The effect of initial conditions on Atmospheric General Circulation Model

A brief description of a nine-level grid-point global atmospheric general circulation model is presented with the emphasis on the physics parameterizations. This model was developed by the modeling group from Institute of Atmospheric Physics as one task of the CO2-Climate cooperation project between Chinese Academy of Sciences and United States Department of Energy. The task was initiated by Qing-Cun Zeng (IAP) and Robert D. Cess (SUNY). The operational design, computer coding and climate simulation tuning of the model were mainly carried out by Xue-Hong Zhang (for dynamics) and the author (for physics) in SUNY at Stony Brook. The final version was frozen in September 1990. Preliminary diagnoses showed that the model reproduces principal features of the observed climatology.

The rapid development of science and technology has enabled finer and finer resolutions in atmospheric general circulation models (AGCMs). Parallelization becomes progressively more critical as the resolution of AGCMs increases. This paper presents a new parallel version of the finite-difference Gridpoint Atmospheric Model of the Institute of Atmospheric Physics (IAP)–State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG; GAMIL) with various parallel optimization strategies, including two-dimensional hybrid parallel decomposition; hybrid parallel programming; parallel communications for coupling the physical packages, land surface, and dynamical core; and a cascading solution to the tridiagonal equations used in the dynamical core. The new parallel version under two different horizontal resolutions (1° and 0.25°) is evaluated. The new parallel version enables GAMIL to achieve higher parallel efficiency and utilize a greater number of CPU cores. GAMIL1° achieves 37.8% parallel efficiency using 960 CPU cores, while GAMIL0.25° achieves 57.5% parallel efficiency.

The purpose of this book is to provide an introduction to numerical modelling of the ocean and the atmosphere. It originates from courses given at Stockholm University and is intended to serve as a textbook for students in meteorology and oceanography with a background in mathematics and physics. Focus is on numerical schemes for the most commonly used equations in oceanography and meteorology as well as on the stability, precision and other properties of these schemes. Simple equations capturing the properties of the primitive equations employed in models of the ocean and atmosphere will be used. These model equations are solved numerically on a grid by discretisation, the derivatives of the differential equations being replaced by finite-difference approximations. The focus will be on the basic numerical methods used for oceanographic and atmospheric modelling. These models are based on the Navier-Stokes equations (including the Coriolis effect) and a tracer equation for heat in both the atmosphere and ocean and tracer equations for humidity and salt in the atmosphere and ocean, respectively. A coupled atmospheric and oceanic general circulation model represents the core part of an Earth System climate model. The book starts by presenting the most common types of partial differential equations and finite difference schemes used in meteorology and oceanography. Subsequently the limitations of these numerical schemes as regards stability, accuracy, presence of computational modes and accuracy the computationally determined phase speed are discussed. The shallow-water equations are discretised for different spatial grids and friction and diffusion terms are introduced. Hereafter implicit and semi-implicit schemes are discussed as well as the semi-Lagrangian technique. Coordinates for atmospheric as well as oceanic models are presented as well as a highly simplified 3D model. A brief description is given of how some atmospheric general circulation models use spectral methods as "horizontal coordinates". Finally, some "pen-and-paper" theoretical exercises and a number of GFD computer exercises are given.

This work presents snapshot simulations of the late 20th and late 21st century Antarctic climate under the RCP8.5 scenario carried out with an empirically bias‐corrected global atmospheric general circulation model (AGCM), forced with bias‐corrected sea‐surface temperatures and sea ice and run with about 100‐km resolution over Antarctica. The bias correction substantially improves the simulated mean late 20th century climate. The simulated atmospheric circulation of the bias‐corrected model compares very favorably to the best available AMIP (Atmospheric Model Intercomparison Project)‐type climate models. The simulated interannual circulation variability is improved by the bias correction. Depending on the metric, a slight improvement or degradation is found in the simulated variability on synoptic timescales. The simulated climate change over the 21st century is broadly similar in the corrected and uncorrected versions of the atmospheric model, and atmospheric circulation patterns are not geographically “pinned” by the applied bias correction. These results suggest that the method presented here can be used for bias‐corrected climate projections. Finally, the authors discuss different possible choices in terms of the place of bias corrections and other intermediate steps in the modeling chain leading from global coupled climate simulations to impact assessment.

The sensitivity of cumulus convection parameterizations is investigated using the CPTEC/COLA Atmospheric General Circulation Model (AGCM) with T62L28 resolution. This model has been used at CPTEC/INPE since 1995 with the Kuo convective scheme for weather and seasonal climate forecasts. In this study, two sets of integrations are performed using climatological Sea Surface Temperature (SST) of the Southern Hemisphere summer season (December, January and February) as bottom boundary conditions. Five integrations with different initial conditions are applied for each ensemble. The study was divided in two groups, one using the adjusted Relaxed Arakawa-Schubert convection scheme considering modifications in the convection physics (ARAS) and the other one using the Kuo convection scheme (KUO). The atmospheric circulation and precipitation model results are compared with NCEP/NCAR reanalysis data and CMAP precipitation data. The results are analyzed mainly over South America and also for the Southern Hemisphere to verify the model response compared to observed data when different convection scheme is applied. The adjusted scheme for RAS suggested in this study, reduced errors in several areas of South America, when comparing with the previous version. Over most of South America areas KUO gives smaller errors than ARAS. Over tropical Pacific Ocean, Southeastern Brazil and south of northeast Brazil, ARAS scheme shows better results.

Extraseasonal ensemble numerical predictions of winter climate over China

The sea surface temperature (SST)‐forced predictability in precipitation is investigated in terms of the seasonal mean modes (SMMs) for June–July (JJ) and the sub‐seasonal mode (SSM) using the 24‐year six‐member ensembles simulation with an atmospheric general circulation model (AGCM). The SSM was defined as a 20‐day window length by an extended empirical orthogonal function (EOF) over the East Asian monsoon region. For the JJ mean rainfall, the first EOF of the model ensemble is considered to be the forced SST mode, with being an El Niño Southern Oscillation (ENSO) related pattern from the regressed SST field. The first mode of model was well correlated with the second mode of the observation in the spatial and temporal variation.In the potential predictability, the analysis of variance (ANOVA) was used for the mean mode and the SSM. The potential predictability for the SMM is improved over the north of the 30°N. Compared with JJ mean mode, the SSM has more SST‐forced variance and less internal variability. Copyright © 2008 Royal Meteorological Society

Atmospheric general circulation models were originally developed for such practical applications as weather forecasts and climate prediction. They can also be used to investigate the basic properties of atmospheric circulations by performing numerical experiments under suitable conditions. This kind of usage is similar to certain kinds of laboratory experiments: baroclinic disturbances can be studied using a rotating annulus by changing the temperature difference between inner and outer annuli or rotation speed; and various aspects of atmospheric general circulation can also be examined using atmospheric general circulation models by artificially changing their governing parameters.

A land surface model (LSM) has been included in the ECMWF Hamburg version 4 (ECHAM4) atmospheric general circulation model (AGCM). The LSM is an early version of the Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) and it replaces the simple land surface scheme previously included in ECHAM4. The purpose of this paper is to document how a more exhaustive consideration of the land surface–vegetation processes affects the simulated boreal summer surface climate. To investigate the impacts on the simulated climate, different sets of Atmospheric Model Intercomparison Project (AMIP)-type simulations have been performed with ECHAM4 alone and with the AGCM coupled with ORCHIDEE. Furthermore, to assess the effects of the increase in horizontal resolution the coupling of ECHAM4 with the LSM has been implemented at different horizontal resolutions. The analysis reveals that the LSM has large effects on the simulated boreal summer surface climate of the atmospheric model. Considerable impacts are found in the surface energy balance due to changes in the surface latent heat fluxes over tropical and midlatitude areas covered with vegetation. Rainfall and atmospheric circulation are substantially affected by these changes. In particular, increased precipitation is found over evergreen and summergreen vegetated areas. Because of the socioeconomical relevance, particular attention has been devoted to the Indian summer monsoon (ISM) region. The results of this study indicate that precipitation over the Indian subcontinent is better simulated with the coupled ECHAM4–ORCHIDEE model compared to the atmospheric model alone.

Abstract. Climate proxy data provide noisy, and spatially incomplete information on some aspects of past climate states, whereas palaeosimulations with climate models provide global, multi-variable states, which may however differ from the true states due to unpredictable internal variability not related to climate forcings, as well as due to model deficiencies. Using data assimilation for combining the empirical information from proxy data with the physical understanding of the climate system represented by the equations in a climate model is in principle a promising way to obtain better estimates for the climate of the past. Data assimilation has been used for a long time in weather forecasting and atmospheric analyses to control the states in atmospheric General Circulation Models such that they are in agreement with observation from surface, upper air, and satellite measurements. Here we discuss the similarities and the differences between the data assimilation problem in palaeoclimatology and in weather forecasting, and present and conceptually compare three data assimilation methods that have been developed in recent years for applications in palaeoclimatology. All three methods (selection of ensemble members, Forcing Singular Vectors, and Pattern Nudging) are illustrated by examples that are related to climate variability over the extratropical Northern Hemisphere during the last millennium. In particular it is shown that all three methods suggest that the cold period over Scandinavia during 1790–1820 is linked to anomalous northerly or easterly atmospheric flow, which in turn is related to a pressure anomaly that resembles a negative state of the Northern Annular Mode.

This article uses an atmospheric general circulation model to show that the cooler North Atlantic sea surface temperature anomalies in summer can produce a Rossby wave‐like teleconnection pattern strengthening the Siberian High in winter and next spring. The stronger Siberian High enhances the continent northerlies over East Asia and the associated cyclonic circulation over the western North Pacific, which provides a tropical westerly background for occurrence of the westerly wind bursts. The stronger northerlies over East Asia can also induce frequent cold surges, which tend to produce the equatorial westerly wind anomalies in the western Pacific for initiating an El Niño event. Copyright © 2013 Royal Meteorological Society

Capabilities of the Grid-point Atmospheric Model of Institute of Atmospheric Physics/National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (IAP/LASG; GAMIL) and the Spectral Atmospheric General Circulation Model of IAP/LASG (SAMIL) in simulating the Spring Persistent Rainfall (SPR) in East Asia are evaluated by the Global Precipitation Climatology Project (GPCP). Similar to that observed in the Coupled Model Intercomparison Project Phases 3 and 5 (CMIP3/CMIP5) ensembles, rainfall amount in both GAMIL and SAMIL are slightly overestimated over the SPR domain and are underestimated over the SPR center. For the spatial structure, the northeast–southwest tilted rain belt could not be effectively reproduced by GAMIL, and the main rain belt shifts to the north of its normal position in SAMIL. According to the Taylor diagram, SAMIL generally shows better performances in simulating the SPR pattern. Similar to other atmospheric general circulation models (AGCMs) employed in CMIP3/CMIP5, the thermal contrast between East Asia and the South China Sea is overestimated in SAMIL and GAMIL. The overestimated meridional thermal contrast intensifies the southerly water vapor supply and favors the northward shifting of main rain belt.

The paper introduces a moist, deterministic test case of intermediate complexity for Atmospheric General Circulation Models (AGCMs). We suggest pairing an AGCM dynamical core with simple physical parameterizations to test the evolution of a single, idealized, initially weak vortex into a tropical cyclone. The initial conditions are based on an initial vortex seed that is in gradient‐wind and hydrostatic balance. The suggested “simple‐physics” package consists of parameterizations of bulk aerodynamic surface fluxes for moisture, sensible heat and momentum, boundary layer diffusion, and large‐scale condensation. Such a configuration includes the important driving mechanisms for tropical cyclones, and leads to a rapid intensification of the initial vortex over a forecast period of ten days. The simple‐physics test paradigm is not limited to tropical cyclones, and can be universally applied to other flow fields. The physical parameterizations are described in detail to foster model intercomparisons. The characteristics of the intermediate‐complexity test case are demonstrated with the help of four hydrostatic dynamical cores that are part of the Community Atmosphere Model version 5 (CAM 5) developed at the National Center for Atmospheric Research (NCAR). In particular, these are the Finite‐Volume, Spectral Element, and spectral transform Eulerian and semi‐Lagrangian dynamical cores that are coupled to the simple‐physics suite. The simulations show that despite the simplicity of the physics forcings the models develop the tropical cyclone at horizontal grid spacings of about 55 km and finer. The simple‐physics simulations reveal essential differences in the storm's structure and strength due to the choice of the dynamical core. Similar differences are also seen in complex full‐physics aqua‐planet experiments with CAM 5 which serve as a motivator for this work. The results suggest that differences in complex full‐physics simulations can be, at least partly, replicated in simplified model setups. The simplified experiments might therefore provide easier access to an improved physical understanding of how the dynamical core and moist physical parameterizations interact. It is concluded that the simple‐physics test case has the potential to close the gap between dry dynamical core assessments and full‐physics aqua‐planet experiments, and can shed light on the role of the dynamical core in the presence of moisture processes.

The mean evolution of the West African monsoon (WAM) circulation and its interannual variability have been studied using an ensemble of 21 simulations (common period 1961–1994) performed with four different atmospheric general circulation models (AGCMs) (European Center/Hamburg (ECHAM) 3, ECHAM 4, Action de Recherche Petite Echelle Grande Echelle (ARPEGE), and Goddard Institute for Space Studies (GISS)) and forced by the same observed sea surface temperature (SST) data set. The results have been compared with European Centre for Medium‐Range Weather Forecasts reanalyses (ERA‐40). The climatological means of WAM winds for the AGCMs are similar to the ERA‐40 ones. However, the AGCMs tend to underestimate the southern wind component at low levels around 10°N compared to the ERA‐40. The simulated Tropical Easterly Jet (TEJ) is usually shifted northward and also too weak for ECHAM 3 and ECHAM 4 compared to ERA‐40. The interannual variability of an atmospheric WAM index (WAMI) is quite successfully reproduced (the correlations between the mean ensemble of each AGCM and ERA‐40 time series over 1961–1994 range between 0.51 and 0.64). In particular, the four AGCMs reproduce quite well the mean teleconnection structure with El Niño–Southern Oscillation, i.e., a strong (weak) monsoon during La Niña (El Niño) events, even if the largest absolute correlations between WAMI and SST in the eastern and central equatorial Pacific are weaker than in ERA‐40. On a yearly basis, WAMI is more predictable and skillful during the cold ENSO years than during the warm ENSO ones. The unskillful warm ENSO events are associated with a significant cooling over the equatorial Atlantic and Western Pacific Ocean and a significant warming in the tropical Indian Ocean.

Key Points Objective parameter estimation using Ensemble Kalman Filter techniques Cumulus‐convection scheme of an atmospheric general circulation model Applying numerical weather prediction techniques to a climate model