Low (web) Quality - BALTEX

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Simulating aerosol effects on the dynamics and microphysics of
precipitation systems with spectral bin and bulk parameterization schemes
L. Ruby Leung 1 , Alexander P. Khain 2 , and Barry Lynn 2
1 Pacific Northwest National Laboratory, Richland, WA, USA; Email: Ruby.Leung@pnl.gov
2 The Hebrew University of Jerusalem, Israel
1. Introduction
An increase in atmospheric aerosol particles serving as cloud
condensation nuclei (CCN) can increase the concentration and
reduce the size of cloud droplets. Although the relationship
between aerosol concentrations and cloud drop size is
relatively well established through observations and modeling,
the effects of aerosols on precipitation are less well understood
because aerosols can influence both the microphysical and
dynamical structure of clouds. Despite the importance of
evaluating how air pollution may potentially influence the
global and regional hydrologic cycle, to date, simulating the
effects of aerosols on precipitation remains a challenge, as
simulation results and observations often diverge in their
quantification of aerosol effects on precipitation. This study
compares two microphysical schemes and their quantification
of aerosol effects on precipitation in different cloud systems
including the squall line and orographic clouds.
2. Results
A new spectral bin microphysical scheme (SBM) was
implemented into the Weather Research and Forecasting
(WRF) (Skamarock et al. 2005) model (referred to as Fast-
SBM), which uses a smaller number of size distribution
functions than the previous (standard) version of SBM
(referred to as Exact-SBM). The Exact-SBM scheme has been
described by Khain et al (2004) and Lynn et al (2007). The
scheme is based on solving the kinetic equation system for size
distributions of seven types of hydrometeors: water drops,
three types of crystals (columnar-, plate- and branch-type),
aggregates (snow), graupel and hail. Each hydrometeor type is
described by a size distribution function defined on the grid of
mass (size) containing 33 mass bins.
The WRF model was applied to a 2D domain to simulate the
structure of a squall line. It was shown that both the Fast SBM
and Exact SBM reproduced the typical structure of an idealized
squall line quite realistically. The schemes simulated similar
dynamical and microphysical structures, and there was
excellent agreement in the simulated precipitation amounts
between the schemes under a very wide range of aerosol
conditions in which initial condensation nuclei concentration
varied from 100 cm -3 to 3000 cm -3 . Moreover, the Fast-SBM
uses about 40% of the computing power of the exact-SBM,
allowing it to be used for “real-time” simulations over limited
domains.
The results of SBM simulations have been compared with a
modified version of the Thompson bulk parameterization
scheme within the same dynamical framework (2D WRF).
The bulk scheme has been extended to simulate the process
of drop nucleation, so that drop concentration is no longer
prescribed a priori, but rather calculated depending on the
prescribed aerosol concentration. This scheme is referred to
as DROP scheme. A large set of sensitivity studies have
been performed, in which the sensitivity of results
(microphysical parameters and precipitation) to aerosol
concentration, droplet nucleation above cloud base, etc. has
been compared with the results from the SBM.
Comparison of the results from the DROP scheme and SBM
scheme shows that the SBM scheme produces more realistic
dynamical and microphysical structure of the squall line.
The addition of the DROP scheme did relatively little to
change the underlying results of the bulk scheme, and
unlike the SBM simulations, that show different
precipitation sensitivities to aerosol concentrations in
relatively clean and polluted environments, the drop
scheme simulates large monotonic decrease in
precipitation with increasing aerosol concentrations.
Both the SBM and DROP schemes are being applied to
simulate orographic clouds to further elucidate the effects
of aerosols on orographic precipitation. Both 2D and 3D
simulations are being performed and compared with
observations collected during the SUPRECIP field
experiment (Rosenfeld et al. 2008) in northern California.
References
Khain A., A. Pokrovsky and M. Pinsky, A. Seifert, and V. Phillips,
2004: Effects of atmospheric aerosols on deep convective
clouds as seen from simulations using a spectral microphysics
mixed-phase cumulus cloud model Part 1: Model description.
J. Atmos. Sci., 61, 2963-2982.
Lynn B., A. Khain, D. Rosenfeld, William L. Woodley, 2007:
Effects of aerosols on precipitation from orographic clouds. J.
Geophys. Res., 112, D10225, doi:10.1029/2006JD007537.
Rosenfeld, D., W.L. Woodley, D. Axisa, E. Freud, J.G. Hudson, A.
Givati, 2008: Aircraft measurements of the impacts of pollution
aerosols on clouds and precipitation over the Sierra Nevada. J.
Geophys. Res., 113, D15203, doi:10.1029/2007JD009544
Skamarock, W.C., Klemp J.B., Dudhia, J., Gill D.O., Barker D.M.,
Wang W., and Powers J.G., 2005: A description of the
Advanced Research WRF Version 2. NCAR Tech Notes-
468+STR.