A modeling study of cloud microphysics: Part I: Effects of Hydrometeor Convergence on Precipitation

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A modeling study of cloud microphysics Part I
Effects of Hydrometeor Convergence on
Precipitation Efficiency.

• C.-H. Sui and Xiaofan Li

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Introduction

• Precipitation is one of the most important
  processes in earth hydrological cycles.
• Precipitation is generated by convective
  processes that are un-resolvable sub-grid scale
  eddies in current atmospheric global models, so
  the various cumulus parameterization schemes have
  been designed.
• The model-generated rainfall often contains large
  uncertainties. Explicit cloud microphysics
  parameterization schemes are employed in
  cloud-resolving mesoscale models, regional and
  even global models.
• In this study, we determine precipitation
  efficiency, especially the effect of hydrometeor
  convergence.

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Model and experiment

• The Goddard Cumulus Ensemble (GCE) model is used
  for this study.
• The 2-D version of the model used by Sui et al.
  (1994, 1998) and further modified by Li et al.
  (1999) is used in this study.
• The cloud microphysics parameterization schemes
  are based on the schemes proposed by Rutledge and
  Hobbes (1983, 1984), Lin et al. (1983), Tao et
  al. (1989), Hsie et al. (1980), and Krueger et
  al. (1995). The corresponding equations are
  described in Li et al. (1999, 2002c).

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• Based on the 6-hourly TOGA COARE observations
  within the Intensive Flux Array (IFA) region.
• The model is integrated from 1992/12/19/0400 LST
  to 1993/01/09/0400 LST (21 days total).
• The horizontal domain is 768 km, the grid mesh of
  1.5 km, the vertical grid resolution ranges from
  about 200 m near the surface to about 1 km about
  100 mb and time step of 12 seconds.

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Time evolution and horizontal distribution of
surface rain rate simulated during 1992
/12/20/0000 LST to 1992/12/21/1200 LST.
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Vertically integrated budgets of water vapor and
clouds
qv the mixing ratio of water vapor,CONVqv
moisture convergence,
surface evaporationSqv SIqv SOqv
the source and sink in the water vapor
budget.SIqv PCNDPDEPPSDEPPGDEPSOq
v PREVP PMLTG PMLTSC qc qr qi
qs qg surface rain rate
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Collection efficiency PRACW/PCND
66 Precipitation efficiency PS/(PCNDPDEP) 67
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Precipitation efficiency

• Cloud Microphysics Precipitation Efficiency
  (CMPE)
• Large-Scale Precipitation Efficiency (LSPE)

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RMS1.3
RMS1.8
96 km
48 km
SIqv or SOqv Local Change of qv (mmh-1)
RMS2.5
24 km
SIqv ---- o SOqv( ) ------
The correlation coefficients are about 0.89 for
all the three cases.PCNDPDEPPSDEPPGDEP
SIqv EsCONVqv
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Conclusions and discussions

• The precipitation efficiency gt 100 occurs in
  light-rain conditions (lt 5 mm hr-1) as a result
  of the additional hydrometeor converging into the
  atmospheric column.
• The dependence of CMPE on the rainfall intensity
  may be explained by the mesoscale-flow patterns
  in the convective and stratiform rain regions of
  the squall line.
• The suggested effect of horizontal hydrometeor
  advection on the precipitation generation and
  distribution is expected to be more important in
  severe weather events like hurricanes/typhoons
  that has spiral rainbands and very strong
  mesoscale circulation.

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Convective ? ? Stratiform hydrometeor
divergence for convective region hydrometeor
convergence for stratiform region.
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