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
We initialize CAM every 6 hours with the ECMWF analyses for the period October 11-22, 2001. For each initialization, werun the model for 5 days obtaining an ensemble of forecasts with various features. However, individual forecasts can begrouped into 2 typical behaviors: either CAM maintains the PBL or the PBL collapses. To illustrate this, we examine 2typical forecasts starting on October 16 at 0UT (PBL maintained) and October 20 at 0UT (PBL collapses).
Forecast of the PBL and cloud layerThe 2 versions of the model CAM3 and CAM3-UW show similarities. In both models, the PBL collapses (maintains) forthe 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 inCAM3-UW go to zero.
CAM3
CAM3-UW
Oct 16 initialization
Oct 20 initialization
Oct 16 initialization
Oct 20 initialization
Figure 4: 5-day evolution of Q, CLOUD and CLDLIQ in CAM3and CAM3-UW for forecasts initialized on Oct 16 and Oct 20.
Correlation with surface fluxes, TKE and omega
We illustrate the relationship between PBL height inCAM3 and some variables in CAM3 and CAM3-UW.
Figure 5: Turbulent Kinetic Energy (TKE) and vertical velocity inCAM3-UW for forecasts initialized on Oct 16 and Oct 20
Oct 16
Oct 20
Figure 6: PBL height and latent heat flux in observations and in CAM3.
October 20 initialization (PBL collapses)
When the PBL collapses, the shallow scheme turns off in CAM3 and the PBL scheme weakensin CAM3-UW.
3. Moisture budgets
We have made a detailed analysis of the budget terms of temperature, moisture and cloud water. Asan illustration, we consider the terms of the moisture budget. The moisture equation can be written:
where TOT is the total tendency, ADV represent the tendency to the advection (sum of the horizontaland vertical advection) and PAR represents the subgrid scale parameterization term. We separatethe parameterization term into its components:
- PBL is the moisture tendency due to the PBL scheme,- SHALLOW is the tendency coming from the shallow convection including the evaporation ofshallow convective precipitation.- CLDWAT is the tendency coming from the prognostic cloud water scheme, which includes theconversion between vapor and condensate in the stratiform cloud and the evaporation of fallingprecipitation and cloud water sedimentation.- DEEP is the deep convection tendency (not active for the EPIC column).
October 16 initialization (PBL maintained)
In CAM3 and CAM3-UW, the advection term dries the upper part of the PBL while theparameterization 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 moisteningthe PBL is different between the 2 models.- CAM3 unphysically maintains the PBL by a mixture of dry convection and shallow convection. ThePBL scheme moves the moisture up and creates a moist layer around 950mb. This moist layertriggers the shallow scheme which ventilates the moisture higher in the atmosphere.
- CAM3-UW behaves more physically. It is the PBL scheme that moves the moisture up in theboundary layer without any significant contribution from the shallow scheme.
PAR = PBL + SHALLOW + CLDWAT (+ DEEP)
TOT = ADV + PAR
or
CAM3Parameterization terms
The tendencies are from:- PBL = PBL scheme- SHALLOW = shallow convection scheme- CLDWAT = prognostic cloud water
CAM3-UWParameterization terms
The tendencies are from:- PBL = PBL scheme- SHALLOW = shallow convection scheme- CLDWAT = prognostic cloud water
CAM3Parameterization terms
The tendencies are from:- PBL = PBL scheme- SHALLOW = shallow convection scheme- CLDWAT = prognostic cloud water
CAM3-UWParameterization terms
The tendencies are from:- PBL = PBL scheme- SHALLOW = shallow convection scheme- CLDWAT = prognostic cloud water
CAM3TOT = ADV + PAR
The tendencies are from:- TOT= total tendency- ADV = advection tendency- PAR = parameterization tendency
ModelsWe 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 ofthe PBL coupled with a shallow cumulus scheme which includes the determination of cloud-base mass flux based onsurface 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.
1. Overview
We illustrate the way CAM and CAM3-UW represent regions ofpersistent stratocumulus with forecast simulations of a column in theSouth Eastern Pacific (20S-85W).
MotivationStratocumulus clouds play an important role in the seasonal cycle of theEastern Pacific and the global climate by exerting a strong cooling effect onthe surface.
These clouds are very complex to parameterize in GCMs because :- they are only a few hundred meters thick. Therefore, they are difficult torepresent with the current climate model vertical resolution.- they are maintained by a complex set of interactions between the cloudlayer and its environment, which are not always well understood.
Figure 1: Some processes controlling stratocumulus.
Stratocumulus
Buoyancyflux
subsidence
Potential temperature
Inversionjump
BL height
sfc
Entrainement of dry air
LW cooling
The Eastern Pacific Investigation of Climate (EPIC) columnThis location has been chosen because of the availability of observationaldatasets and accurate analyses.
• the WHOI buoy provides a long-term time-series of surface meteorologicalvariables.
• the 2001 EPIC cruise provides a comprehensive dataset of remote sensingand surface measurements for Oct 16-21, 2001.
• the MK ECMWF analyses provide a realistic state of the EPIC column.
EPIC
Figure 2: The EPIC column (20S-85W).Time-height cross-section of potentialtemperature (THETA) and moisture (q) fromradiosondes and ECMWF analysis.Observations shows a very stable PBL undera sharp inversion. ECMWF analyses provide arealistic state for the EPIC column even if theheight of the PBL and the strength of theinversion are underestimated.
Forecast frameworkIn the CAPT protocol, we realistically initialize CAM with analyses and wethen run the model in forecast mode to determine the drift from theanalyses and/or available field data. This method allows us to diagnosemodel parameterization deficiencies.
Figure 3: Forecast runs framework
Strategy
If the model is initialized realistically,
we assume the error comes from theparameterizations deficiencies.
Advantages
Full feedback <=> SCM
Limitations
Accuracy of the atmospheric state ?
Initialize realistically
Operational ECMWF analysis(Martin Koehler PBL)
CAM
5-day forecast
Starting daily at 00 UT
(also forecasts at 6,12,18 UT)
EPIC 2001 cruise
WHOI buoy