Longwave Cloud Forcing Research Articles

Comparison of cloud forcing derived from the Earth Radiation Budget Experiment with that simulated by the NCAR community climate model

A comprehensive comparison of top of atmosphere radiative quantities from the Earth Radiation Budget Experiment (ERBE) with the same quantities from the latest version of the National Center for Atmospheric Research Community Climate Model (CCM) is presented. The ERBE data set offers a unique collection of top of atmosphere radiation fields for it includes clear‐sky radiation fields that have in the past not been available for such comparisons. Comparison between ERBE clear‐sky longwave fluxes and CCMl indicates larger outgoing flux in the model than measured by ERBE. This overestimation is ascribed to the prevalent dryness of the model. Comparison of clear sky albedo indicates good agreement over oceans and land. Total outgoing longwave flux also reflects the moisture deficiency of the model, but differences due to an underprediction of “effective” high cloud are also apparent. The clear‐sky and total fluxes are combined to form the cloud radiative forcing from the ERBE data and the CCM. Comparison of shortwave cloud radiative forcing indicates deficiencies in the model where marine stratus clouds are absent. Large longwave cloud forcing over the tropical deep convective regions in Indonesia and South America are present in the model but are underestimated compared with the ERBE results. Three regions, located over Indonesia, the equatorial Pacific, and the North Atlantic, are considered in detail. For the Indonesian region where deep convection is present, we consider the statistical correlation between the longwave cloud forcing and the shortwave cloud forcing from the ERBE data and CCM. Results indicate a near cancellation between the SWCF and LWCF for these regions, whereas the model predicts a net cooling. Another major area of discrepancy is over the North Atlantic and Pacific oceans where ERBE shows that clouds significantly reduce the solar heating of the oceans. While the model simulates this cooling, the magnitude is underpredicted by more than a factor of 2.

Cloud-Radiative Forcing and Climate: Results from the Earth Radiation Budget Experiment

The study of climate and climate change is hindered by a lack of information on the effect of clouds on the radiation balance of the earth, referred to as the cloud-radiative forcing. Quantitative estimates of the global distributions of cloud-radiative forcing have been obtained from the spaceborne Earth Radiation Budget Experiment (ERBE) launched in 1984. For the April 1985 period, the global shortwave cloud forcing [-44.5 watts per square meter (W/m(2))] due to the enhancement of planetary albedo, exceeded in magnitude the longwave cloud forcing (31.3 W/m(2)) resulting from the greenhouse effect of clouds. Thus, clouds had a net cooling effect on the earth. This cooling effect is large over the mid-and high-latitude oceans, with values reaching -100 W/m(2). The monthly averaged longwave cloud forcing reached maximum values of 50 to 100 W/m(2) over the convectively disturbed regions of the tropics. However, this heating effect is nearly canceled by a correspondingly large negative shortwave cloud forcing, which indicates the delicately balanced state of the tropics. The size of the observed net cloud forcing is about four times as large as the expected value of radiative forcing from a doubling of CO(2). The shortwave and longwave components of cloud forcing are about ten times as large as those for a CO(2) doubling. Hence, small changes in the cloud-radiative forcing fields can play a significant role as a climate feedback mechanism. For example, during past glaciations a migration toward the equator of the field of strong, negative cloud-radiative forcing, in response to a similar migration of cooler waters, could have significantly amplified oceanic cooling and continental glaciation.

An analysis of cloud radiation forcing as calculated from ERBE, AVHRR and Nimbus-7 ERB and cloud data

Longwave cloud radiative forcing, defined as the cloud free flux minus the average flux has been calculated using radiation budget data from NOAA operational satellites, ERBE (Earth Radiation Budget Experiment) and Nimbus-7 cloud and radiation budget observations. Each satellite data set has unique characteristics and different analysis procedures were used for obtaining the cloud forcing from each set. Longwave cloud forcing was computed globally on a 2.5° latitude/longitude grid. An intercomparison of results indicates that the estimates of cloud forcing, while showing some differences, exhibit a high degree of consistency in both magnitude and spatial distribution. This suggests that we can obtain a reasonably accurate depiction of spatial and temporal variability of the longwave cloud forcing suitable for climate studies. A method for calculating cloud forcing anomalies from anomalies in outgoing longwave radiation, available from NOAA operational satellites, is proposed.

The response of a general circulation model to cloud longwave radiative forcing. I: Introduction and initial experiments

A new version of the NCAR Community Climate Model (CCM1) is used to study the effect of cloud radiative forcing on model simulations. Previous attempts to determine the role played by clouds in climate and the general circulation of the atmosphere are reviewed first. The concept of cloud radiative forcing is discussed and the forcings in the shortwave and longwave spectral regions are contrasted. At low latitudes, the cloud longwave forcing is primarily within the atmosphere, rather than at the surface. It is thus appropriate to study its effect with a model which employs fixed sea surface temperatures (s.s.t.s). The impact of cloud longwave forcing is studied in 510-day integrations for constant January conditions. The experiments isolate the forcing by tropical and extra-tropical clouds. Tropical cloud forcing warms the tropical upper troposphere and accelerates the subtropical jets. There are subtle interactions between the forcing, the clear-sky longwave heating and the latent heating in the tropics. These additional diabatic heating terms oppose the forcing in some regions and enhance it in others. The tropical cloud forcing strengthens the precipitation maxima at low latitudes. There are also changes in the extra-tropical flow, including the excitation of a pattern in the 200 mb geopotential height differences which is similar to those found in previous studies of the effect of s.s.t. anomalies. This confirms that the forcing may be as important as latent heat release in determining the atmospheric response to such anomalies. The mode is excited with comparable strength by the extra-tropical cloud forcing. Finally, some concerns regarding the generality of these results and their applicability to other models and the real atmosphere are discussed. These concerns suggest the need for sensitivity studies, some of which are already in progress.

The response of a general circulation model to cloud longwave radiative forcing. I: Introduction and initial experiments

AbstractA new version of the NCAR Community Climate Model (CCM1) is used to study the effect of cloud radiative forcing on model simulations. Previous attempts to determine the role played by clouds in climate and the general circulation of the atmosphere are reviewed first. The concept of cloud radiative forcing is discussed and the forcings in the shortwave and longwave spectral regions are contrasted. At low latitudes, the cloud longwave forcing is primarily within the atmosphere, rather than at the surface. It is thus appropriate to study its effect with a model which employs fixed sea surface temperatures (s.s.t.s).The impact of cloud longwave forcing is studied in 510‐day integrations for constant January conditions. The experiments isolate the forcing by tropical and extra‐tropical clouds. Tropical cloud forcing warms the tropical upper troposphere and accelerates the subtropical jets. There are subtle interactions between the forcing, the clear‐sky longwave heating and the latent heating in the tropics. These additional diabatic heating terms oppose the forcing in some regions and enhance it in others.The tropical cloud forcing strengthens the precipitation maxima at low latitudes. There are also changes in the extra‐tropical flow, including the excitation of a pattern in the 200 mb geopotential height differences which is similar to those found in previous studies of the effect of s.s.t. anomalies. This confirms that the forcing may be as important as latent heat release in determining the atmospheric response to such anomalies. The mode is excited with comparable strength by the extra‐tropical cloud forcing. Finally, some concerns regarding the generality of these results and their applicability to other models and the real atmosphere are discussed. These concerns suggest the need for sensitivity studies, some of which are already in progress.