scholarly journals Radiative Fluxes in the ECHAM5 General Circulation Model

2006 ◽  
Vol 19 (16) ◽  
pp. 3792-3809 ◽  
Author(s):  
Martin Wild ◽  
Erich Roeckner
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Shortwave Radiation ◽  
Radiative Transfer Model ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Transfer Model ◽  
Radiative Fluxes ◽  
Good Agreement

Abstract Radiative fluxes in the ECHAM5 general circulation model (GCM) are evaluated using both surface and satellite-based observations. The fluxes at the top of the atmosphere (TOA) are generally in good agreement with the satellite data. Larger deviations in simulated cloud forcing are found especially at lower latitudes where the shortwave component within the intertropical convergence zone is overestimated during boreal summer and underestimated in the marine stratocumulus regimes, especially during boreal winter. At the surface the biases in the radiative fluxes are significantly smaller than in earlier versions of the same model and in other GCMs. The shortwave clear-sky fluxes are shown to be in good agreement with newly derived observational estimates. Compared to the preceding model version, ECHAM4, the spurious absorption of solar radiation in the cloudy atmosphere disappears due to the higher resolution in the near-infrared bands of the shortwave radiation code. This reduces the biases with respect to collocated surface and TOA observations. It is illustrated that remaining biases in atmospheric absorption may be related to the crude aerosol climatology, which does not account for high loadings of absorbing aerosol such as from biomass burning, whereas the biases disappear in areas and seasons where aerosol effects are less important. In the longwave, the introduction of the Rapid Radiative Transfer Model (RRTM) radiation code leads to an increase in the longwave downward flux at the surface at high latitudes, thereby reducing biases typically found in GCMs. The considerable skill in the simulation of the fluxes at the earth’s surface underlines the suitability of ECHAM5 as an atmospheric component of an integrated earth system model.

scholarly journals Air, Sea, and Land Interactions of the Continental U.S. Hydroclimate

2009 ◽  
Vol 10 (2) ◽  
pp. 353-373 ◽  
Author(s):  
Vasubandhu Misra ◽  
P. A. Dirmeyer
Keyword(s):  
United States ◽  
General Circulation Model ◽  
General Circulation ◽  
Winter Season ◽  
Circulation Model ◽  
Moisture Flux ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Moisture Flux Convergence ◽  
The Mean

Abstract Multidecadal simulations over the continental United States by an atmospheric general circulation model coupled to an ocean general circulation model is compared with that forced by observed sea surface temperature (SST). The differences in the mean and the variability of precipitation are found to be larger in the boreal summer than in the winter. This is because the mean SST differences in the two simulations are qualitatively comparable between the two seasons. The analysis shows that, in the boreal summer season, differences in moisture flux convergence resulting from changes in the circulation between the two simulations initiate and sustain changes in precipitation between them. This difference in precipitation is, however, further augmented by the contributions from land surface evaporation, resulting in larger differences of precipitation between the two simulations. However, in the boreal winter season, despite differences in the moisture flux convergence between the two model integrations, the precipitation differences over the continental United States are insignificant. It is also shown that land–atmosphere feedback is comparatively much weaker in the boreal winter season.

scholarly journals Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

2007 ◽  
Vol 7 (10) ◽  
pp. 2503-2515 ◽  
Author(s):  
C. Cagnazzo ◽  
E. Manzini ◽  
M. A. Giorgetta ◽  
P. M. De F. Forster ◽  
J. J. Morcrette
Keyword(s):  
Radiative Transfer ◽  
General Circulation Model ◽  
Spectral Resolution ◽  
General Circulation ◽  
Circulation Model ◽  
Shortwave Radiation ◽  
Transfer Model ◽  
Ozone Absorption ◽  
Zonal Winds ◽  
Radiation Scheme

Abstract. In order to improve the representation of ozone absorption in the stratosphere of the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased from 4 to 6 bands. Two 20-years simulations with the general circulation model have been performed, one with the standard and the other with the newly introduced parameterization respectively, to evaluate the temperature and dynamical changes arising from the two different representations of the shortwave radiative transfer. In the simulation with the increased spectral resolution in the radiation parameterization, a significant warming of almost the entire model domain is reported. At the summer stratopause the temperature increase is about 6 K and alleviates the cold bias present in the model when the standard radiation scheme is used. These general circulation model results are consistent both with previous validation of the radiation scheme and with the offline clear-sky comparison performed in the current work with a discrete ordinate 4 stream scattering line by line radiative transfer model. The offline validation shows a substantial reduction of the daily averaged shortwave heating rate bias (1–2 K/day cooling) that occurs for the standard radiation parameterization in the upper stratosphere, present under a range of atmospheric conditions. Therefore, the 6 band shortwave radiation parameterization is considered to be better suited for the representation of the ozone absorption in the stratosphere than the 4 band parameterization. Concerning the dynamical response in the general circulation model, it is found that the reported warming at the summer stratopause induces stronger zonal mean zonal winds in the middle atmosphere. These stronger zonal mean zonal winds thereafter appear to produce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In addition, the comparison of the two simulations performed with the general circulation model shows that the increase in the spectral resolution of the shortwave radiation and the associated changes in the cloud optical properties result in a warming (0.5–1 K) and moistening (3%–12%) of the upper tropical troposphere. By comparing these modeled differences with previous works, it appears that the reported changes in the solar radiation scheme contribute to improve the model mean temperature also in the troposphere.

scholarly journals The Sensitivity of Rainfall Estimation to Error Assumptions in a Bayesian Passive Microwave Retrieval Algorithm

2015 ◽  
Vol 54 (2) ◽  
pp. 408-422 ◽  
Author(s):  
Gregory S. Elsaesser ◽  
Christian D. Kummerow
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Mathematical Expression ◽  
Passive Microwave ◽  
Bayesian Probability ◽  
Radiative Transfer Model ◽  
Retrieval Algorithm ◽  
Transfer Model ◽  
Model Errors ◽  
Frequency Distributions

AbstractThe Goddard profiling algorithm (GPROF) uses Bayesian probability theory to retrieve rainfall over the global oceans. A critical component of GPROF and most Bayes theorem–based retrieval frameworks is the specification of uncertainty in the observations being utilized to retrieve the parameter of interest. In the case of GPROF, for any sensor, uncertainties in microwave brightness temperatures (Tbs) arise from radiative transfer model errors, satellite sensor noise and/or degradation, and nonlinear, scene-dependent Tb offsets added during sensor intercalibration procedures. All mentioned sources impact sensors in a varying fashion, in part because of sensor-dependent fields of view. It is found that small errors in assumed Tb uncertainty (ranging from 0.57 K at 10 GHz to 2.29 K at 85 GHz) lead to a 3.6% change in the retrieved global-average oceanic rainfall rate, and 10%–20% (20%–40%) shifts in the pixel-level (monthly) frequency distributions for given rainfall bins. A mathematical expression describing the sensitivity of retrieved rainfall to uncertainty is developed here. The strong global sensitivity is linked to rainfall variance scaling systematically as Tb varies. For ocean scenes, the same emission-dominated rainfall–Tb physics used in passive microwave rainfall retrieval is also responsible for the substantial underestimation (overestimation) of global rainfall if uncertainty is overestimated (underestimated). Proper uncertainties are required to quantify variability in surface rainfall, assess long-term trends, and provide robust rainfall benchmarks for general circulation model evaluations. The implications for assessing global and regional biases in active versus passive microwave rainfall products, and for achieving rainfall product agreement among a constellation of orbiting microwave radiometers [employed in the Global Precipitation Measurement (GPM) mission], are also discussed.

Validation of General Circulation Model Radiative Fluxes Using Surface Observations

1995 ◽  
Vol 8 (5) ◽  
pp. 1309-1324 ◽  
Author(s):  
Martin Wild ◽  
Atsumu Ohmura ◽  
Hans Gilgen ◽  
Erich Roeckner
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Radiative Fluxes ◽  
Surface Observations

Parameterization of PBL Processes in an Atmospheric General Circulation Model: Description and Preliminary Assessment

2009 ◽  
Vol 137 (3) ◽  
pp. 1061-1082 ◽  
Author(s):  
Celal S. Konor ◽  
Gabriel Cazes Boezio ◽  
Carlos R. Mechoso ◽  
Akio Arakawa
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Atmospheric General Circulation Model ◽  
Potential Temperature ◽  
Circulation Model ◽  
Surface Fluxes ◽  
Shortwave Radiation ◽  
Model Description ◽  
Atmospheric General Circulation ◽  
Seasonal And Diurnal Variations

Abstract This paper presents the basic features of a newly developed planetary boundary layer (PBL) parameterization, and the performance assessment of a version of the University of California, Los Angeles (UCLA), Atmospheric General Circulation Model (AGCM) to which the parameterization is incorporated. The UCLA AGCM traditionally uses a framework in which a sigma-type vertical coordinate for the PBL shares a coordinate surface with the free atmosphere at the PBL top. This framework facilitates an explicit representation of processes concentrated near the PBL top, which is crucially important especially for predicting PBL clouds. In the new framework, multiple layers are introduced between the PBL top and earth’s surface, allowing for predictions of the vertical profiles of potential temperature, total water mixing ratio, and horizontal winds within the PBL. The vertically integrated “bulk” turbulent kinetic energy (TKE) is also predicted for the PBL. The PBL-top mass entrainment is determined through an equation including the effects of TKE and the radiative and evaporative cooling processes concentrated near the PBL top. The surface fluxes are determined from an aerodynamic formula in which the velocity scale depends both on the square root of TKE and the grid-scale PBL velocity at the lowermost model layer. The turbulent fluxes within the PBL are determined through an approach that includes the effects of both large convective and small diffusive eddies. AGCM simulations with the new formulation of PBL are analyzed with a focus on the seasonal and diurnal variations. The simulated seasonal cycle of stratocumulus over the eastern oceans is realistic, as are the diurnal cycles of the PBL depth and precipitation over land. The simulated fluxes of latent heat, momentum, and shortwave radiation at the ocean surface and baroclinic activity in the middle latitudes show significant improvements over the previous versions of the AGCM based on the single-layer PBL.

scholarly journals Subtropical Dipole Modes Simulated in a Coupled General Circulation Model

2012 ◽  
Vol 25 (12) ◽  
pp. 4029-4047 ◽  
Author(s):  
Yushi Morioka ◽  
Tomoki Tozuka ◽  
Sebastien Masson ◽  
Pascal Terray ◽  
Jing-Jia Luo ◽  
...  
Keyword(s):  
General Circulation Model ◽  
Latent Heat ◽  
Mixed Layer ◽  
General Circulation ◽  
Circulation Model ◽  
South Atlantic ◽  
Shortwave Radiation ◽  
Coupled General Circulation Model ◽  
Austral Spring ◽  
Negative Pole

Abstract The growth and decay mechanisms of subtropical dipole modes in the southern Indian and South Atlantic Oceans and their impacts on southern African rainfall are investigated using results from a coupled general circulation model originally developed for predicting tropical climate variations. The second (most) dominant mode of interannual sea surface temperature (SST) variations in the southern Indian (South Atlantic) Ocean represents a northeast–southwest oriented dipole, now called subtropical dipole mode. The positive (negative) SST interannual anomaly pole starts to grow in austral spring and reaches its peak in February. In austral late spring, the suppressed (enhanced) latent heat flux loss associated with the variations in the subtropical high causes a thinner (thicker) than normal mixed layer thickness that, in turn, enhances (reduces) the warming of the mixed layer by the climatological shortwave radiation. The positive (negative) pole gradually decays in austral fall because the mixed layer cooling by the entrainment is enhanced (reduced), mostly owing to the larger (smaller) temperature difference between the mixed layer and the entrained water. The increased (decreased) latent heat loss due to the warmer (colder) SST also contributes to the decay of the positive (negative) pole. Although further verification using longer observational data is required, the present coupled model suggests that the South Atlantic subtropical dipole may play a more important role in rainfall variations over the southern African region than the Indian Ocean subtropical dipole.

scholarly journals Impact of an improved radiation scheme in the MAECHAM5 General Circulation Model

2006 ◽  
Vol 6 (6) ◽  
pp. 11067-11092 ◽  
Author(s):  
C. Cagnazzo ◽  
E. Manzini ◽  
M. A. Giorgetta ◽  
P. M. De F. Forster
Keyword(s):  
General Circulation Model ◽  
Spectral Resolution ◽  
General Circulation ◽  
Middle Atmosphere ◽  
Circulation Model ◽  
Shortwave Radiation ◽  
Optical Parameters ◽  
Discrete Ordinate ◽  
Zonal Winds ◽  
Radiation Scheme

Abstract. In order to improve the representation of the shortwave radiative transfer in the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased and extended in the UV-B and UV-C bands. The upgraded shortwave parameterization is first validated offline with a 4 stream discrete-ordinate line-by-line model. Thereafter, two 20-years simulations with the MAECHAM5 middle atmosphere general circulation model are performed to evaluate the temperature changes and the dynamical feedbacks arising from the newly introduced parameterization. The offline clear-sky comparison of the standard and upgraded parameterizations with the discrete ordinate model shows considerable improvement for the upgraded parameterization in terms of shortwave fluxes and heating rates. In the simulation with the upgraded ratiation parameterization, we report a significant warming of almost the entire atmosphere, largest at 1 hPa at the stratopause, and stronger zonal mean zonal winds in the middle atmosphere. The warming at the summer stratopause alleviates the cold bias present in the model when the standard radiation scheme is used. The stronger zonal mean zonal winds induce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In the troposphere, the changes in the spectral resolution and the associated changes in the cloud optical parameters introduce a relatively small warming and, consistenly, a moisteneing. The warming occurs mostly in the upper troposphere and can contribute to a possible improvement of the model temperature climatology.

scholarly journals Towards a better representation of the solar cycle in general circulation models

2007 ◽  
Vol 7 (1) ◽  
pp. 45-64
Author(s):  
K. M. Nissen ◽  
K. Matthes ◽  
U. Langematz ◽  
B. Mayer
Keyword(s):  
Solar Cycle ◽  
High Resolution ◽  
Heating Rate ◽  
General Circulation ◽  
General Circulation Models ◽  
Short Wave ◽  
Radiative Transfer Model ◽  
Transfer Model ◽  
Radiation Scheme ◽  
Good Agreement

Abstract. It is shown that a high-resolution short-wave (SW) heating rate parameterization is necessary to simulate solar cycle variations in atmospheric models. The improved Freie Universität Berlin (FUB) high-resolution radiation scheme (FUBRad) is introduced and compared to the 4-band ECHAM5 SW radiation scheme of Fouquart and Bonnel (FB). Both schemes are validated against the detailed radiative transfer model libRadtran. FUBRad produces realistic heating rate variations during the solar cycle and a temperature response that is in good agreement with observations. The SW heating rate response with the FB scheme is about 20 times smaller than with FUBRad and cannot produce the observed temperature signal. Comparison of the total short-wave heating rates under moderate solar conditions shows good agreement between FUBRad, FB and libRadtran up to 80 km indicating that both parameterizations are well suited for climate integrations that do not take solar variability into account. The FUBRad scheme has been implemented as a sub-submodel of the Modular Earth Submodel System (MESSy).

scholarly journals Modulation of Tropical Intraseasonal Oscillations by Ocean–Atmosphere Coupling

2006 ◽  
Vol 19 (3) ◽  
pp. 366-391 ◽  
Author(s):  
K. Rajendran ◽  
A. Kitoh
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Boreal Summer ◽  
Coupled Modes ◽  
Structure Amplitude ◽  
Ocean Atmosphere ◽  
Intraseasonal Oscillations ◽  
The Impact ◽  
Tropical Intraseasonal Oscillations

Abstract The impact of ocean–atmosphere coupling on the structure and propagation characteristics of 30–60-day tropical intraseasonal oscillations (TISOs) is investigated by analyzing long-term simulations of the Meteorological Research Institute coupled general circulation model (CGCM) and its stand-alone atmospheric general circulation model (AGCM) version forced with SSTs derived from the CGCM and comparing them with recent observation datasets [Global Precipitation Climatology Project (GPCP) precipitation, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), and Reynolds SST]. Composite events of (i) eastward propagating Madden–Julian oscillations (MJOs) during boreal winter and (ii) northward propagating intraseasonal oscillations (NPISOs) during boreal summer, constructed based on objective criteria, show that the three-dimensional structure, amplitude, and speed of propagation, and the phase relationship among surface fluxes, SST, and convection, are markedly improved in the CGCM simulation. Consistent with the frictional wave conditional instability of the second kind mechanism, successive development of low-level convergence to the east (north) of deep convection was found to be important for eastward (northward) propagation of MJO (NPISO). Complex interaction between large-scale dynamics and convection reveals the importance of atmospheric dynamics and suggests that they are intrinsic modes in the atmosphere where coupling is not essential for their existence. However, as in observations, realistic coupling in the CGCM is found to result in the evolution of TISOs as coupled modes through a coherent coupled feedback process. This acts as an amplifying mechanism for the existing propagating convective anomalies and plays an important modifying role toward a more realistic simulation of TISOs. In contrast, the simulated TISOs in its atmosphere-alone component lack many of the important features associated with their amplitude, phase, and life cycle. Thus, a realistic representation of the interaction between sea surface and the atmospheric boundary layer is crucial for a better simulation of TISOs.

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