Simplified Arakawa-Schubert Scheme Research Articles

Sensitivity of Simulated Conditions to Different Parameterization Choices Over Complex Terrain in Central Chile

This study evaluates the performance of fourteen high-resolution WRF runs with different combinations of parameterizations in simulating the atmospheric conditions over the complex terrain of central Chile during austral winter and spring. We focus on the validation of results for coastal, interior valleys, and mountainous areas independently, and also present an in-depth analysis of two synoptic-scale events that occurred during the study period: a frontal system and a cut-off low. The performance of the simulations decreases from the coast to higher altitudes, even though the differences are not very clear between the coast and interior valleys for 10 m wind speeds and precipitation. The simulated vertical profiles show a warmer and drier boundary layer and a cooler and moister free atmosphere than observed. The choice of the land-surface model has the largest positive impact on near-surface variables with the five-layer thermal diffusion scheme showing the smallest errors. Precipitation is more sensitive to the choice of cumulus parameterizations, with the simplified Arakawa–Schubert scheme generally providing the best performance for absolute errors. When examining the performance of the model simulating rain/no-rain events for different thresholds, also the cumulus parameterizations better represented the false alarm ratio (FAR) and the bias score (BS). However, the Morrison microphysics scheme resulted in the best critical success index (CSI), while the probability of detection (POD) was better in the simulation without analysis nudging. Overall, these results provide guidance to other researchers and help to identify the best WRF configuration for a specific research or operational goal.

Sensitivity of the simulation of extreme precipitation events in China to different cumulus parameterization schemes and the underlying mechanisms

The ability of climate models to capture extreme precipitation events is crucially important, but most of the existing models contain significant biases for the simulation of extreme precipitation. To understand the causes of these biases, we used five different cumulus parameterization schemes in the regional Climate–Weather Research and Forecasting (CWRF) model to investigate its performance and biases in the simulation of extreme precipitation events in China. In general, the ensemble cumulus parameterization (ECP) scheme was the most skillful in reproducing the spatial distribution of the 95th percentile daily precipitation (P95) and the other four schemes either overestimated (the Kain-Fritsch Eta and Tiedtke schemes) or underestimated (the Betts-Miller-Janjic and New Simplified Arakawa-Schubert schemes) P95. Compared with the observational data, ECP scheme significantly improved the simulation of extreme precipitation in China and had the highest correlation and the smallest root-mean-square error in most areas and seasons. To clarify the underlying physical processes of P95 simulation biases, we established a regression model of extreme precipitation based on ECP scheme. This showed that P95 in North China is mainly affected by moisture convergence, planetary boundary layer height and lifting condensation level (relative importance 18–32%). In Central China, the vertical upward motion of water vapor, sensible heat flux and planetary boundary layer height (relative importance 18–30%) are main factors associated with P95. In South China, the vertical upward motion and horizontal transport of water vapor are predominant (relative importance 26–37%). In addition, the net surface energy, surface and atmospheric radiation flux, total precipitable water, convective available potential energy and cloud water path also have a high correlation with P95 (the second most important factor; relative importance 14–31%). The influence of each factor on the simulation of P95 is different when using different cumulus parameterization schemes and the interaction among the different factors determines the ability of CWRF model to simulate extreme precipitation. These results provide important references for future model evaluations and improvements.

The Impact of Scale-Aware Parameterization on the Next-Generation Global Prediction System in Taiwan for Front Predictions

In order to improve the precipitation forecast of the next-generation Global Prediction System with the Finite-Volume Cubed-Sphere Dynamical Core in Taiwan’s Central Weather Bureau, this study modified the convective processes in the New Simplified Arakawa-Schubert scheme based on the methodology of scale-aware parameterization developed in Kwon and Hong (2017) and investigated its impacts on a front event, which propagated across Taiwan and produced heavy rainfall in late May of 2020. Results show that the modified scale-aware parameterization has significantly improved the intensity and the spatial distribution of frontal precipitation forecasts due to the proper definition of convective updraft fraction. However, the synoptic-scale features perform a larger warm bias with the modified scale-aware parameterization. Therefore, further modification of the scale-aware capability of convective cloud water detrainment is proposed to reduce the heating from microphysical processes and result in a better overall performance for the medium-range weather forecasts.

Characteristics of warm cores of tropical cyclones in a 25-km-mesh regional climate simulation over CORDEX East Asia domain

In this study, the characteristics of the tropical cyclone (TC) warm cores in high resolution reanalysis dataset (ERA5) and a 25-km-mesh regional climate simulation over CORDEX East Asia domain during 1988–2009 are analyzed. The Kain–Fritsch scheme with new convection trigger function (Ma and Tan, Atmos Res 92:190–211, 2009; KFMT), the renewed Kain-Fritsch convective parameterization (Kain, J Appl Meteorol 43:170–181, 2004; KF) and the simplified Arakawa-Schubert scheme (Arakawa and Schubert, J Atmos Sci 31:674–701, 1974; SAS) are employed to illustrate the impact of cumulus parameterization schemes (CPSs) on the representation of warm core in the regional climate simulation. The TC intensity and warm core strength in ERA5 reanalysis is largely weaker than those in the regional climate simulations. In ERA5 reanalysis and three regional climate simulations, the warm core strength shows a significant positive correlation with TC intensity, but the warm core height has no correlation with TC intensity, especially during the intensifying stage of TCs. The results also show that CPS has a great impact on the warm core structures of simulated TCs. The TC warm core strength is strongest in KFMT experiment and weakest in SAS experiment, which is consistent with the differences in the TC intensities. The TC warm core heights in the KF and KFMT experiments are significantly higher than that in SAS experiment. These differences of TC warm core strength and height are mainly due to the discrepancy in the simulation of convective activities with different CPSs. The results of this study point out that the ability of climate model to simulate TC intensity is strongly related to the model capability to simulate the TC thermal structure, which is influenced by convection representations in the model.

Precipitation over East Asia simulated by ECHAM6.3 with different schemes of cumulus convective parameterization

This study investigates the applicability of five cumulus parameterization schemes (CPSs) for precipitation simulation in East Asia with the atmospheric model ECHAM6.3. Besides the built-in Tiedtke–Nordeng–Tiedtke (TNT) scheme, four updated schemes are implemented in ECHAM6.3: two modified Tiedtke schemes, the simplified Arakawa–Schubert scheme (SAS), and the Betts–Miller–Janjic scheme (BMJ). An overall assessment of seasonal climatology shows that the SAS and the two modified Tiedtke schemes have clear advantages over the default TNT scheme. The spatial pattern of precipitation climatology and seasonal variations are successfully reproduced by all CPSs. Biases of rainfall distribution are strongly modulated by the simulation of atmospheric circulation systems over East Asia. The main characteristics of summer monsoonal precipitation, including the increase in rainfall and rain belt migration under the influence of the East Asian summer monsoon (EASM), are successfully captured. However, there are large model diversities in the location and intensity of rain centers around Southeast China and the Korean Peninsula. In addition, all simulations of Meiyu rain band migration are farther north than observed. The SAS scheme is the best of the five CPSs in simulating the timing and intensity of seasonal rainfall variations. The results also suggest that the environmental feedback of a CPS is a key factor that determines the applicability of the CPS in a particular climate model.

Forecasts of MJO Events during DYNAMO with a Coupled Atmosphere-Ocean Model: Sensitivity to Cumulus Parameterization Scheme

An operational weather forecast model, coupled to an oceanic model, was used to predict the initiation and propagation of two major Madden-Julian Oscillation (MJO) events during the dynamics of the MJO (DYNAMO) campaign period. Two convective parameterization schemes were used to understand the sensitivity of the forecast to the model cumulus scheme. The first is the Tiedtke (TDK) scheme, and the second is the Simplified Arakawa-Schubert (SAS) scheme. The TDK scheme was able to forecast the MJO-1 and MJO-2 initiation at 15- and 45-day lead, respectively, while the SAS scheme failed to predict the convection onset in the western equatorial Indian Ocean (WEIO). The diagnosis of the forecast results indicates that the successful prediction with the TDK scheme is attributed to the model capability to reproduce the observed intraseasonal outgoing longwave radiation-sea surface temperature (OLR-SST) relationship. On one hand, the SST anomaly (SSTA) over the WEIO was induced by surface heat flux anomalies associated with the preceding suppressed-phase MJO. The change of SSTA, in turn, caused boundary layer convergence and ascending motion, which further induced a positive column-integrated moist static energy (MSE) tendency, setting up a convectively unstable stratification for MJO initiation. The forecast with the SAS scheme failed to reproduce the observed OLR-SST-MSE relation. The propagation characteristics differed markedly between the two forecasts. Pronounced eastward phase propagation in the TDK scheme is attributed to a positive zonal gradient of the MSE tendency relative to the MJO center, similar to the observed, whereas a reversed gradient appeared in the forecast with the SAS scheme with dominant westward propagation. The difference is primarily attributed to anomalous vertical and horizontal MSE advection.

Analysis of Extreme Rainfall Event with Different Microphysics and Parameterization Schemes in WRF Model

In the present study, the imitation of heavy rainfall event which occurred over Jharkhand during 18 August 2016 was taken as a case study. Weather Research and Forecasting (WRF) model has been utilized for this study. National Centers for Environmental Prediction (NCEP) analysis data is compared with GSMaP data with different combination of physical parameterization scheme like microphysics (MP) and cumulus parameterization (CP). In the present study, three MP schemes: Kessler scheme, Lin et al. scheme and WRF Single-moment 6-class scheme with combination of three CP schemes: Betts-Miller-Janjic scheme, Multi-scale Kain-Fritsch scheme and New simplified Arakawa-Schubert scheme have been used. The model predicted humidity, temperature and precipitation were compared with the GSMaP product. The model nicely depicted the cloud pattern and recognized the rain event spatially. The obtained result shows that the model overestimates the precipitation for all the schemes.

Sensitivity of AGCM‐simulated regional JJAS precipitation to different convective parameterization schemes

ABSTRACTThis study examines the effects of convective parameterization schemes (CPSs) on the simulated June–September (JJAS) precipitation climatology, variability and predictability over southwestern Arabian Peninsula and northeast Africa during 1981–2014 within an atmospheric global climate model framework. The three CPSs used are: the simplified Arakawa–Schubert (SAS) scheme; the SAS scheme with Tokioka modification (STOK) and the Emanuel (EMAN) scheme coupled with a probability distribution function‐based cloud parameterization scheme. SAS and STOK overestimate JJAS total precipitation over the selected domain compared to observations, while EMAN underestimates. EMAN provides the most realistic distribution of precipitation and spatial distribution of convective precipitation despite the tendency for underestimating the total precipitation. The principal patterns of precipitation variability (as measured by the leading empirical orthogonal functions) in EMAN and STOK are clearly related to the observed pattern, while the maximum variability in SAS occurs in a completely different location. The El Niño Southern Oscillation‐related JJAS precipitation teleconnection simulated by the SAS scheme is very weak as compared to observations and other two CPSs. The signal‐to‐noise ratio estimated by SAS and STOK is very low as compared to EMAN. Correlation analysis shows that EMAN performs better than the other two CPSs. Low (high) outgoing longwave radiation (OLR) values are indicative of enhanced (suppressed) convection and hence more (less) cloud coverage. The root mean square error estimated for OLR in EMAN is lower than that of the other two CPSs. The vertical structure of specific humidity and temperature shows less error in EMAN than that of the other two CPSs, which could be a reason for better predictability over the region of interest.

A Mass-Flux Cumulus Parameterization Scheme across Gray-Zone Resolutions

AbstractA method that enables a mass-flux cumulus parameterization scheme (CPS) to work seamlessly in various model grids across CPS gray-zone resolutions is proposed. The convective cloud-base mass flux, convective inhibition, and convective detrainment in the simplified Arakawa–Schubert (SAS) scheme are modified to be functions of the convective updraft fraction. The combination of two updraft fractions is used to modulate the cloud-base mass flux; the first one depends on the horizontal grid space and the other is a function of the grid-scale and convective vertical velocity. The convective inhibition and detrainment of hydrometeors are also modified to be a function of the grid-size-dependent convective updraft fraction.A set of sensitivity experiments with the Weather Research and Forecasting (WRF) Model is conducted for a heavy rainfall case over South Korea. The results show that the revised SAS CPS outperforms the original SAS. At 3 and 1 km, the precipitation core over South Korea is well reproduced by the experiments with the revised SAS scheme. On the contrary, the simulated precipitation is widespread in the case of the original SAS experiment and there are multiple spurious cores when the CPS is removed at those resolutions. The modified mass flux at the cloud base is found to play a major role in organizing the grid-scale precipitation at the convective core. A 1-month simulation at 3 km confirms that the revised scheme produces slightly better summer monsoonal precipitation results as compared to the typical model setup without CPS.

An improvement in mass flux convective parameterizations and its impact on seasonal simulations using a coupled model

A new closure and a modified detrainment for the simplified Arakawa–Schubert (SAS) cumulus parameterization scheme are proposed. In the modified convective scheme which is named as King Abdulaziz University (KAU) scheme, the closure depends on both the buoyancy force and the environment mean relative humidity. A lateral entrainment rate varying with environment relative humidity is proposed and tends to suppress convection in a dry atmosphere. The detrainment rate also varies with environment relative humidity. The KAU scheme has been tested in a single column model (SCM) and implemented in a coupled global climate model (CGCM). Increased coupling between environment and clouds in the KAU scheme results in improved sensitivity of the depth and strength of convection to environmental humidity compared to the original SAS scheme. The new scheme improves precipitation simulation with better representations of moisture and temperature especially during suppressed convection periods. The KAU scheme implemented in the Seoul National University (SNU) CGCM shows improved precipitation over the tropics. The simulated precipitation pattern over the Arabian Peninsula and Northeast African region is also improved.

Impacts of the triggering function of cumulus parameterization on warm-season diurnal rainfall cycles at the Atmospheric Radiation Measurement Southern Great Plains site

In this study, we investigated the impacts of the triggering function of the deep convection scheme on diurnal rainfall variation in the middle latitudes by using the single-column version of the Community Atmospheric Model (SCAM). Using the climate statistics of a long-term ensemble analysis of SCAM simulations, we quantified and validated the diurnal rainfall climatological regimes at the Atmospheric Radiation Measurement Southern Great Plains (SGP) site. The results showed that the averaged diurnal rainfall cycle simulated using the default Zhang-Mcfarlane (ZM) scheme of the SCAM peaks near noon, which is far earlier than the observed nighttime peak phase. This bias was due to the ZM scheme, which produced spurious daytime rainfall, even during days in which only light rainfall was observed. By contrast, using a weather-focused scheme, the Simplified Arakawa-Schubert (SAS) scheme, we successfully simulated the nocturnal peak of the diurnal cycle. Experiments conducted on the ZM and SAS schemes featuring different triggering functions revealed that the relaxation of launching parcels above the planetary boundary layer (PBL) and the inclusion of convective inhibition (CIN) were crucial designs for the model to capture the nocturnal rainfall events of the SGP. The inclusion of CIN reduces spurious weak convective events, and the allowance of launching parcels being above the PBL better captures convective cloud base. The results of this study highlight the modulatory effect of low-level inhomogeneity on the diurnal variation of convection over midlatitudes and the importance of the triggering function of the deep convection scheme in capturing those variations.

Modification of cumulus convection and planetary boundary layer schemes in the GRAPES global model

Cumulus convection is a key linkage between hydrological cycle and large-scale atmospheric circulation. Cumulus parameterization scheme is an important component in numerical weather and climate modeling studies. In the Global/Regional Assimilation and Prediction Enhanced System (GRAPES), turbulent mixing and diffusion approach is applied in its shallow convection scheme. This method overestimates the vertical transport of heat and moisture fluxes but underestimates cloud water mixing ratio over the region of stratocumulus clouds. As a result, the simulated low stratocumulus clouds are less than observations. To overcome this problem, a mass flux method is employed in the shallow convection scheme to replace the original one. Meanwhile, the deep convection scheme is adjusted correspondingly. This modification is similar to that in the US NCEP Global Forecast System (GFS), which uses the simplified Arakawa Schubert Scheme (SAS). The planetary boundary layer scheme (PBL) is also revised by considering the coupling between the PBL and stratocumulus clouds. With the modification of both the cumulus and PBL schemes, the GRAPES simulation of shallow convective heating rate becomes more reasonable; total amounts of stratocumulus clouds simulated over the eastern Pacific and their vertical structure are more consistent with observations; the underestimation of stratocumulus clouds simulated by original schemes is less severe with the revised schemes. Precipitation distribution in the tropics becomes more reasonable and spurious precipitation is effectively suppressed. The westward extension and northward movement of the western Pacific subtropical high simulated with the revised schemes are more consistent with Final Operational Global Analysis (FNL) than that simulated with the original schemes. The statistical scores for the global GRAPES forecast are generally improved with the revised schemes, especially for the simulation of geopotential height in the Northern Hemisphere and winds in the tropics. Root mean square errors (RMSEs) decrease in the lower and upper troposphere with the revised schemes. The above results indicate that with the revised cumulus and PBL schemes, model biases in the tropics decrease and the global GRAPES performance is greatly improved.

Improvements in the representation of the Indian summer monsoon in the NCEP climate forecast system version 2

A new triggering mechanism for deep convection based on the heated condensation framework (HCF) is implemented into the National Centers for Environmental Prediction climate forecast system version 2 (CFSv2). The new trigger is added as an additional criterion in the simplified Arakawa–Schubert scheme for deep convection. Seasonal forecasts are performed to evaluate the influence of the new triggering mechanism in the representation of the Indian summer monsoon in the CFSv2. The HCF trigger improves the seasonal representation of precipitation over the Indian subcontinent. The new triggering mechanism leads to a significant, albeit relatively small, improvement in the bias of seasonal precipitation totals. In addition, the new trigger improves the representation of the seasonal precipitation cycle including the monsoon onset, and the probability distribution of precipitation intensities. The mechanism whereby the HCF improves convection over India seems to be related not only to a better representation of the background state of atmospheric convection but also to an increase in the frequency in which SAS is triggered. As a result, there was an increase in convective precipitation over India favored by the availability of moist convective instability. The increase in precipitation intensity leads to a reduction in the dry bias.

Simulation of the Summer Monsoon Rainfall over East Asia Using the NCEP GFS Cumulus Parameterization at Different Horizontal Resolutions

Abstract The most recent version of the simplified Arakawa–Schubert (SAS) cumulus scheme in the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) (GFS SAS) is implemented in the Weather Research and Forecasting (WRF) Model with a modification of the triggering condition and the convective mass flux in order to make it dependent on the model’s horizontal grid spacing. The East Asian summer monsoon season of 2006 is selected in order to evaluate the performance of the modified GFS SAS scheme. In comparison to the original GFS SAS scheme, the modified GFS SAS scheme shows overall better agreement with the observations in terms of the simulated monsoon rainfall. The simulated precipitation from the original GFS SAS scheme is insensitive to the model’s horizontal grid spacing, which is counterintuitive because the portion of the resolved clouds in a grid box should increase as the model grid spacing decreases. This behavior of the original GFS SAS scheme is alleviated by the modified GFS SAS scheme. In addition, three different cumulus schemes (Grell and Freitas, Kain and Fritsch, and Betts–Miller–Janjić) are chosen to investigate the role of a horizontal resolution on the simulated monsoon rainfall. Although the forecast skill of the surface rainfall does not always improve as the spatial resolution increases, the improvement of the probability density function of the rain rate with the smaller grid spacing is robust regardless of the cumulus parameterization scheme.

The impact of revised simplified Arakawa–Schubert convection parameterization scheme in CFSv2 on the simulation of the Indian summer monsoon

Keeping the systematic bias of the climate forecast system model version 2 (CFSv2) in mind, an attempt is made to improve the Indian summer monsoon (ISM) rainfall variability in the model from diurnal through daily to seasonal scale. Experiments with default simplified Arakawa–Schubert (SAS) and a revised SAS schemes are carried out to make 15 years climate run (free run) to evaluate the model fidelity with revised SAS as compared to default SAS. It is clearly seen that the revised SAS is able to reduce some of the biases of CFSv2 with default SAS. Improvement is seen in the annual seasonal cycle, onset and withdrawal but most importantly the rainfall probability distribution function (PDF) has improved significantly. To understand the reason behind the PDF improvement, the diurnal rainfall simulation is analysed and it is found that the PDF of diurnal rainfall has significantly improved with respect to even a high resolution CFSv2 T382 version. In the diurnal run with revised SAS, the PDF of rainfall over central India has remarkably improved. The improvement of diurnal cycle of total rainfall has actually been contributed by the improvement of diurnal cycle of convection and associated convective rainfall. This is reflected in outgoing longwave radiation and high cloud diurnal cycle. This improvement of convective cycle has resolved a long standing problem of dry bias by CFSv2 over Indian land mass and wet bias over equatorial Indian Ocean. Besides the improvement, there are some areas where there are still scopes for further development. The cold tropospheric temperature bias, low cloud fractions need further improvement. To check the role of shallow convection, another free run is made with revised SAS along with shallow convection (SC). The major difference between the new and old SC schemes lies in the heating and cooling behavior in lower-atmospheric layers above the planetary boundary layer. However, the inclusion of revised SC scheme could not show much improvement as compared to revised SAS with deep convection. Thus, it seems that revised SAS with deep convection can be a potentially better parameterization scheme for CFSv2 in simulating ISM rainfall variability.

Sensitivity of simulated intraseasonal oscillation to four convective parameterization schemes in a coupled climate model

This paper investigates the sensitivity of a simulated tropical precipitation climatology focusing on the intraseasonal oscillation (ISO) to four convective parameterization schemes: simplified Arakawa-Schubert (SAS), relaxation Arakawa-Schubert (RAS), new Kain-Fritsch (KF2), National Center for Atmospheric Research (NCAR) Climate Model version 3 (CCM). An 8-year boreal summer climatology from 1997 to 2004 is constructed using an ocean-atmosphere coupled global climate model (GCM). The simulated tropical precipitation climatology shows that all four experiments capture the observed climatology fairly well, with the pattern correlation coefficients greater than 0.8. The ensemble mean of results from the four experiments does not reveal a benefit in reproducing the observed precipitation climatology or the ISO signals. Although the KF2 scheme has been most widely tested and updated in mesoscale modeling communities, its capability in tropical climate simulation is shown to be relatively good in terms of sea surface temperature (SST) and precipitation. Results from the SAS and KF2 schemes show similar patterns in terms of climatology and ISO signals, with a greater precipitation variance than that from other experiments. The ISO signals from the RAS run show relatively realistic ISO signals, but with too strong intensity. Our study implies that the appropriate partitioning of deep convection due to cumulus parameterization scheme and stratiform precipitation due to microphysics scheme should be taken into account when developing or revising physics algorithms in coupled GCMs.

Boreal summer intraseasonal variability simulated in the NCEP climate forecast system: insights from moist static energy budget and sensitivity to convective moistening

The NCEP Climate Forecast System (CFS) with the relaxed Arakawa Schubert (RAS, hereafter referred to as CTRL) convection scheme of Moorthi and Suarez exhibits better performance in representing boreal summer tropical intraseasonal variability as compared with a simulation using simplified Arakawa–Schubert scheme. The intraseasonal moist static energy (MSE) budget is analyzed in this version of the CFS model (CTRL), which produces realistic eastward and northward propagation characteristics. The moist and thermodynamic processes involved in the maintenance and propagation of the poleward moving intraseasonal oscillation (ISO) disturbances are examined here. Budget diagnostics show that horizontal MSE advection is the principal component of the budget, contributing to the poleward movement of the convection. The injection of MSE moistens the atmosphere north of the convective area causing the poleward movement of convection by destabilization of the atmosphere. The moistening process is mainly contributed by the climatological wind acting on the anomalous moisture gradient as confirmed from the examination of moisture advection equation. While surface enthalpy fluxes (consisting of radiative and surface turbulent heat fluxes) maintain the ISO anomalies, they oppose the MSE tendency due to horizontal advection thus regulating the poleward propagation characteristics. In addition, the model results show that wind–evaporation feedback dominates over cloud–radiation feedback for ISO propagation; this is in contrast to our estimates using the newly available European Centre for Medium Range Weather Forecasts Interim reanalysis. Sensitivity experiments suggest that intraseasonal variability in the CFS model with the RAS scheme is highly sensitive to the parameterization of both the shallow convection and the convective rain evaporation and downdrafts. Removal of these components adversely affects the propagation characteristics and greatly reduces the amplitude of intraseasonal variability. Our results support the primary importance of the moisture preconditioning ahead of the ISO and the physical relationship between moisture and precipitation. For realistic ISO simulations, models need to represent these features appropriately.

Precipitation in Boreal Summer Simulated by a GCM with Two Convective Parameterization Schemes: Implications of the Intraseasonal Oscillation for Dynamic Seasonal Prediction

Abstract In this paper, the intraseasonal oscillation (ISO) and its possible link to dynamical seasonal predictability within a general circulation model framework is investigated. Two experiments with different convection scheme algorithms, namely, the simplified Arakawa–Schubert (SAS) and the relaxed Arakawa–Schubert (RAS) convection algorithms, were designed to compare seasonal simulations from 1979 to 2002 on a seasonal model intercomparison project (SMIP)-type simulation test bed. Furthermore, the wave characteristics (wave intensity, period, and propagation) of the simulated ISO signal provided by the model with two different convection schemes for extended boreal summers from 1997 to 2004 were compared to the observational ISO signal. Precipitation in the boreal summer was fairly well simulated by the model irrespective of the convection scheme used, but the RAS run outperformed the SAS run with respect to tabulated skill scores. Decomposition of the interannual variability of boreal summer precipitation based on observations and model results demonstrates that the seasonal predictability of precipitation is dominated by the intraseasonal component over the warm pool area and the SST-forced signal over the equatorial Pacific Ocean, implying that the seasonal mean anomalies are more predictable under active ISO conditions as well as strong ENSO conditions. Comparison of the ISO simulations with the observations revealed that the main features, such as the intensity of precipitation variance in the intraseasonal time scale and the evolution of propagating ISOs, were reproduced fairly well by the model; however, the wave characteristics associated with the ISO signals were better captured by the experiment with the RAS scheme than the SAS scheme. This study further suggests that accurate simulation of the ISO can improve the seasonal predictability of dynamical seasonal prediction systems.

Test of a convectively forced gravity wave drag parameterization in a general circulation model

The influence of gravity wave drag induced by cumulus convection (GWDC) on a simulated boreal summer climate was evaluated in a general circulation model. For this, the GWDC scheme developed by Chun and Baik was implemented into a version of the National Centers for Environmental Prediction (NCEP) global spectral model (GSM). Ensemble simulations with the two different convection schemes, the simplified Arakawa-Schubert (SAS) scheme and Community Climate Model (CCM) convection scheme, were conducted for the boreal summer of 1996. A cloud factor to modulate the stress intensity with respect to the cloud type was introduced in this study, in order to prevent unrealistic behaviors of the GWDC scheme in GSM. The effect of gravity wave drag on the zonal mean of wind and temperature fields was focused. On the whole, the effect of GWDC in this study is positive on the simulated seasonal climate. It is evident that biases in temperature in the polar region as well as in the zonal and meridional winds in the upper atmosphere are reduced. The percentage of reduction of the bias in zonal winds is about 10–20%. Such a response of the GWDC forcing widely appears not only in tropical regions but also in mid-latitude regions. These characteristics are prominent in the case of the SAS scheme, which is due to the various convective cloud types. The magnitude of GWDC forcing is generally small, but still positive, in the case of the CCM scheme, which is due to rather homogeneous cloud types. It is also found that the role of a particular GWDC forcing depends upon the inherent systematic biases of a particular model. It is concluded that incorporation of the GWDC parameterization in GCMs should be taken into account to improve the seasonal prediction.

Improved Forecasts of the Diurnal Cycle in the Tropics Using Multiple Global Models. Part I: Precipitation

Abstract The Tropical Rainfall Measuring Mission (TRMM) satellite supplemented with the Defense Meteorological Satellites Program (DMSP) microwave dataset provides accurate rain-rate estimates. Furthermore, infrared radiances from the geostationary satellites provide the possibility for mapping the diurnal change of tropical rainfall. Modeling of the phase and amplitude of the tropical rainfall is the theme of this paper. The present study utilizes a suite of global multimodels that are identical in all respects except for their cumulus parameterization algorithms. Six different cumulus parameterizations are tested in this study. These include the Florida State University (FSU) Modified Kuo Scheme (KUO), Goddard Space Flight Center (GSFC) Relaxed Arakawa–Schubert Scheme (RAS1), Naval Research Laboratory–Navy Operational Global Atmospheric Prediction System (NRL–NOGAPS) Relaxed Arakawa–Schubert Scheme (RAS2), NCEP Simplified Arakawa–Schubert Scheme (SAS), NCAR Zhang–McFarlane Scheme (ZM), and NRL–NOGAPS Emanuel Scheme (ECS). The authors carried out nearly 600 experiments with these six versions of the T170 Florida State University global spectral model. These are 5-day NWP experiments where the diurnal change datasets were archived at 3-hourly intervals. This study includes the estimation of skills of the phase and amplitudes of the diurnal rain using these member models, their ensemble mean, a multimodel superensemble, and those from a single unified model. Test results are presented for the global tropics and for some specific regions where the member models show difficulty in predicting the diurnal change of rainfall. The main contribution is the considerable improvement of the modeling of diurnal rain by deploying a multimodel superensemble and by constructing a single unified model. The authors also present a comparison of these findings on the modeling of diurnal rain from another suite of multimodels that utilized different versions of cloud radiation algorithms (instead of different cumulus parameterization schemes) toward defining the suite of multimodels. The principal result is that the superensemble does provide a future forecast for the total daily rain and for the diurnal change of rain through day 5 that is superior to forecasts provided by the best model. The training of the superensemble with good observed estimates of rain, such as those from TRMM, is necessary for such forecasts.