Forecast simulations of Southeast Pacific Stratocumulus with CAM3 and CAM3-UW.
Cécile Hannay(1), Jeffrey Kiehl (1), Dave Williamson(1), Jerry Olson(1), Jim Hack(1) and Chris Bretherton (2).
(1): National Center for Atmospheric Research, Boulder, Colorado
(2): Department of Atmospheric Science, University of Washington, Seattle, Washington.
2. Evolution of the 5-day forecasts
1. Overview
We illustrate the way CAM and CAM3-UW represent regions of
persistent stratocumulus with forecast simulations of a column in the
South Eastern Pacific (20S-85W).
Motivation
Stratocumulus clouds play an important role in the seasonal cycle of the
Eastern Pacific and the global climate by exerting a strong cooling effect on
the surface.
These clouds are very complex to parameterize in GCMs because :
- they are only a few hundred meters thick. Therefore, they are difficult to
represent with the current climate model vertical resolution.
- they are maintained by a complex set of interactions between the cloud
layer and its environment, which are not always well understood.
We initialize CAM every 6 hours with the ECMWF analyses for the period October 11-22, 2001. For each initialization, we
run the model for 5 days obtaining an ensemble of forecasts with various features. However, individual forecasts can be
grouped into 2 typical behaviors: either CAM maintains the PBL or the PBL collapses. To illustrate this, we examine 2
typical forecasts starting on October 16 at 0UT (PBL maintained) and October 20 at 0UT (PBL collapses).
Forecast of the PBL and cloud layer
The 2 versions of the model CAM3 and CAM3-UW show similarities. In both models, the PBL collapses (maintains) for
the Oct 16 (Oct 20) initialization. When the PBL collapses, the model becomes very moist near the surface.
There are differences between the 2 versions of the model:
- CAM3 produces an unrealistically thick layer of clouds that sometimes extends to the surface. CAM3 produces some
‘empty’ clouds (clouds with very low or no liquid water content).
- CAM3-UW clouds are more realistic and lay on a single level. CAM3-UW better represents the diurnal cycle of the PBL,
due to the entrainment of dry air at the top of the PBL. When the PBL collapses, the cloud fraction and cloud water in
CAM3-UW go to zero.
Potential temperature
subsidence
Entrainement of dry air
Inversion
jump
LW cooling
3. Moisture budgets
CAM3
Oct 16 initialization
Oct 20 initialization
CAM3-UW
Oct 16 initialization
Oct 20 initialization
Stratocumulus
or
TOT = ADV + PAR
where TOT is the total tendency, ADV represent the tendency to the advection (sum of the horizontal
and vertical advection) and PAR represents the subgrid scale parameterization term. We separate
the parameterization term into its components:

PAR = PBL + SHALLOW + CLDWAT (+ DEEP)
- PBL is the moisture tendency due to the PBL scheme,
- SHALLOW is the tendency coming from the shallow convection including the evaporation of
shallow convective precipitation.
- CLDWAT is the tendency coming from the prognostic cloud water scheme, which includes the
conversion between vapor and condensate in the stratiform cloud and the evaporation of falling
precipitation and cloud water sedimentation.
- DEEP is the deep convection tendency (not active for the EPIC column).
In CAM3 and CAM3-UW, the advection term dries the upper part of the PBL while the
parameterization term moistens it. The 2 models show similar patterns for these 2 terms. However,
splitting the parameterization term into its components reveals that the mechanism for moistening
the PBL is different between the 2 models.
- CAM3 unphysically maintains the PBL by a mixture of dry convection and shallow convection. The
PBL scheme moves the moisture up and creates a moist layer around 950mb. This moist layer
triggers the shallow scheme which ventilates the moisture higher in the atmosphere.
sfc
Figure 1: Some processes controlling stratocumulus.
The Eastern Pacific Investigation of Climate (EPIC) column
This location has been chosen because of the availability of observational
datasets and accurate analyses.
• the WHOI buoy provides a long-term time-series of surface meteorological
variables.
• the 2001 EPIC cruise provides a comprehensive dataset of remote sensing
and surface measurements for Oct 16-21, 2001.
• the MK ECMWF analyses provide a realistic state of the EPIC column.
- CAM3-UW behaves more physically. It is the PBL scheme that moves the moisture up in the
boundary layer without any significant contribution from the shallow scheme.
CAM3
TOT = ADV + PAR
The tendencies are from:
- TOT= total tendency
- ADV = advection tendency
- PAR = parameterization tendency
Figure 2: The EPIC column (20S-85W).
Time-height cross-section of potential
temperature (THETA) and moisture (q) from
radiosondes and ECMWF analysis.
Observations shows a very stable PBL under
a sharp inversion. ECMWF analyses provide a
realistic state for the EPIC column even if the
height of the PBL and the strength of the
inversion are underestimated.
EPIC
q
q
 V  q    PAR
t
p
October 16 initialization (PBL maintained)
Buoyancy
flux
BL height
We have made a detailed analysis of the budget terms of temperature, moisture and cloud water. As
an illustration, we consider the terms of the moisture budget. The moisture equation can be written:
CAM3
Parameterization terms
The tendencies are from:
- PBL = PBL scheme
- SHALLOW = shallow convection scheme
- CLDWAT = prognostic cloud water
Figure 4: 5-day evolution of Q, CLOUD and CLDLIQ in CAM3
and CAM3-UW for forecasts initialized on Oct 16 and Oct 20.
Correlation with surface fluxes, TKE and omega
CAM3-UW
We illustrate the relationship between PBL height in
CAM3 and some variables in CAM3 and CAM3-UW.
Oct 16
Parameterization terms
The tendencies are from:
- PBL = PBL scheme
- SHALLOW = shallow convection scheme
- CLDWAT = prognostic cloud water
Oct 20
October 20 initialization (PBL collapses)
When the PBL collapses, the shallow scheme turns off in CAM3 and the PBL scheme weakens
in CAM3-UW.
Forecast framework
In the CAPT protocol, we realistically initialize CAM with analyses and we
then run the model in forecast mode to determine the drift from the
analyses and/or available field data. This method allows us to diagnose
model parameterization deficiencies.
CAM3
Parameterization terms
The tendencies are from:
- PBL = PBL scheme
- SHALLOW = shallow convection scheme
- CLDWAT = prognostic cloud water
Initialize realistically
Operational ECMWF analysis
(Martin Koehler PBL)
Strategy
If the model is initialized realistically,
we assume the error comes from the
parameterizations deficiencies.
Figure 5: Turbulent Kinetic Energy (TKE) and vertical velocity in
CAM3-UW for forecasts initialized on Oct 16 and Oct 20
Figure 6: PBL height and latent heat flux in observations and in CAM3.
CAM
Advantages
Full feedback <=> SCM
5-day forecast
Starting daily at 00 UT
(also forecasts at 6,12,18 UT)
EPIC 2001 cruise
WHOI buoy
Limitations
Accuracy of the atmospheric state ?
Figure 3: Forecast runs framework
Models
We use 2 versions of CAM with different parameterizations of PBL and shallow cumulus.
• the standard CAM3 which uses Holtslag-Boville (1993) for the boundary layer and Hack (1994) for the shallow convection.
• the CAM3-UW uses the turbulence scheme of Grenier-Bretherton (2001) which includes explicit entrainment at the top of
the PBL coupled with a shallow cumulus scheme which includes the determination of cloud-base mass flux based on
surface layer turbulent kinetic energy (TKE) and convective inhibition near the cloud base.
We use 3 vertical resolutions (26, 30 and 60 levels). Here we present the 30-level results.
CAM3-UW
Parameterization terms
The tendencies are from:
- PBL = PBL scheme
- SHALLOW = shallow convection scheme
- CLDWAT = prognostic cloud water