Abstract
Abstract. The NCEP Global Forecast System (GFS) model has an important systematic error shared by many other models: stratocumuli are missed over the subtropical eastern oceans. It is shown that this error can be alleviated in the GFS by introducing a consideration of the low-level inversion and making two modifications in the model's representation of vertical mixing. The modifications consist of (a) the elimination of background vertical diffusion above the inversion and (b) the incorporation of a stability parameter based on the cloud-top entrainment instability (CTEI) criterion, which limits the strength of shallow convective mixing across the inversion. A control simulation and three experiments are performed in order to examine both the individual and combined effects of modifications on the generation of the stratocumulus clouds. Individually, both modifications result in enhanced cloudiness in the Southeast Pacific (SEP) region, although the cloudiness is still low compared to the ISCCP climatology. If the modifications are applied together, however, the total cloudiness produced in the southeast Pacific has realistic values. This nonlinearity arises as the effects of both modifications reinforce each other in reducing the leakage of moisture across the inversion. Increased moisture trapped below the inversion than in the control run without modifications leads to an increase in cloud amount and cloud-top radiative cooling. Then a positive feedback due to enhanced turbulent mixing in the planetary boundary layer by cloud-top radiative cooling leads to and maintains the stratocumulus cover. Although the amount of total cloudiness obtained with both modifications has realistic values, the relative contributions of low, middle, and high layers tend to differ from the observations. These results demonstrate that it is possible to simulate realistic marine boundary clouds in large-scale models by implementing direct and physically based improvements in the model parameterizations.
Highlights
The climatology of the tropical and subtropical Pacific Ocean south of the Equator is characterized by large east-west gradients in sea surface temperature (SST), with values increasing from ∼20 ◦C along the South American coast to ∼29 ◦C in the Western Pacific warm pool
The present paper reports on a series of studies aimed at improving the simulation of stratocumulus in the operational Global Forecast System (GFS)/Climate Forecast System (CFS) when running in simulation mode, i.e., for a period long enough to establish a model climatology
When compared with the CMAP climatology most of the main precipitation features are reproduced in the four experiments, such as the rain bands associated with the Intertropoical Convergence Zone (ITCZ), South Pacific Convergence Zone (SPCZ), mid-latitude oceanic storm tracks, and maxima over the continents at monsoon locations
Summary
The climatology of the tropical and subtropical Pacific Ocean south of the Equator is characterized by large east-west gradients in sea surface temperature (SST), with values increasing from ∼20 ◦C along the South American coast to ∼29 ◦C in the Western Pacific warm pool. Underprediction of stratus results in overestimation of heat flux into the ocean and may be the primary reason why ocean-atmosphere coupled models show positive SST biases of several degrees off the coast of Peru (Mechoso et al, 1995; Wang et al, 2005; de Szoeke et al, 2006) These model difficulties with marine boundary layer clouds in the eastern tropical oceans are evident in the 2003 version of the operational GFS, which has been used since 2003 as the atmospheric component of the Climate Forecast System (CFS) to produce operational climate forecasts at NCEP (Saha et al, 2006).
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