The purpose of this research is to address the cloud feedback in a future climate scenario predicted using global climate models. To understand the variability of low-level clouds in the current climate, variations in cloud cover as well as relationship between cloud cover and other variables are examined using the adjusted International Satellite Cloud Climatology Project (ISCCP) data and Intergovernmental panel on climate change (IPCC) Fourth Assessment Report (AR4) models.

The study focuses on the low-level cloud amount, the variability of which is critical in balancing earth’s radiation budget. The correlations of the observed low-level cloud cover anomalies with a variety of variables suggest that low-level clouds in tropical marine areas (persistent low-level cloud regions) are associated with a cool sea surface temperature, stronger stability, higher sea level pressure, and subsidence. An increase in SST causes a reduction in lower tropospheric stability, which in turn allows for more vertical motion within and around the cloud deck that leads to increased dry air entrainment. This brings about a reduction in cloudiness and a transition from low-level to high-level cloud types. Higher SLP can also produce more subsidence aloft, thereby increasing LTS independent of SST.

Obtaining a better understanding of the physical processes that control cloud response to climate variability and the evaluation of cloud feedbacks simulated by current models should help to assess which of the model estimates of cloud feedback is the most reliable. To more closely examine the evidence of low-level cloud responses, the correlations between cloud cover and various meteorological variables in the four cloud regions with persistent low-level cloud cover are computed for each model and compared with observations. Models are selected according to whether they have the correct sign of correlation relative to observations. Two models passed the cloud-meteorology correlation test, exhibiting considerably good agreement with observed net cloud radiative forcing. These models show a reduction in cloud cover throughout much of the Pacific in response to global warming by greenhouse gas forcing (i.e., a positive feedback). In this study, we give a concise answer to the question of whether low-level clouds act as a positive or negative feedback to climate change.

1. INTRODUCTION

2. METHODOLOGY AND DATA

2.1 Description of adjusted ISCCP data 9 2.2 Description of model simulations

3. OBSERVATIONAL GLOBAL CHARACTERISTICS OF CLOUDS

3.1 Spatial distribution and temporal variation of cloud cover

3.2 Large-scale environment

3.3 Variations of low cloud cover correlated with meteorological variables

4. EVALUATION OF MODEL SIMULATIONS

4.1 Model evaluation for the simulated cloud variations

4.2 Change of cloud amount (and CRF) by global warming

5. SUMMARY