scholarly journals Implementing the Delta-Four-Stream Approximation for Solar Radiation Computations in an Atmosphere General Circulation Model

2008 ◽  
Vol 65 (7) ◽  
pp. 2448-2457 ◽  
Author(s):  
Tarek Ayash ◽  
Sunling Gong ◽  
Charles Q. Jia
Keyword(s):  
Solar Radiation ◽  
General Circulation ◽  
Climate Modeling ◽  
Circulation Model ◽  
General Circulation Models ◽  
Computational Method ◽  
Radiation Budget ◽  
High Latitudes ◽  
Clear Sky ◽  
Winter Hemisphere

Abstract Proper quantification of the solar radiation budget and its transfer within the atmosphere is of utmost importance in climate modeling. The delta-four-stream (DFS) approximation has been demonstrated to offer a more accurate computational method of quantifying the budget than the simple two-stream approximations widely used in general circulation models (GCMs) for radiative-transfer computations. Based on this method, the relative improvement in the accuracy of solar flux computations is investigated in the simulations of the third-generation Canadian Climate Center atmosphere GCM. Relative to the computations of the DFS-modified radiation scheme, the GCM original-scheme whole-sky fluxes at the top of the atmosphere (TOA) show the largest underestimations at high latitudes of a winter hemisphere on the order of 4%–6% (monthly means), while the largest overestimations of the same order are found over equatorial regions. At the surface, even higher overestimations are found, exceeding 20% at subpolar regions of a winter hemisphere. Flux differences between original and DFS schemes are largest in the tropics and at high latitudes, where the monthly zonal means and their dispersions are within 5 W m−2 at the TOA and 10 W m−2 at the surface in whole sky, but differences may be as large as 20 and −40 W m−2. In clear sky, monthly zonal means and their dispersions remain within 2 W m−2, but may be as large as 25 and −12 W m−2. Such differences are found to be mostly determined by variations in cloud optical depth and solar zenith angle, and by aerosol loading in a clear sky.

Comparison of general circulation models to Earth Radiation Budget Experiment data: Computation of clear-sky fluxes

1992 ◽  
Vol 97 (D18) ◽  
pp. 20421 ◽  
Author(s):  
Robert D. Cess ◽  
Gerald L. Potter ◽  
W. Lawrence Gates ◽  
Jean-Jacques Morcrette ◽  
Lisa Corsetti
Keyword(s):  
General Circulation ◽  
General Circulation Models ◽  
Radiation Budget ◽  
Experiment Data ◽  
Clear Sky ◽  
Earth Radiation Budget ◽  
Earth Radiation

scholarly journals Ice-sheets’ surface mass-balance evaluation in the UGAMP GCM: the climate of Antarctica

1996 ◽  
Vol 23 ◽  
pp. 167-173
Author(s):  
I. Marsiat
Keyword(s):  
Mass Balance ◽  
General Circulation ◽  
Ice Sheets ◽  
Circulation Model ◽  
General Circulation Models ◽  
High Latitudes ◽  
Accumulation Pattern ◽  
Surface Mass ◽  
The Antarctic ◽  
Good Agreement

General Circulation Models (GCMs) will be more and more used for coupled climatic simulations involving ice sheets. It is therefore of prime importance to evaluate the performance of these models in simulating the mass balance and climate over ice sheets. The Antarctic climate simulated with the U.K. Universities Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM, hereafter referred to as the UGCM) is in good agreement with the available observations. In particular, the accumulation pattern appears fairly reasonable. Some imperfections are related to the surface temperature and energy budget but without severe consequences for the atmosphere behaviour. Refining the snow-related parameterizations could improve the results of the model in high latitudes.

scholarly journals The influence of eruption season on the global aerosol evolution and radiative impact of tropical volcanic eruptions

2011 ◽  
Vol 11 (8) ◽  
pp. 22443-22481 ◽  
Author(s):  
M. Toohey ◽  
K. Krüger ◽  
U. Niemeier ◽  
C. Timmreck
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Volcanic Eruptions ◽  
High Latitudes ◽  
Cumulative Impact ◽  
Clear Sky ◽  
The Difference ◽  
Global Mean ◽  
Eruption Magnitude

Abstract. Simulations of tropical volcanic eruptions using a general circulation model with coupled aerosol microphysics are used to assess the influence of season of eruption on the aerosol evolution and radiative impacts at the Earth's surface. This analysis is presented for eruptions with SO2 injection magnitudes of 17 and 700 Tg, the former consistent with estimates of the 1991 Mt. Pinatubo eruption, the later a near-"super eruption". For each eruption magnitude, simulations are performed with eruptions at 15° N, at four equally spaced times of year, and sensitivity to eruption season is quantified as the difference between the maximum and minimum cumulative anomalies. Eruption season has a significant influence on aerosol optical depth (AOD) and clear-sky shortwave (SW) radiative flux anomalies for both eruption magnitudes. The sensitivity to eruption season for both fields is generally weak in the tropics, but increases in the mid- and high latitudes, reaching maximum values of ~80 %. Global mean AOD and clear-sky SW anomalies show sensitivity to eruption season on the order of 15–20 %, which results from differences in aerosol effective radius for the different eruption seasons. Smallest aerosol size and largest cumulative impact result from a January eruption for the Pinatubo-magnitude, and from a July eruption for the near-super eruption. In contrast to AOD and clear-sky SW anomalies, all-sky SW anomalies are found to be insensitive to season of eruption for the Pinatubo-magnitude eruption experiment, due to the reflection of solar radiation by clouds in the mid- to high latitudes. However, differences in all-sky SW anomalies between eruptions in different seasons are significant for the larger eruption magnitude, and the ~15 % sensitivity to eruption season of the global mean all-sky SW anomalies is comparable to the sensitivity of global mean AOD and clear-sky SW anomalies. Our estimates of sensitivity to eruption season are larger than previously reported estimates: implications regarding volcanic AOD timeseries reconstructions and their use in climate models are discussed.

scholarly journals Evaluation of autoconversion and accretion enhancement factors in general circulation model warm-rain parameterizations using ground-based measurements over the Azores

2018 ◽  
Vol 18 (23) ◽  
pp. 17405-17420 ◽  
Author(s):  
Peng Wu ◽  
Baike Xi ◽  
Xiquan Dong ◽  
Zhibo Zhang
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Climate Modeling ◽  
Circulation Model ◽  
General Circulation Models ◽  
Precipitation Intensity ◽  
Equivalent Model ◽  
Warm Rain ◽  
Enhancement Factors ◽  
Precipitating Clouds

Abstract. A great challenge in climate modeling is how to parameterize subgrid cloud processes, such as autoconversion and accretion in warm-rain formation. In this study, we use ground-based observations and retrievals over the Azores to investigate the so-called enhancement factors, Eauto and Eaccr, which are often used in climate models to account for the influence of subgrid variance of cloud and precipitation water on the autoconversion and accretion processes. Eauto and Eaccr are computed for different equivalent model grid sizes. The calculated Eauto values increase from 1.96 (30 km) to 3.2 (180 km), and the calculated Eaccr values increase from 1.53 (30 km) to 1.76 (180 km). Comparing the prescribed enhancement factors in Morrison and Gettleman (2008, MG08) to the observed ones, we found that a higher Eauto (3.2) at small grids and lower Eaccr (1.07) are used in MG08, which might explain why most of the general circulation models (GCMs) produce too-frequent precipitation events but with too-light precipitation intensity. The ratios of the rain to cloud water mixing ratio (qr/qc) at Eaccr=1.07 and Eaccr=2.0 are 0.063 and 0.142, respectively, from observations, further suggesting that the prescribed value of Eaccr=1.07 used in MG08 is too small to simulate precipitation intensity correctly. Both Eauto and Eaccr increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (CLWP>75 gm−2) than those in non-precipitating clouds (CLWP<75 gm−2). Therefore, the selection of Eauto and Eaccr values in GCMs should be regime- and resolution-dependent.

scholarly journals The influence of eruption season on the global aerosol evolution and radiative impact of tropical volcanic eruptions

2011 ◽  
Vol 11 (23) ◽  
pp. 12351-12367 ◽  
Author(s):  
M. Toohey ◽  
K. Krüger ◽  
U. Niemeier ◽  
C. Timmreck
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Volcanic Eruptions ◽  
Radiative Flux ◽  
High Latitudes ◽  
Cumulative Impact ◽  
Clear Sky ◽  
Global Mean ◽  
Eruption Magnitude

Abstract. Simulations of tropical volcanic eruptions using a general circulation model with coupled aerosol microphysics are used to assess the influence of season of eruption on the aerosol evolution and radiative impacts at the Earth's surface. This analysis is presented for eruptions with SO2 injection magnitudes of 17 and 700 Tg, the former consistent with estimates of the 1991 Mt. Pinatubo eruption, the later a near-"super eruption". For each eruption magnitude, simulations are performed with eruptions at 15° N, at four equally spaced times of year. Sensitivity to eruption season of aerosol optical depth (AOD), clear-sky and all-sky shortwave (SW) radiative flux is quantified by first integrating each field for four years after the eruption, then calculating for each cumulative field the absolute or percent difference between the maximum and minimum response from the four eruption seasons. Eruption season has a significant influence on AOD and clear-sky SW radiative flux anomalies for both eruption magnitudes. The sensitivity to eruption season for both fields is generally weak in the tropics, but increases in the mid- and high latitudes, reaching maximum values of ~75 %. Global mean AOD and clear-sky SW anomalies show sensitivity to eruption season on the order of 15–20 %, which results from differences in aerosol effective radius for the different eruption seasons. Smallest aerosol size and largest cumulative impact result from a January eruption for Pinatubo-magnitude eruption, and from a July eruption for the near-super eruption. In contrast to AOD and clear-sky SW anomalies, all-sky SW anomalies are found to be insensitive to season of eruption for the Pinatubo-magnitude eruption experiment, due to the reflection of solar radiation by clouds in the mid- to high latitudes. However, differences in all-sky SW anomalies between eruptions in different seasons are significant for the larger eruption magnitude, and the ~15 % sensitivity to eruption season of the global mean all-sky SW anomalies is comparable to the sensitivity of global mean AOD and clear-sky SW anomalies. Our estimates of sensitivity to eruption season are larger than previously reported estimates: implications regarding volcanic AOD timeseries reconstructions and their use in climate models are discussed.

scholarly journals Ice-sheets’ surface mass-balance evaluation in the UGAMP GCM: the climate of Antarctica

1996 ◽  
Vol 23 ◽  
pp. 167-173 ◽  
Author(s):  
I. Marsiat
Keyword(s):  
Mass Balance ◽  
General Circulation ◽  
Ice Sheets ◽  
Circulation Model ◽  
General Circulation Models ◽  
High Latitudes ◽  
Accumulation Pattern ◽  
Surface Mass ◽  
The Antarctic ◽  
Good Agreement

General Circulation Models (GCMs) will be more and more used for coupled climatic simulations involving ice sheets. It is therefore of prime importance to evaluate the performance of these models in simulating the mass balance and climate over ice sheets. The Antarctic climate simulated with the U.K. Universities Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM, hereafter referred to as the UGCM) is in good agreement with the available observations. In particular, the accumulation pattern appears fairly reasonable. Some imperfections are related to the surface temperature and energy budget but without severe consequences for the atmosphere behaviour. Refining the snow-related parameterizations could improve the results of the model in high latitudes.

Tripleclouds: An Efficient Method for Representing Horizontal Cloud Inhomogeneity in 1D Radiation Schemes by Using Three Regions at Each Height

2008 ◽  
Vol 21 (11) ◽  
pp. 2352-2370 ◽  
Author(s):  
Jonathan K. P. Shonk ◽  
Robin J. Hogan
Keyword(s):  
General Circulation ◽  
Microwave Radiometer ◽  
Circulation Model ◽  
General Circulation Models ◽  
Scaling Factor ◽  
Heating Rates ◽  
Internal Heating ◽  
Clear Sky ◽  
Horizontal Inhomogeneity ◽  
Vertical Level

Abstract Radiation schemes in general circulation models currently make a number of simplifications when accounting for clouds, one of the most important being the removal of horizontal inhomogeneity. A new scheme is presented that attempts to account for the neglected inhomogeneity by using two regions of cloud in each vertical level of the model as opposed to one. One of these regions is used to represent the optically thinner cloud in the level, and the other represents the optically thicker cloud. So, along with the clear-sky region, the scheme has three regions in each model level and is referred to as “Tripleclouds.” In addition, the scheme has the capability to represent arbitrary vertical overlap between the three regions in pairs of adjacent levels. This scheme is implemented in the Edwards–Slingo radiation code and tested on 250 h of data from 12 different days. The data are derived from cloud retrievals using radar, lidar, and a microwave radiometer at Chilbolton, southern United Kingdom. When the data are grouped into periods equivalent in size to general circulation model grid boxes, the shortwave plane-parallel albedo bias is found to be 8%, while the corresponding bias is found to be less than 1% using Tripleclouds. Similar results are found for the longwave biases. Tripleclouds is then compared to a more conventional method of accounting for inhomogeneity that multiplies optical depths by a constant scaling factor, and Tripleclouds is seen to improve on this method both in terms of top-of-atmosphere radiative flux biases and internal heating rates.

scholarly journals Compensation of Hemispheric Albedo Asymmetries by Shifts of the ITCZ and Tropical Clouds

2014 ◽  
Vol 27 (3) ◽  
pp. 1029-1045 ◽  
Author(s):  
Aiko Voigt ◽  
Bjorn Stevens ◽  
Jürgen Bader ◽  
Thorsten Mauritsen
Keyword(s):  
General Circulation ◽  
Hemispheric Asymmetry ◽  
Thermal Inertia ◽  
Circulation Model ◽  
Radiation Budget ◽  
Convection Scheme ◽  
Clear Sky ◽  
Atmosphere General Circulation Model ◽  
Slab Ocean ◽  
Radiative Feedbacks

Abstract Despite a substantial hemispheric asymmetry in clear-sky albedo, observations of Earth’s radiation budget reveal that the two hemispheres have the same all-sky albedo. Here, aquaplanet simulations with the atmosphere general circulation model ECHAM6 coupled to a slab ocean are performed to study to what extent and by which mechanisms clouds compensate hemispheric asymmetries in clear-sky albedo. Clouds adapt to compensate the imposed asymmetries because the intertropical convergence zone (ITCZ) shifts into the dark surface hemisphere. The strength of this tropical compensation mechanism is linked to the magnitude of the ITCZ shift. In some cases the ITCZ shift is so strong as to overcompensate the hemispheric asymmetry in clear-sky albedo, yielding a range of climates for which the hemisphere with lower clear-sky albedo has a higher all-sky albedo. The ITCZ shift is sensitive to the convection scheme and the depth of the slab ocean. Cloud–radiative feedbacks explain part of the sensitivity to the convection scheme as they amplify the ITCZ shift in the Tiedtke (TTT) scheme but have a neutral effect in the Nordeng (TNT) scheme. A shallower slab ocean depth, and thereby reduced thermal inertia of the underlying surface and increased seasonal cycle, stabilizes the ITCZ against annual-mean shifts. The results lend support to the idea that the climate system adjusts so as to minimize hemispheric albedo asymmetries, although there is no indication that the hemispheres must have exactly the same albedo.

Atmospheric general circulation models of the Jurassic

1993 ◽  
Vol 341 (1297) ◽  
pp. 317-326 ◽  
Keyword(s):  
General Circulation ◽  
Cloud Cover ◽  
Atmospheric General Circulation Model ◽  
Circulation Model ◽  
General Circulation Models ◽  
High Latitudes ◽  
Geological Data ◽  
First Order ◽  
Atmospheric General Circulation ◽  
Atmospheric General Circulation Models

An atmospheric general circulation model (GCM) is used to simulate the climate of the Jurassic. It is found that the model gives first order agreement with geological data but that closer comparison is limited by uncertainties in the model and in the imposed boundary conditions. The model results suggest that the relative warmth of high latitudes is strongly influenced by changing sea ice and cloud cover. The implied oceanic heat transports are surprisingly small.

scholarly journals Optimizing Parameters in an Atmospheric General Circulation Model

2005 ◽  
Vol 18 (17) ◽  
pp. 3527-3535 ◽  
Author(s):  
C. A. Severijns ◽  
W. Hazeleger
Keyword(s):  
Cost Function ◽  
General Circulation Model ◽  
General Circulation ◽  
Atmospheric General Circulation Model ◽  
Circulation Model ◽  
General Circulation Models ◽  
Radiation Budget ◽  
Nonlinear Dependence ◽  
Atmospheric General Circulation ◽  
The Cost

Abstract An efficient method to optimize the parameter values of the subgrid parameterizations of an atmospheric general circulation model is described. The method is based on the downhill simplex minimization of a cost function computed from the difference between simulated and observed fields. It is used to find optimal values of the radiation and cloud-related parameters. The model error is reduced significantly within a limited number of iterations (about 250) of short integrations (5 yr). The method appears to be robust and finds the global minimum of the cost function. The radiation budget of the model improves considerably without violating the already well simulated general circulation. Different aspects of the general circulation, such as the Hadley and Walker cells improve, although they are not incorporated into the cost function. It is concluded that the method can be used to efficiently determine optimal parameters for general circulation models even when the model behavior has a strong nonlinear dependence on these parameters.

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