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Abstract
A parameterization package for cloud-radiation interaction is incorporated into a spectral general circulation model (GCM). Fractional cloud amount is predicted quasi-empirically; cloud optical depth is specified for warm clouds and anvil cirrus, but depends on temperature for other subfreezing clouds; the long- and shortwave cloud optical properties are linked to the cloud optical depth. The model's time-mean clouds and its radiative, thermal, and dynamical response to cloud-radiation interaction are investigated for the extended forecast range, primarily by performing two sets of 30-day integrations from real initial conditions for three Northern Hemisphere (NH) winter and three NH summer cases: (i) CLDRADI, with cloud-radiation interaction; and (ii) LONDON, with this GCM's traditional specification of climatological zonal-mean cloud amount and global-mean cloud optical properties.
The 30-day mean CLDRADI fields of total and high cloud amount and corresponding outgoing longwave radiation (OLR) fields are plausible in many respects, especially in the tropics where the latter exhibit South Pacific convergence zone (SPCZ)-like and some intertropical convergence zone (ITCZ)-like features, in qualitative agreement with Nimbus-7 and Earth Radiation Budget Experiment (ERBE) observations. Also, the predicted monthly mean OLR anomalies (relative to model climatology) respond to interannual variations in sea surface temperature. Cloud amount and cloud optical depth are apparently underestimated, however, over the higher-latitude oceans, especially over the Southern Hemisphere (SH) circumpolar low pressure belt and Antarctica. The zonal mean bias in shortwave and net radiation remains large at high latitudes in the summer hemisphere, despite the improved longitudinal structure in the tropics.
Cloud-radiation interaction elicits a cirrus warming response, which reduces the tropical upper-tropospheric cold bias by ∼1–2 K. Over Antarctica, the warm bias in SH summer and cold bias in SH winter are both considerably reduced. During NH winter, the tropical upper troposphere experiences a significant westerly acceleration, including a sign reversal of the zonal-mean zonal wind. By being more conducive to meridional propagation, CLDRADI's tropical westerlies may contribute to the amplification of the quasi-stationary planetary waves in the SH summer extratropics. Otherwise, the impact of cloud-radiation interaction on extratropical geopotential height is generally minimal at extended range.
Abstract
A parameterization package for cloud-radiation interaction is incorporated into a spectral general circulation model (GCM). Fractional cloud amount is predicted quasi-empirically; cloud optical depth is specified for warm clouds and anvil cirrus, but depends on temperature for other subfreezing clouds; the long- and shortwave cloud optical properties are linked to the cloud optical depth. The model's time-mean clouds and its radiative, thermal, and dynamical response to cloud-radiation interaction are investigated for the extended forecast range, primarily by performing two sets of 30-day integrations from real initial conditions for three Northern Hemisphere (NH) winter and three NH summer cases: (i) CLDRADI, with cloud-radiation interaction; and (ii) LONDON, with this GCM's traditional specification of climatological zonal-mean cloud amount and global-mean cloud optical properties.
The 30-day mean CLDRADI fields of total and high cloud amount and corresponding outgoing longwave radiation (OLR) fields are plausible in many respects, especially in the tropics where the latter exhibit South Pacific convergence zone (SPCZ)-like and some intertropical convergence zone (ITCZ)-like features, in qualitative agreement with Nimbus-7 and Earth Radiation Budget Experiment (ERBE) observations. Also, the predicted monthly mean OLR anomalies (relative to model climatology) respond to interannual variations in sea surface temperature. Cloud amount and cloud optical depth are apparently underestimated, however, over the higher-latitude oceans, especially over the Southern Hemisphere (SH) circumpolar low pressure belt and Antarctica. The zonal mean bias in shortwave and net radiation remains large at high latitudes in the summer hemisphere, despite the improved longitudinal structure in the tropics.
Cloud-radiation interaction elicits a cirrus warming response, which reduces the tropical upper-tropospheric cold bias by ∼1–2 K. Over Antarctica, the warm bias in SH summer and cold bias in SH winter are both considerably reduced. During NH winter, the tropical upper troposphere experiences a significant westerly acceleration, including a sign reversal of the zonal-mean zonal wind. By being more conducive to meridional propagation, CLDRADI's tropical westerlies may contribute to the amplification of the quasi-stationary planetary waves in the SH summer extratropics. Otherwise, the impact of cloud-radiation interaction on extratropical geopotential height is generally minimal at extended range.
Abstract
The sensitivity of a coupled general circulation model (CGCM) to tropical marine stratocumulus (MSc) clouds and low-level clouds over the tropical land is examined. The hypothesis that low-level clouds play an important role in determining the strength and position of the Walker circulation and also on the strength and phase of the El Niño–Southern Oscillation (ENSO) is studied using a Geophysical Fluid Dynamics Laboratory (GFDL) experimental prediction CGCM. In the Tropics, a GFDL experimental prediction CGCM exhibits a strong bias in the western Pacific where an eastward shift in the ascending branch of the Walker circulation diminishes the strength and expanse of the sea surface temperature (SST) warm pool, thereby reducing the east–west SST gradient, and effectively weakening the trade winds. These model features are evidence of a poorly simulated Walker circulation, one that mirrors a “perpetual El Niño” state. One possible factor contributing to this bias is a poor simulation of MSc clouds in the eastern equatorial Pacific (which are essential to a proper SST annual cycle). Another possible contributing factor might be radiative heating biases over the land in the Tropics, which could, in turn, have a significant impact on the preferred locations of maximum convection in the Tropics. As a means of studying the sensitivity of a CGCM to both MSc clouds and to varied radiative forcing over the land in the Tropics, low-level clouds obtained from the International Satellite Cloud Climatology Project (ISCCP) are prescribed. The experiment sets consist of one where clouds are fully predicted, another where ISCCP low-level clouds are prescribed over the oceans alone, and a third where ISCCP low-level clouds are prescribed both over the global oceans and over the tropical landmasses. A set of ten 12-month hindcasts is performed for each experiment.
The results show that the combined prescription of interannually varying global ocean and climatological tropical land low-level clouds into the CGCM results in a much improved simulation of the Walker circulation over the Pacific Ocean. The improvement to the tropical circulation was also notable over the Indian and Atlantic basins as well. These improvements in circulation led to a considerable increase in ENSO hindcast skill in the first year by the CGCM. These enhancements were a function of both the presence of MSc clouds over the tropical oceans and were also due to the more realistic positioning of the regions of maximum convection in the Tropics. This latter model feature was essentially a response to the change in radiative forcing over tropical landmasses associated with a reduction in low cloud fraction and optical depth when ISCCP low-level clouds were prescribed there. These results not only underscore the importance of a reasonable representation of MSc clouds but also point out the considerable impact that radiative forcing over the tropical landmasses has on the simulated position of the Walker circulation and also on ENSO forecasting.
Abstract
The sensitivity of a coupled general circulation model (CGCM) to tropical marine stratocumulus (MSc) clouds and low-level clouds over the tropical land is examined. The hypothesis that low-level clouds play an important role in determining the strength and position of the Walker circulation and also on the strength and phase of the El Niño–Southern Oscillation (ENSO) is studied using a Geophysical Fluid Dynamics Laboratory (GFDL) experimental prediction CGCM. In the Tropics, a GFDL experimental prediction CGCM exhibits a strong bias in the western Pacific where an eastward shift in the ascending branch of the Walker circulation diminishes the strength and expanse of the sea surface temperature (SST) warm pool, thereby reducing the east–west SST gradient, and effectively weakening the trade winds. These model features are evidence of a poorly simulated Walker circulation, one that mirrors a “perpetual El Niño” state. One possible factor contributing to this bias is a poor simulation of MSc clouds in the eastern equatorial Pacific (which are essential to a proper SST annual cycle). Another possible contributing factor might be radiative heating biases over the land in the Tropics, which could, in turn, have a significant impact on the preferred locations of maximum convection in the Tropics. As a means of studying the sensitivity of a CGCM to both MSc clouds and to varied radiative forcing over the land in the Tropics, low-level clouds obtained from the International Satellite Cloud Climatology Project (ISCCP) are prescribed. The experiment sets consist of one where clouds are fully predicted, another where ISCCP low-level clouds are prescribed over the oceans alone, and a third where ISCCP low-level clouds are prescribed both over the global oceans and over the tropical landmasses. A set of ten 12-month hindcasts is performed for each experiment.
The results show that the combined prescription of interannually varying global ocean and climatological tropical land low-level clouds into the CGCM results in a much improved simulation of the Walker circulation over the Pacific Ocean. The improvement to the tropical circulation was also notable over the Indian and Atlantic basins as well. These improvements in circulation led to a considerable increase in ENSO hindcast skill in the first year by the CGCM. These enhancements were a function of both the presence of MSc clouds over the tropical oceans and were also due to the more realistic positioning of the regions of maximum convection in the Tropics. This latter model feature was essentially a response to the change in radiative forcing over tropical landmasses associated with a reduction in low cloud fraction and optical depth when ISCCP low-level clouds were prescribed there. These results not only underscore the importance of a reasonable representation of MSc clouds but also point out the considerable impact that radiative forcing over the tropical landmasses has on the simulated position of the Walker circulation and also on ENSO forecasting.
