Tropical and Subtropical Cloud Transitions in Weather and Climate Prediction Models: The GCSS/WGNE Pacific Cross-Section Intercomparison (GPCI)

Abstract

A model evaluation approach is proposed in which weather and climate prediction models are analyzed along a Pacific Ocean cross section, from the stratocumulus regions off the coast of California, across the shallow convection dominated trade winds, to the deep convection regions of the ITCZ-the Global Energy and Water Cycle Experiment Cloud System Study/Working Group on Numerical Experimentation (GCSS/ WGNE) Pacific Cross-Section Intercomparison (GPCI). The main goal of GPCI is to evaluate and help understand and improve the representation of tropical and subtropical cloud processes in weather and climate prediction models. In this paper, a detailed analysis of cloud regime transitions along the cross section from the subtropics to the tropics for the season June-July-August of 1998 is presented. This GPCI study confirms many of the typical weather and climate prediction model problems in the representation of clouds: underestimation of clouds in the stratocumulus regime by most models with the corresponding consequences in terms of shortwave radiation biases; overestimation of clouds by the 40-yr ECMWF Re-Analysis (ERA-40) in the deep tropics (in particular) with the corresponding impact in the outgoing longwave radiation; large spread between the different models in terms of cloud cover, liquid water path and shortwave radiation; significant differences between the models in terms of vertical cross sections of cloud properties (in particular), vertical velocity, and relative humidity. An alternative analysis of cloud cover mean statistics is proposed where sharp gradients in cloud cover along the GPCI transect are taken into account. This analysis shows that the negative cloud bias of some models and ERA-40 in the stratocumulus regions [as compared to the first International Satellite Cloud Climatology Project (ISCCP)] is associated not only with lower values of cloud cover in these regimes, but also with a stratocumulus-to-cumulus transition that occurs too early along the trade wind Lagrangian trajectory. Histograms of cloud cover along the cross section differ significantly between models. Some models exhibit a quasi-bimodal structure with cloud cover being either very large (close to 100%) or very small, while other models show a more continuous transition. The ISCCP observations suggest that reality is in-between these two extreme examples. These different patterns reflect the diverse nature of the cloud, boundary layer, and convection parameterizations in the participating weather and climate prediction models.

An A-train and MERRA view of cloud, thermodynamic, and dynamic variability within the subtropical marine boundary layer

“…Strong (poor) spatial correspondence between radiance skewness and AIRS ECF (MODIS cloud fraction) was found, suggesting infraredbased ECF is a potentially valuable and unappreciated diagnostic for MBL cloud characterization. The mean MBL depth derived from AIRS (Martins et al, 2010) shows a characteristic transition from shallow MBLs (920-970 hPa) near the coast to deeper MBLs (830-880 hPa) away from the coast and is a well-observed feature of the stratocumulus-tocumulus transition (e.g., Teixeira et al, 2011). The AIRSderived dMSE between 700 and 1000 hPa agree very well with ERA-Interim (Kubar et al, 2012).…”

“…MERRA is on average moister than AIRS by ∼ 5 % in the NH, nearly identical to AIRS in the SEA, and a much more spatially heterogeneous difference is observed in the SEP from the coastal proximity westward between 8 and 12 • S. Bretherton et al (2010) demonstrate that the free troposphere in the SEP westward of 75 • W is characteristically very dry (0.1 g kg −1 ) with sporadic filaments of moist air (as high as 3-6 g kg −1 ) up to an altitude of 2.5 km. In addition, these moist filaments have been observed with GPS-RO refractivity profiles by von Engeln et al (2007). The vertical structure of RH obtained from VOCALS-REx radiosondes implies a well-mixed MBL near the coast with MBL decoupling west of 80 • W. Myers and Norris (2015) showed that 700 hPa is drier in the SH subtropics than in the NH using ERA-Interim data.…”

Characteristics of the Cloud Top Heights of Marine Boundary Layer Clouds and the Frequency of Marine Fog over Mid-Latitudes

“…The typical characteristic that the cloud layers are lower near California than near Peru is also well captured in the data. Karlsson et al (2010) showed cloud-top-height evolution along the GPCI line (GCSS/WGNE Pacific cross-section intercomparison; GCSS: GEWEX cloud system study; WGNE: working group on numerical experimentation; GEWEX: Global Energy and Water Exchanges Project; see Teixeira et al 2011) off California using Multi-angle Imaging SpectroRadiometer (MISR) data. Stratocumulus cloud structure along 20°S observed by a cloud radar and upward-pointing lidar aboard an aircraft is shown in Bretherton et al (2010), Rahn and Garreaud (2010), and Abel et al (2010).…”

Interpretation of the positive low-cloud feedback predicted by a climate model under global warming

“…4 CGILS focuses on three cases of boundary-layer clouds occurring along a transect ranging from California to Hawaii (Teixeira et al 2011) and representative of stratus, stratocumulus and shallow cumulus cloud types (Karlsson et al 2010). For each case, idealized large-scale conditions representative of the present-day climate are derived from European Centre for Medium-Range Weather Forecasts (ECMWF) analysis, and idealized large-scale forcings representative of global warming conditions are derived by prescribing a ?2 K SST increase and by assuming that the tropical temperature profile follows a moist adiabat, that the relative humidity remains constant, that profiles of horizontal heat and moisture advection are unchanged, and that large-scale subsidence is changed so as to balance the radiative cooling above the boundary layer (Zhang and Bretherton 2008).…”