Transcript Get to Know Snow!

Explicit Forecasts of Winter Precipitation
using an improved bulk microphysics scheme
Gregory Thompson and Roy Rasmussen
Collaborators: Bill Hall, Matt Garvert, Brian Colle, Axel Seifert, Istvan Geresdi,…
Mount Washington, Oregon
Funded by FAA to support
aircraft icing research
WMO Cloud Modeling Workshop
Hamburg, Germany 12-16 July 2004
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
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Idealized 2-D simulations
original R-G
sounding
increase moisture
to create deeper
and colder clouds
modified
sounding
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
MM5 microphysics testing in 2-d
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
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
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Bin versus Control and Final bulk schemes
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
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.
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Key results of 2D bulk micro experiments
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
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
M-P
y-intercept
Key results of 2D bulk micro experiments
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
ratio
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Key results of 2D bulk micro experiments
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.
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
2-D Workshop case3
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
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.
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
30 Jan 1998 – NASA Twin Otter flight
Canada
Lake Erie
Ohio
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Pennsylvania
30 Jan 1998 – Cleveland, OH sounding
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
30 Jan 1998 – cloud-top temperature
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
30 Jan 1998 – liquid water content
>95% of water mass in drops less than 30 microns
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Future:
Expand analysis/verification:
• how to verify snow/graupel mixing ratios
• surface precipitation
• composite pattern, lots of cases
• include AIRS2 and other cases?
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Future:
Expand analysis/verification:
• how to verify snow/graupel mixing ratios
• surface precipitation
• composite pattern, lots of cases
• include AIRS2 and other cases?
Comparisons to other models:
• bin/explicit model of Geresdi
• THIS WORKSHOP
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Future:
Expand analysis/verification:
• how to verify snow/graupel mixing ratios
• surface precipitation
• composite pattern, lots of cases
• include AIRS2 and other cases?
Comparisons to other models:
• bin/explicit model of Geresdi
• THIS WORKSHOP
Put into WRF model:
• std-version + 2-moment
• Hall/Seifert modifications
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
Future:
Expand analysis/verification:
• how to verify snow/graupel mixing ratios
• surface precipitation
• composite pattern, lots of cases
• include AIRS2 and other cases?
Comparisons to other models:
• bin/explicit model of Geresdi
• THIS WORKSHOP
Put into WRF model:
• std-version + 2-moment
• Hall/Seifert modifications
Other micro species:
• aggregate category?
• drizzle category?
• hail category?
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany
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).
WMO Cloud Modeling Workshop, 12-16 July 2004, Hamburg, Germany