Hongyan Zhu

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UM Convection Scheme

• Hongyan Zhu

BMRC
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Bulk cloud model ----
Updraft
Mass
Heat
Moisture
Liquid water content
E -- Mixing entrainment N -- Mixing detrainment
(turbulent mixing at the edge of the cloud) D
Forced detrainment . Q -- conversion of water
vapour to liquid water and ice. PREP -- liquid
water and ice precipitated.
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Cloud Base Massflux
Mass flux of the plume in the initial convective
layer (Mi) is simply related to the stability of
the lowest convecting layer by the empirical
formulation
Where c is 3.33 e-4 b 0.2 K
The initial mass flux is proportion to the excess
buoyancy of the parcel starting from layer k in
layer k1. This closure is still available as
option at 3c in UMUI, operationally, CAPE closure
is now used.
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CAPE closure
In CAPE closure scheme , the cloud base mass flux
is calculated based on the reduction to zero of
convectively available potential energy (CAPE)
over a given timescale. Based on Fritsch and
Ahappell (1980). Assuming steady state clouds
.i.e. constant parcel properties.
Q1,Q2 indicate rates of change of theta and q
calculated by the model, scaled by the original
cloud base mass flux calculated using the
original closure.
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Tiedtke
where Mn-1 are the mass fluxes from a previous
first guess updraft/downdraft computation
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In the operational model, the CAPE timescale can
be modified in the case of high relative humidity
to reflect the fact that humid environments are
more unstable to convection this modification is
found to reduce the incidence of grid point
storms.
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Entrainment / Detrainment
Simpson and Wiggert (1969) and Simpson (1971)
suggested a fraction entrainment coefficient that
depends upon cloud radius R
( height coordinate cloud model R is about 500m
for shallow cloud and 2000m for deep cloud) (
sigma- coordinate cloud model )
(except at the lowest layer, where Ae is 1.)
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Mixing Detrainment
Forced Detrainment
If on ascending from layer k to k1 the excess
buoyancy of the parcel is less than a minimum
value (b) i.e.,

• Cloud model eqs.

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MOTIVATION for adaptivity
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IFS-Tiedtke SCM
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Adaptive detrainment scheme
Forced detrainment caused the buoyancy excess of
the plume to decrease with height. To obtain
less abrupt detraiment, the rate of decrease with
height is set to be a fixed fraction of the
buoyancy excess in the absence of forced
detrainment
The buoyancy excess b is set to be
-- is the buoyancy excess
-- 0.75
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The Mixing detrainment coefficient for deep
convection has been set to be depend on the
relative humidity due to the reason that mixing
detrainment ocurs by evaporation at cloud
boundaries.
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Adaptive entrainment scheme
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Entrainment/Detrainment in ECMWF Convection scheme
Updraught
Turbulent entrainment/detrainment
e and d are generally given in units (1/m) since
(Simpson 1971) defined entrainment in plume with
radius R as e0.2/R for convective clouds R
is of order 1500 m for deep and R100 or 50 m for
shallow
Organized entrainment is linked to moisture
convergence, but only applied in lower part of
the cloud (this part of scheme is questionable)
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Entrainment/Detrainment in ECMWF Convection Scheme
Organized detrainment
Only when negative buoyancy (K decreases with
height), compute mass flux at level z?z with
following relation
with
and
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Plan and cooperation work with UK Met office
1 Relationship between MJO and convection scheme.
Power Spectra Plot of Precipitation
MMF
CAM
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Plan and cooperation work with UK Met office

• 1 A number of things might be relevant to the
  MJO in the convection scheme
• a Sensitivity of convection scheme to the
  humidity
• b Closure
• C Stratiform heating profile.
• 2 TWP-ICE comparisons using SCM.

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Thank You!
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• Options available for CAPE closure
• At 4A, various options for the CAPE timescale
  are available.
• The default option. CAPE timescale is reset
  depending on RH with minimum time scale of 5 m.
• The CAPE timescale is reset depending on RH, but
  the minimum timescale is limited to the
  convection time step.
• Fixed CPAE timescale as set in UMUI panel. (not
  recommended)
• Grid-box scaled CAPE timescale. (designed for
  use with high resolution models 94km and 12km)
• Vertical velocity dependence CAPE timescale. The
  vertical dependent CAPE timescale works by
  reducing the CAPE timescale as the maximum
  vertical velocity increases, hence stabilising
  the model profiles at locations where the
  explicit convection is too strong.