I. Description of Physical Processes
II. Comparison of Model Implementations
III. Critical Evaluation
One of the more difficult aspects of forecasting is convection. This is especially true with computer model forecasts. Efforts have been made to model convective outbreaks. However, the great number of variables required to simulate convection, and the fact that it often occurs on scales too small to be tracked well by models, mean simulation of this process is virtually impossible. Actions and motions at this scale have proved to be better handled through parameterization. Parameterization is a way to, out of mathematical convenience and necessity, represent the physical effects of a process (in this case convection) in terms of simplified parameters, rather than actually resolving the process itself. Thus, if the parameters satisfy certain conditions, convection is forecasted by the model.
The National Weather Service (NWS) forecast models concern themselves with convection, primarily because of the significant weather it can produce. Models should be able to show the forecaster the following pieces of information through parameterization of convection:
1. a quantitative measure of instability
2. a triggering mechanism for convection
3. the net effect of latent heating on the local atmosphere
4. the amount of
water vapor available in a convective situation
Steps are being made to improve the parameterization of convection in all
models. The result will hopefully be a reliable forecasting tool for
thunderstorm genesis.
The various models each follow somewhat different schemes in
dealing with convection. Here's a look at how the models parameterize
convection:
A. Shallow convection (Nonprecipitating):
1. Medium Range Forecast Model (MRF):
This spectral model is run daily using
observed data at 00Z and extends out to 16 days. These plots are updated
once a day at 10 a.m. (EST) based on the 00Z model run. (0.7 x 0.7 degree resolution)
 Shallow convection occurs where convective instability exsists but no convection occurs.
 The convection calculations are performed after each leapfrog time step and can be considered as adjustments. Deep convection is done first, then shallow convection, as is the case with the ETA.
 Effects of nonprecipitating cumulus clouds, carried out in a manner similar to Tiedtke, utilize a subroutine that carries out an enhanced vertical diffusion of specific heat and humidity.
 Only model columns that contain conditionally unstable layers near the surface are eligible for this process.
 No convergence is explicitly required, more vigorous vertical mixing of water vapor occurs (in synoptically inactive regions, the water vapor would otherwise tend to accumulate near the surface).
 The shallow convective process acts to simulate the increased turbulence and attendant vertical eddy transport. These are known to be properties of shallow cumulus clouds and their local environment. This process is also treated in the same way as the model's basic vertical diffusion.
 Shallow convection calculation follows these five guidelines:
1. Cloud base is determined from values of lifting
condensation level that have been received from the deep convection subroutine,
which has been previously called. A value for cloud top (the highest unstable
layer) is also passed, but stronger restrictions (below) apply.
2. In columns for which deep convection has been performed,
shallow convection is not allowed.
3. Shallow convective "clouds" may occupy, at
most, sigma layers two through six, producing relatively strong diffusion
within the cloud and weak diffusion above and below it, as governed by an
imposed K profile.
4. Values of K are tapered in the vertical (to prevent
development of unrealistic kinks in the T and q profiles) as follows: at the
base of the cloud layer, K = 1.5m^{2}s^{1}; at the top, K = 1;
for the nexttotop layer, K = 3; for intermediate layers, if any: K=5 .
5. There must be at least one potentially unstable layer
within the conditionally unstable column.
Note: K = diffusion coefficient (m^{2}s^{1})
2. ETA:
This model uses a vertical coordinate
system (eta coordinates) with series of steps of varying heights, allowing
improved accuracy when dealing with topographic effects. This model runs
out to 60 hours. This model uses the
BettsMillerJanjic (BMJ) scheme. There
is also an experimental run using the KainFritsch (KF) scheme. Output
from the experimental runs can be found at http://www.nssl.noaa.gov/etakf/.
 Determines the mixing line between two saturation points, one at the cloud base, and one at the cloud top. The model uses the mixing line to create modified soundings reflecting the moist mixing process.
 Determines the most unstable parcel and establishes the LCL.
 Calculates cloud depth.
 If the cloud depth resulting from lifting the most unstable parcel is >10 hPa deep, < 200 hPa deep, and covers at least 2 model layers then the shallow convection ETA is triggered.
 Modifies the temperature profile so there is no latent heat release. (See Figure 1 )
 Modifies moisture profile so that no precipitation will reach the ground.
 Results in condensation at cloud bottom and moisture moving upward to evaporate at cloud top. So there will be net drying at the cloud base, and net moistening will occur at the top.
 Whole modification process takes about 40 minutes of real time.
B. Deep convection:
1. Medium Range Forecast (MRF):
This spectral model is run daily using observed data at 00Z and extends out to 16 days. These plots are updated once a day at 10 a.m. (EST) based on the 00Z model run.
 Simplified ArakawaSchubert (SAS) Scheme that models deep convection. This is based on rate of destabilization or quasiequilibrium.
 In order to be active it requires moisture convergence and deep conditional instability.
 Moisture convergence between cloud base and cloud top is separated into a rain producing portion and a humidity increasing portion. The convective heating and moistening of the environment is distributed with height between cloud base and cloud top.
 No ceiling is imposed on the deep convection. The medium range model is allowed to carry water vapor only in the lowest 12 sigma layers, which begin at the surface and gradually thicken into the mid troposphere.
 No convective moistening is allowed above layer 12 (only restriction).
 Either layer 2 (sigma = .981) or layer 3 (sigma = .960), or a combination of the two must produce the air entering the convective column.
 Layer one is not considered because it has a thickness of only 10 mb and is subject to surface processes that produce rapid variations in temperature and humidity.
 Now considers evaporation
 ALSO NOT CONSIDERED
 ice phase
 vertical fluxes of heat, momentum and moisture
 the effects of water loading
 cloud water, although the convective portion of a column
is referred to as the "cloud"
2. Aviation Model (AVN):
This model currently gives forecast
information out to 84 hours at 6z and 18z and 126 hours at 00z and 12z. Starting in March 2002 these lengths will
increase to 384 hours (global model).
Currently it uses T170L42 resolution four times a day and plots are 6
paneled, but it will change in May 2002 to T254L64 resolution. It
is the same numerical model as MRF, the only difference is in the data cutoff
times.
 Uses 42 layers
 0.7 x 0.7 degree resolution
 Uses the Simplified ArakawaSchubert (SAS) convective parameterization scheme
 Convection initiation within a column considers time rate of change in
stability as primary convective
trigger
presence of positive buoyancy, and Cap strength
 Refer to MRF description for details.
3. Nested Grid Model (NGM):
There have been no improvements
implemented since 1990 and none are planned. The NGM differs from other
models because of its use of horizontal nesting. The model makes its own
forecasts of conditions at the boundaries of the inner grids. The NGM does not have cumulus
parameterization.
 Moist convection also from modified Kuo scheme*
 Convection occurs if all of following conditions are met:
1. Sufficient moisture convergence in bottom six
model layers
2. Parcel originating in any of the bottom four layers
would become buoyant if lifted
3. Total moisture convergence into column below cloud
top is positive
 Moisture is distributed in the vertical through latent heating and moistening. Precipitation and latent heating are produced from 80% of the available moisture. The remaining 20% is used for moistening.
 Precipitation is allowed to fall and evaporate. If precipitation remains after evaporation has raised the relative humidity in a layer to about 48%, it is allowed to fall into the next layer. That process continues until all of the precipitation evaporates or it reaches the ground.
 One type of vertical mixing is dry convective adjustment. When a superadiabatic layer develops, the temperature profile is adjusted to be adiabatic so as to conserve the enthalpy of the column, a process also employed in the ETA.
* The Kuo scheme stabilizes the atmosphere at higher levels in the vertical
than would be done by a gridscale process. This model forecasts moderate
precipitation when actually it is more likely to be light or heavy.
4. ETA:
This model uses a vertical coordinate
system (eta coordinates) with series of steps of varying heights, allowing
improved accuracy when dealing with topographic effects. This model runs
out to 60. This model uses the
BettsMillerJanjic (BMJ) scheme. There
is also an experimental run using the KainFritsch (KF) scheme. Output
from the experimental runs can be found at http://www.nssl.noaa.gov/etakf/.
 The most unstable parcel from the LCL to the EL triggers the convective parameter
 Calculates cloud depth, deep parameterization continues if greater than 200 mb
 The BMJ deep convective scheme nudges the environment toward a welldefined reference profile (See Figure 2 )
 Temperature profile is modified to be near that of the moist adiabat to allow for slight instability ( See Figure 3 )
 Moisture profile defines the distance a parcel needs to be lifted to reach saturation
 Measures cloud efficiency which indicates enthalpy (heat potential) transport upward and produces as little precipitation as possible.
 Enthalpy is conserved because latent heat release will balance the net change in moisture due to condensation.
 To get precipitation, the modified moisture profile must become drier and the modified temperature profile must become warmer
 Precipitation is directly calculated from the amount of latent heat produced by the modification of the soundings
 Adjustment is made so that enthalpy is unchanged but allows for latent heat of condensation to be released and precipitation to fall.
 Entire modification process takes 40 minutes of real time
5. Rapid Update Cycle (RUC):
This model is designed for shortterm
forecasts and is updated frequently  every hour (it is considered the
"nowcast" of weather models). RUC operates on sigma coordinates
near the ground and on a theta coordinate system aloft. This model is sometimes
more accurate than others because it uses a wider range of available
observations. The RUC2 was active as
of 1998 with a 40 km resolution. About to come out (early 2001) is the
new RUC with 20 km resolution. Since the Grell scheme operates well with
resolutions of 10 to 30 km, the new RUC will forecast heavy rainfall events
even better.
 Uses isentropic values
 Does have some convective parameters
 Uses Grell convective parameterization scheme The Grell Scheme is based on the rate of destabilization or quasi equilibrium. This is a single cloud scheme with updraft downdraft fluxes and compensating motion that determines the heating and moistening profiles. This scheme is useful for smaller grid scales (e.g., 1030km), and it tends to allow a balance between the resolved scale rainfall and the convective rainfall. (http://www.epa.gov/asmdnerl/models3/doc/science/ch03.pdf )
 Includes downdraft detrainment, cloud top levels, minimum cloud depth, and capping criteria
 Inclusion of downdrafts results in smaller scale details in warm season precipitation patterns
 Evaporation of precipitation occurs with convective precipitation (not stable precipitation)
 Includes analysis and forecast grids with perceptible water, CAPE (multilayered), and CIN

Contains forecast sub grids for convective precipitation
during last forecast period
C. WRF
(Deep or Shallow Convection) The
WRF model is an experimental model so it can be used in both deep and shallow
convection. The NCEP and NSSL versions
are included for comparison/contrast. The ability exists for both Deep and Shallow Convection to be included
on either one of these models depending on the convective parameterization used
in the model. The NCEP version is
available on the web (http://rain.mmm.ucar.edu/mm5/pages/wrf.html)
for various initializations and fields.
NSSL also has its version available on the web (http://vicksburg.nssl.noaa.gov/wrf/).
1.
NCEP version
·BettsMillerJanjic convective parameterization
·NCEP 3class simple ice microphysics
·MRF PBL scheme
·5layer slab ground temperature
·RRTM longwave radiation
·MM5 shortwave radiation
·Thermal diffusion surface physics
·22 km grid length, 28 vertical levels, Time step 120 s
It adjusts the sounding toward a predetermined, postconvective reference profile derived from climatology.
Trigger: Three conditions are required to trigger convection:
·
At least some CAPE
·
Convective cloud depth
exceeding a threshold value
·
Moist soundings to activate
.
General
WRF NCEP facts
The dynamics uses a terrainfollowing heightbased, conservative fluxform of the fully compressible nonhydrostatic equations.
The numerics is 3rd order RungeKutta in time, 5th order horizontally and 3rd order vertically in space.
The model is initialized directly from the Eta analysis via the WRF Standard Initialization package.
The display uses NetCDF output with NCL graphics.
2. NSSL version
· Modified KF convective parameterization
· MRF PBL scheme
· NCEP 3 class simple ice microphysics
· RRTM longwave radiation
· Dudhia shortwave radiation
· Thermal diffusion surface physics
· 34 km grid length, 31 vertical levels
· Time step 180s
· Model domain and resolution are very similar
to those of the Eta model.
KF Convective
Parametrization
This is a complex scheme designed to rearrange mass in a column so that CAPE is consumed.
Trigger: The following conditions must be met for the scheme to trigger convection:
· The sounding
has CAPE for source parcels from a lowlevel layer 50 to 100 hPa thick
· The cap is
small enough for a parcel to penetrate given a boost of a few m/s (a function
of largescale vertical motion at LCL)
· The convective
cloud depth exceeds a threshold
Just as all people have unique skills and
abilities, the convective parameterization schemes of the various models will
do different things well and different things poorly. Often, the skill of
the model depends on the exact location and time for which it is forecasting. For example, the AVN under predicts mesoscale convective
events across the Great Plains during the warmest part of the year. The
ETA parameterizes convection differently over water than land, thus it often
overestimates precipitation along the Gulf and Atlantic coasts. The ETA
is also plagued by a greater amount of convective feedback than the NGM.
The MRF now accounts for factors such as evaporation of falling rain like the
RUC and NGM. The NGM and the MRF employ a variant of the Grell scheme
which stabilizes the atmosphere at higher levels in the vertical than would be
done by a gridscale process causing precipitation events to be forecast as
moderate when they actually are light or heavy. Of course, no model can accurately forecast
precipitation (or anything) everywhere all the time, but the problems
with the various models are fairly well documented. Using this
information and relying on more than one model can allow forecasters to predict
convective precipitation events with a good degree of accuracy. Still,
any parameterization scheme is going to be flawed because it is limited in its
ability to describe what truly occurs in the complex and chaotic atmosphere.
Anthes, T.A., 1977. A cumulus parameterization scheme utilizing a one dimensional cloud model. Monthly Weather Review, 105, 270286.
AVN Version of the Global Spectral Model web site Feb 2002 http://www.srh.noaa.gov/ftproot/ssd/nwpmodel/html/avn.htm#changes
Baldwin, Mike, et. al., 1998. Convective Parameterization in Models: Background Information. http://www.comet.ucar.edu/nwplessons/precipproclesson3/.
Benjamin, Stanley G., Kevin J. Brundage, and Lauren L. Morone, NOAA/ERL Forecast Systems Laboratory, NWS/NOAA, 1994: Implementation of the Rapid Update Cycle. http://maps.fsl.noaa.gov/tpbruc.cgi.
Black, Thomas L., 1994: The New NMC Mesoscale Eta Model: Description and Forecast Examples. Weather and Forecasting, 9, 265278.
Developing Meteorological Fields. Environmental Protection Agency. 1999 http://www.epa.gov/asmdnerl/models3/doc/science/ch03.pdf.
NCAR WRF web site. Weather Research and Forecasting Model. 2002 http://rain.mmm.ucar.edu/mm5/pages/wrf.html .
Grell, G. 1993. Prognostic evaluation of assumptions used by cumulus parameterizations. Monthly Weather Review, 121, 764787.
Hoke, James H., Norman A. Phillips, Geoffrey J. DiMego, James J. Tucillo, Joseph G. Sela, 1989: The Regional Analysis and Forecast System of the National Meteorological Center. Weather and Forecasting, 4, 323334.
Information about the MRF. http://sgi62.wwb.noaa.gov:8080/web2/web2/chap5.html.
Kuo, H.L. 1965. On formulation and intensification of tropical cyclones through latent heat release by cumulus convection. Journal of Atmospheric Science, 22, 4063.
NCAR WRF website. Weather
Research and Forecasting Model. 2002 http://rain.mmm.ucar.edu/mm5/pages/wrf.html
NOAA/NMC, 1995: Description of Models.
NSSL WRF website. RealTime WRF model runs from NSSL. May 23. 2001 http://vicksburg.nssl.noaa.gov/wrf
Operational Models Matrix web site.
2002 http://meted.ucar.edu/nwp/pcu2/index.htm
Rogers, Eric, et. al.,
2000. Changes to the NCEP Meso Eta
Analysis and Forecast System: Assimilaltion of satellite radiances and increase
in resolution. http://sgi62.wwb.noaa.gov:8080/ETA22TPB/.
Staudenmaier Jr., Mike. WRSSD/NWSFO SLC. The Convective Parameterization Scheme in the MESOETA Model. http://nimbo.wrh.noaa.gov/wrhq/96TAs/TA9623/ta9623.html.
Tiedtke, M., ECMWF, 1983. The sensitivity of the timemean largescale flow to cumulus convection in the ECMWF model. Workshop on Convection in LargeScale Numerical Models. 28 Nov  1 Dec 1983, pp. 297316.
UCAR website. How Models Produce Precipitation & Clouds: Convective Parameterization. http://meted.ucar.edu/nwp/pcu1/ic3/frameset.htm .
WRF model information. Weather research and forecasting
model. http://www.wrfmodel.org/Welcome.html
Jeff Jamison,
Julie Jasien, and Greg
Ostermeier
April 30, 1998
http://www.met.tamu.edu/class/metr452/models/convection.html
Julie Slater, Dan
Stillman
February 1, 1999
Brad Armstrong, Bruce
Sherbon, and Christine Meleo
February 15, 2000
Dave Long, Jennifer Derrick, Nathan
Cheneaux
February 14, 2001
Robert Osborn, Stephen Latimer,
Travis Wyatt, and Michael Griffin
February 18, 2002
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