Low-Latitude Cloud Feedbacks CPT at Stony Brook
1. CPT SCM Intercomparison and Analysis of Low Cloud Simulations and Their Feedbacks

(To be posted)

Participants can download idealized SCM/CRM/LES forcing data here (login id and passwd are the same as those at UW, NCAR, and GFDL).

2. Slowing down of the Walker circulation and its impact on climate feedbacks of low clouds in the Eastern tropical Pacific.

We analyzed the response of the Walker circulation to warming SST. We used both model simulation and reanalysis products as well as ship-based measurements in this study. We found that the surface pressure gradients across the Pacific have described in all reanalysis and ship-based and products in the last fifty years. All model simulations are consistent with observations in this measure, suggesting slowing down of the Walker circulation in a warmer climate Associated with these decreases are the reductions of the 200hp velocity potentials. The models clearly show reduction of subsidence in the Eastern tropical and subtropical Pacific. Specific analysis procedure and results can be found here.

Our next plan is to study the impact of the reduced subsidence on the variation of low clouds and their climate feedbacks. This will be carried out in two phases. One is to analyze the variation of subsidence rate and low clouds from different GCM simulations. The second is to use a single-column model with specified large-scale forcing and SST condition. In this second task, we will reduce the rate of subsidence and increase the SST to evaluate the response low clouds in the CAM SCM. Results will be compared with similar experiment from Min Zhao and Isaac Held at GFDL with the AM2 and from other CPT members.

This work has been carried out by Song Hua and Minghua Zhang under the NSF CPT collaborative grant to Stony Brook.

3. Evaluation of simulated clouds against satellite data from ISCCP

This project focuses on two themes. The first theme is to assess the robustness of simulated clouds from the three US atmospheric GCMs used by the CPT (the NCAR CAM3, the GFDL AM2, and the GSFC GMAO) against satellite observations from the ISCCP project (Rossow et al. 1996). Model clouds are sampled by using an ISCCP simulator as developed by Webb et al. (2001) and Klein and Jakob (1999). Clouds are categorized according to cloud top altitudes and their optical thickness as specified in this table.
Several common model biases are found in all three models. These include the overestimation of optically thick high-top clouds, and underestimation of optically thin middle and low clouds. The CAM3 also overestimated optically thick middle and low clouds but underestimated optical intermediate middle-top clouds. The GSFC model also significantly overestimated optically intermediate high-top clouds (cirrostratus), but underestimated optically thick low clouds. The GFDL GCM overestimated optically thick middle clouds but underestimated optically intermediate middle clouds.

Detailed comparison of simulated clouds from the three models with satellite data, for each ISCCP cloud type, can be found here.

The second theme is to examine the variation of cloud types in climate change experiments, in particular what cloud types contribute the most to the model cloud feedbacks. We hope that results from the first theme on the comparison between model cloud types and ISCCP data can tell us what confidence we can put in the diagnosed cloud feedbacks in the models, and what cloud types need to be improved to improve the confidences.

Two climate change experiments have been carried out with the CAM, in which perturbations of SSTs from other coupled models are used as forcing. In both experiments, the model showed negative cloud feedback due to dominance of the reduction of shortwave cloud forcing. All experiments show increase of low cloud clouds, but decrease of high and middle clouds. The increase low cloud amount however is from the optically thick low clouds, while the decreased middle and high clouds are from the optically thin clouds. Results from these experiments can be found here.

Our next plan is to modify the CAM so that its basic cloud distributions, in particular the optically thick low clouds, become closer to ISCCP measurements, and then carry out similar climate change experiments to study the dependence of the cloud feedback on the model basic cloud climatology.

This work has been carried out by Wuyin Lin and Minghua Zhang under the NSF CPT collaborative grant to Stony Brook .
 

Link to CPT at University Of Washington (with links to other CPT members)