Representation of tropical subseasonal variability of precipitation in global reanalyses

Tropical subseasonal variability of precipitation from five global reanalyses (RAs) is evaluated against Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) observations. The RAs include the three generations of global RAs from the National Center for Environmental Prediction (NCEP), and two other RAs from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC). The analysis includes comparisons of the seasonal means and subseasonal variances of precipitation, and probability densities of rain intensity in selected areas. In addition, the space–time power spectrum was computed to examine the tropical Madden-Julian Oscillation (MJO) and convectively coupled equatorial waves (CCEWs). The modern RAs show significant improvement in their representation of the mean state and subseasonal variability of precipitation when compared to the two older NCEP RAs: patterns of the seasonal mean state and the amplitude of subseasonal variability are more realistic in the modern RAs. However, the probability density of rain intensity in the modern RAs show discrepancies from observations that are similar to what the old RAs have. The modern RAs show higher coherence of CCEWs with observed variability and more realistic eastward propagation of the MJO precipitation. The modern RAs, however, exhibit common systematic deficiencies including: (1) variability of the CCEWs that tends to be either too weak or too strong, (2) limited coherence with observations for waves other than the MJO, and (3) a systematic phase lead or lag for the higher-frequency waves.     

Representation of tropical storms in the northwestern pacific by the Modern-Era Retrospective analysis for research and applications

This study examines the tropical storms simulated in the Modern-Era Retrospective analysis for Research and Applications (MERRA) global atmospheric reanalysis for the recent 12 years (1998–2009), focusing on the tropical storm activity over the Northwestern Pacific. For validation, the International Best Track Archive for Climate Stewardship (IBTrACS) dataset is used as an observational counterpart. Climatological-mean features of the tropical storm genesis, tracks and their maximum intensity are the primary interests in this study. Regarding the genesis location of tropical storms, MERRA is reasonable in resolving major development regions over the South China Sea and the Northwestern Pacific close to the Philippines. The seasonal variation of the number of storms is also reproduced in a realistic way in MERRA, with peak values occurring from July to September. In addition, MERRA tends to reproduce the observed interannual variation of the number of tropical storms during the 12-years, though with a limited accuracy. The simulated paths toward higher latitudes are also reasonable in MERRA, where the reanalysis corresponds well with the observations in resolving frequent paths of westward moving storms and recurving storms toward the northeast. Regarding the intensity, MERRA captures the linear relationship between the minimum center pressure and the maximum wind speed near the surface at the maximum development. Some discrepancies from the observed features are found in the reanalysis, such as less frequent development of storms over the South China Sea and less frequent paths over this region. The reanalysis also does not attain the observed maximum intensity for the resolved tropical storms, particularly underestimating the center pressure. These deficiencies are likely related to limitations in the horizontal resolution and the parameterized physics of the data assimilation system.     

Cloud Radiative Effects and Changes Simulated by the Coupled Model Intercomparison Project Phase 5 Models

Using 32 CMIP5(Coupled Model Intercomparison Project Phase 5) models, this study examines the veracity in the simulation of cloud amount and their radiative effects(CREs) in the historical run driven by observed external radiative forcing for 1850–2005, and their future changes in the RCP(Representative Concentration Pathway) 4.5 scenario runs for2006–2100. Validation metrics for the historical run are designed to examine the accuracy in the representation of spatial patterns for climatological mean, and annual and interannual variations of clouds and CREs. The models show large spread in the simulation of cloud amounts, specifically in the low cloud amount. The observed relationship between cloud amount and the controlling large-scale environment are also reproduced diversely by various models. Based on the validation metrics,four models—ACCESS1.0, ACCESS1.3, Had GEM2-CC, and Had GEM2-ES—are selected as best models, and the average of the four models performs more skillfully than the multimodel ensemble average.All models project global-mean SST warming at the increase of the greenhouse gases, but the magnitude varies across the simulations between 1 and 2 K, which is largely attributable to the difference in the change of cloud amount and distribution. The models that simulate more SST warming show a greater increase in the net CRE due to reduced low cloud and increased incoming shortwave radiation, particularly over the regions of marine boundary layer in the subtropics. Selected best-performing models project a significant reduction in global-mean cloud amount of about-0.99% K~(-1)and net radiative warming of 0.46 W m~(-2)K~(-1), suggesting a role of positive feedback to global warming.     

Mechanisms of diurnal precipitation over the US Great Plains: a cloud resolving model perspective

The mechanisms of summertime diurnal precipitation in the US Great Plains were examined with the two-dimensional (2D) Goddard Cumulus Ensemble (GCE) cloud-resolving model (CRM). The model was constrained by the observed large-scale background state and surface flux derived from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program’s Intensive Observing Period (IOP) data at the Southern Great Plains (SGP). The model, when continuously-forced by realistic surface flux and large-scale advection, simulates reasonably well the temporal evolution of the observed rainfall episodes, particularly for the strongly forced precipitation events. However, the model exhibits a deficiency for the weakly forced events driven by diurnal convection. Additional tests were run with the GCE model in order to discriminate between the mechanisms that determine daytime and nighttime convection. In these tests, the model was constrained with the same repeating diurnal variation in the large-scale advection and/or surface flux. The results indicate that it is primarily the surface heat and moisture flux that is responsible for the development of deep convection in the afternoon, whereas the large-scale upward motion and associated moisture advection play an important role in preconditioning nocturnal convection. In the nighttime, high clouds are continuously built up through their interaction and feedback with long-wave radiation, eventually initiating deep convection from the boundary layer. Without these upper-level destabilization processes, the model tends to produce only daytime convection in response to boundary layer heating. This study suggests that the correct simulation of the diurnal variation in precipitation requires that the free-atmospheric destabilization mechanisms resolved in the CRM simulation must be adequately parameterized in current general circulation models (GCMs) many of which are overly sensitive to the parameterized boundary layer heating.     

Korea Institute of Ocean Science and Technology Earth System Model and Its Simulation Characteristics

Ocean Science Journal - We document the performance of a new earth system model developed at Korea Institute of Ocean Science and Technology, called the KIOST-ESM, based on a low-resolution...     

Validation of the experimental hindcasts produced by the GloSea4 seasonal prediction system

Using 14 year (1996–2009) ensemble hindcast runs produced with the Global Seasonal Forecasting System version 4 (GloSea4), this study evaluates the spatial and temporal structure of the hindcast climatology and the prediction skill of major climate variability. A special focus is on the fidelity of the system to reproduce and to forecast phenomena that are closely related to the East Asian climate. Overall the GloSea4 system exhibits realistic representations of the basic climate even though a few model deficiencies are identified in the sea surface temperature and precipitation. In particular, the capability of GloSea4 to capture the seasonal migration of rain belt associated with Changma implies a good potential for the Asian summer monsoon prediction. It is found that GloSea4 is as skillful as other state-of-the-art seasonal prediction systems in forecasting climate variability including the El-Nino/southern oscillation (ENSO), the East Asian summer monsoon, the Arctic Oscillation (AO), and the Madden-Julian Oscillation (MJO). The results presented in this study will provide benchmark evaluation for next seasonal prediction systems to be developed at the Korea Meteorological Administration.     

Simulation of the intraseasonal variability over the Eastern Pacific ITCZ in climate models

During boreal summer, convective activity over the eastern Pacific (EPAC) inter-tropical convergence zone (ITCZ) exhibits vigorous intraseasonal variability (ISV). Previous observational studies identified two dominant ISV modes over the EPAC, i.e., a 40-day mode and a quasi-biweekly mode (QBM). The 40-day ISV mode is generally considered a local expression of the Madden-Julian Oscillation. However, in addition to the eastward propagation, northward propagation of the 40-day mode is also evident. The QBM mode bears a smaller spatial scale than the 40-day mode, and is largely characterized by northward propagation. While the ISV over the EPAC exerts significant influences on regional climate/weather systems, investigation of contemporary model capabilities in representing these ISV modes over the EPAC is limited. In this study, the model fidelity in representing these two dominant ISV modes over the EPAC is assessed by analyzing six atmospheric and three coupled general circulation models (GCMs), including one super-parameterized GCM (SPCAM) and one recently developed high-resolution GCM (GFDL HIRAM) with horizontal resolution of about 50?km. While it remains challenging for GCMs to faithfully represent these two ISV modes including their amplitude, evolution patterns, and periodicities, encouraging simulations are also noted. In general, SPCAM and HIRAM exhibit relatively superior skill in representing the two ISV modes over the EPAC. While the advantage of SPCAM is achieved through explicit representation of the cumulus process by the embedded 2-D cloud resolving models, the improved representation in HIRAM could be ascribed to the employment of a strongly entraining plume cumulus scheme, which inhibits the deep convection, and thus effectively enhances the stratiform rainfall. The sensitivity tests based on HIRAM also suggest that fine horizontal resolution could also be conducive to realistically capture the ISV over the EPAC, particularly for the QBM mode. Further analysis illustrates that the observed 40-day ISV mode over the EPAC is closely linked to the eastward propagating ISV signals from the Indian Ocean/Western Pacific, which is in agreement with the general impression that the 40-day ISV mode over the EPAC could be a local expression of the global Madden-Julian Oscillation (MJO). In contrast, the convective signals associated with the 40-day mode over the EPAC in most of the GCM simulations tend to originate between 150°E and 150°W, suggesting the 40-day ISV mode over the EPAC might be sustained without the forcing by the eastward propagating MJO. Further investigation is warranted towards improved understanding of the origin of the ISV over the EPAC.     

Examinations of cloud variability and future change in the coupled model intercomparison project phase 3 simulations

Low-level cloud variability is critical to the radiation balance of Earth due to its wide spatial coverage. Using the adjusted International Satellite Cloud Climatology Project (ISCCP) observations of Clement et al. (2009), and the Coupled Model Intercomparison Project Phase 3 (CMIP3) model simulations, this study examines the observed and the simulated low-cloud variations and their relationships with large-scale environmental variables. From the observational analysis, significant correlations are found between low clouds and those of sea surface temperature (SST), lower tropospheric stability (LTS), and sea level pressure (SLP) over tropical marine areas of low cloud prevailing regions during most of the year. Increase of SST coincides with the reduction of LTS and increased vertical motion, which tends to reduce low-level clouds in subtropical oceans. Among the 14 models investigated, CGCM3 and HadGEM1 exhibit more realistic representation of the observed relationship between low-level clouds and large-scale environments. In future climate projection, these two models show a good agreement in the reduction of low-cloud throughout much of the global oceans in response to greenhouse gas forcing, suggesting a positive low-cloud feedback in a climate change context.