growth have

major impacts on the mass

of supercooled cloud water

and formation of freezing

drizzle.

snow depositional growth M-P

y-intercept



3. The assumed graupel size distribution and

method of transforming rimed snow into

graupel have major impacts on the mass of

supercooled cloud water and formation of

freezing drizzle.

Snow depositional growth

Snow riming growth

relative ratio1

:31:1



4. A proper simulation of drizzle

using a single-moment

scheme and exponential size

distribution requires an

increase in the rain intercept

parameter, thereby reducing

rain terminal velocities to

values more characteristic of

drizzle.

orig. rain

new rain

graupel

snow


2-D Workshop case3


Case Studies

• 1990 Feb 13-14 (WISP)shallow, post-frontal, upslope cloud w/widespread FZDZ.

• 1998 Jan 30 (NASA-SLDRP)shallow stratoCu, primarily CLW w/slight FZDZ.

• 1998 Feb 04-05 (SLDRP)deep and dynamic snowstorm w/classic FZRA.

• 2001 Feb 01-02 (IMPROVE-1)deep PacNW frontal system, abundant precip.

• 2001 Nov 28 (IMPROVE-2)deep PacNW frontal system plus orographics.

• 2001 Dec 13-14 (IMPROVE-2)deep PacNW frontal system plus orographics.

http://www.rap.ucar.edu/~gthompsn/workshop2004/







30 Jan 1998 – NASA Twin Otter flight

Canada

Lake Erie

PennsylvaniaOhio


30 Jan 1998 – Cleveland, OH sounding


30 Jan 1998 – cloud-top temperature


30 Jan 1998 – liquid water content

>95% of water mass in drops less than 30 microns


Future:

Expand analysis/verification:• how to verify snow/graupel mixing ratios• surface precipitation• composite pattern, lots of cases• include AIRS2 and other cases?


Future:


Comparisons to other models:• bin/explicit model of Geresdi• THIS WORKSHOP


Future:


Put into WRF model:• std-version + 2-moment• Hall/Seifert modifications



Future:


Put into WRF model:• std-version + 2-moment• Hall/Seifert modifications

Other micro species:• aggregate category?• drizzle category?• hail category?



Microphysics: more detailed reports

• Thompson, Rasmussen, & Manning, 2004: Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. Part I: Description and sensitivity analysis. Mon. Wea. Rev., 132, 519-542.

• Thompson, Bernstein, & Rasmussen, 2004: Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. Part II: Case studies. Mon. Wea. Rev., in preparation.

• JAS - IMPROVE special issue (late 2004).

Explicit Forecasts of Winter Precipitation using an improved bulk microphysics scheme

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