U.K. HiGEM Shaffrey, L. C.; Stevens, I.; Norton, W. A. ...
Journal of climate,
04/2009, Letnik:
22, Številka:
8
Journal Article
Recenzirano
Odprti dostop
This article describes the development and evaluation of the U.K.’s new High-Resolution Global Environmental Model (HiGEM), which is based on the latest climate configuration of the Met Office ...Unified Model, known as the Hadley Centre Global Environmental Model, version 1 (HadGEM1). In HiGEM, the horizontal resolution has been increased to 0.83° latitude × 1.25° longitude for the atmosphere, and 1/3° × 1/3° globally for the ocean. Multidecadal integrations of HiGEM, and the lower-resolution HadGEM, are used to explore the impact of resolution on the fidelity of climate simulations.
Generally, SST errors are reduced in HiGEM. Cold SST errors associated with the path of the North Atlantic drift improve, and warm SST errors are reduced in upwelling stratocumulus regions where the simulation of low-level cloud is better at higher resolution. The ocean model in HiGEM allows ocean eddies to be partially resolved, which dramatically improves the representation of sea surface height variability. In the Southern Ocean, most of the heat transports in HiGEM is achieved by resolved eddy motions, which replaces the parameterized eddy heat transport in the lower-resolution model. HiGEM is also able to more realistically simulate small-scale features in the wind stress curl around islands and oceanic SST fronts, which may have implications for oceanic upwelling and ocean biology.
Higher resolution in both the atmosphere and the ocean allows coupling to occur on small spatial scales. In particular, the small-scale interaction recently seen in satellite imagery between the atmosphere and tropical instability waves in the tropical Pacific Ocean is realistically captured in HiGEM. Tropical instability waves play a role in improving the simulation of the mean state of the tropical Pacific, which has important implications for climate variability. In particular, all aspects of the simulation of ENSO (spatial patterns, the time scales at which ENSO occurs, and global teleconnections) are much improved in HiGEM.
Saharan dust storms have often been observed from space, but the full impact on the Earth's radiation balance has been difficult to assess, due to limited observations from the surface. We present ...the first simultaneous observations from space and from a comprehensive new mobile facility in Niamey, Niger, of a major dust storm in March 2006. The results indicate major perturbations to the radiation balance both at the top of the atmosphere and at the surface. Combining the satellite and surface data, we also estimate the impact on the radiation balance of the atmosphere itself. Using independent data from the mobile facility, we derive the optical properties of the dust and input these and other information into two radiation models to simulate the radiative fluxes. We show that the radiation models underestimate the observed absorption of solar radiation in the dusty atmosphere.
The global radiation balance of the atmosphere is still poorly observed, particularly at the surface. We investigate the observed radiation balance at (1) the surface using the ARM Mobile Facility in ...Niamey, Niger, and (2) the top of the atmosphere (TOA) over West Africa using data from the Geostationary Earth Radiation Budget (GERB) instrument on board Meteosat‐8. Observed radiative fluxes are compared with predictions from the global numerical weather prediction (NWP) version of the Met Office Unified Model (MetUM). The evaluation points to major shortcomings in the NWP model's radiative fluxes during the dry season (December 2005 to April 2006) arising from (1) a lack of absorbing aerosol in the model (mineral dust and biomass burning aerosol) and (2) a poor specification of the surface albedo. A case study of the major Saharan dust outbreak of 6–12 March 2006 is used to evaluate a parameterization of mineral dust for use in the NWP models. The model shows good predictability of the large‐scale flow out to 4–5 days with the dust parameterization providing reasonable dust uplift, spatial distribution, and temporal evolution for this strongly forced dust event. The direct radiative impact of the dust reduces net downward shortwave (SW) flux at the surface (TOA) by a maximum of 200 W m−2 (150 W m−2), with a SW heating of the atmospheric column. The impacts of dust on terrestrial radiation are smaller. Comparisons of TOA (surface) radiation balance with GERB (ARM) show the “dusty” forecasts reduce biases in the radiative fluxes and improve surface temperatures and vertical thermodynamic structure.
The African Monsoon Multidisciplinary Analysis (AMMA) is a major international campaign investigating far‐reaching aspects of the African monsoon, climate and the hydrological cycle. A special ...observing period was established for the dry season (SOP0) with a focus on aerosol and radiation measurements. SOP0 took place during January and February 2006 and involved several ground‐based measurement sites across west Africa. These were augmented by aircraft measurements made by the Facility for Airborne Atmospheric Measurements (FAAM) aircraft during the Dust and Biomass‐burning Experiment (DABEX), measurements from an ultralight aircraft, and dedicated modeling efforts. We provide an overview of these measurement and modeling studies together with an analysis of the meteorological conditions that determined the aerosol transport and link the results together to provide a balanced synthesis. The biomass burning aerosol was significantly more absorbing than that measured in other areas and, unlike industrial areas, the ratio of excess carbon monoxide to organic carbon was invariant, which may be owing to interaction between the organic carbon and mineral dust aerosol. The mineral dust aerosol in situ filter measurements close to Niamey reveals very little absorption, while other measurements and remote sensing inversions suggest significantly more absorption. The influence of both mineral dust and biomass burning aerosol on the radiation budget is significant throughout the period, implying that meteorological models should include their radiative effects for accurate weather forecasts and climate simulations. Generally, the operational meteorological models that simulate the production and transport of mineral dust show skill at lead times of 5 days or more. Climate models that need to accurately simulate the vertical profiles of both anthropogenic and natural aerosols to accurately represent the direct and indirect effects of aerosols appear to do a reasonable job, although the magnitude of the aerosol scattering is strongly dependent upon the emission data set.
Broadband shortwave and longwave radiative fluxes observed both at the surface and from space during the Radiative Atmospheric Divergence using ARM Mobile Facility, GERB data and AMMA Stations ...(RADAGAST) experiment in Niamey, Niger, in 2006 are presented. The surface fluxes were measured by the Atmospheric Radiation Measurement (ARM) Program Mobile Facility (AMF) at Niamey airport, while the fluxes at the top of the atmosphere (TOA) are from the Geostationary Earth Radiation Budget (GERB) instrument on the Meteosat‐8 satellite. The data are analyzed as daily averages, in order to minimize sampling differences between the surface and top of atmosphere instruments, while retaining the synoptic and seasonal changes that are the main focus of this study. A cloud mask is used to identify days with cloud versus those with predominantly clear skies. The influence of temperature, water vapor, aerosols, and clouds is investigated. Aerosols are ubiquitous throughout the year and have a significant impact on both the shortwave and longwave fluxes. The large and systematic seasonal changes in temperature and column integrated water vapor (CWV) through the dry and wet seasons are found to exert strong influences on the longwave fluxes. These influences are often in opposition to each other, because the highest temperatures occur at the end of the dry season when the CWV is lowest, while in the wet season the lowest temperatures are associated with the highest values of CWV. Apart from aerosols, the shortwave fluxes are also affected by clouds and by the seasonal changes in CWV. The fluxes are combined to provide estimates of the divergence of radiation across the atmosphere throughout 2006. The longwave divergence shows a relatively small variation through the year, because of a partial compensation between the seasonal variations in the outgoing longwave radiation (OLR) and surface net longwave radiation. A simple model of the greenhouse effect is used to interpret this result in terms of the dependence of the normalized greenhouse effect at the TOA and of the effective emissivity of the atmosphere at the surface on the CWV. It is shown that, as the CWV increases, the atmosphere loses longwave energy to the surface with about the same increasing efficiency with which it traps the OLR. When combined with the changes in temperature, this maintains the atmospheric longwave divergence within the narrow range that is observed. The shortwave divergence is mainly determined by the CWV and aerosol loadings and the effect of clouds is much smaller than on the component fluxes.
Over much of the world, heating of the surface by sunlight is balanced predominately by evaporative cooling. However, at the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) in Niamey, ...Niger, evaporation makes a significant contribution to the surface energy balance only at the height of the rainy season, when precipitation has replenished the reservoir of soil moisture. The AMF was placed at Niamey from late 2005 to early 2007 to provide measurements of surface fluxes in coordination with geostationary satellite retrievals of radiative fluxes at the top of the atmosphere, as part of the RADAGAST experiment to calculate atmospheric radiative divergence. We use observations at the mobile facility to investigate how the surface adjusts to radiative forcing throughout the year. The surface response to solar heating varies with changes in atmospheric water vapor associated with the seasonal reversal of the West African monsoon, which modulates the greenhouse effect and the ability of the surface to radiate thermal energy directly to space. During the dry season, sunlight is balanced mainly by longwave radiation and the turbulent flux of sensible heat. The ability of longwave radiation to cool the surface drops after the onset of southwesterly surface winds at Niamey, when moist, oceanic air flows onshore, increasing local column moisture and atmospheric opacity. Following the onset of southwesterly flow, evaporation remains limited by the supply of moisture from precipitation. By the height of the rainy season, however, sufficient precipitation has accumulated that evaporation is controlled by incident sunlight, and radiative forcing of the surface is balanced comparably by the latent, sensible, and longwave fluxes. Evaporation increases with the leaf area index, suggesting that plants are a significant source of atmospheric moisture and may tap moisture stored beneath the surface that accumulated during a previous rainy season. Surface radiative forcing during a dust aerosol outbreak is balanced comparably by net surface longwave and the sensible heat flux during the dry season, with the sensible flux increasing in importance with the onset of the summer monsoon winds. Measurements of surface fluxes by the AMF indicate broader features of the West African monsoon circulation and should be used to evaluate model simulations of the Sahel climate.
An overview is presented of the meteorological and thermodynamic data obtained during the Radiative Atmospheric Divergence using Atmospheric Radiation Measurement (ARM) Mobile Facility, Geostationary ...Earth Radiation Budget (GERB) data, and African Monsoon Multidisciplinary Analysis (AMMA) stations (RADAGAST) experiment in Niamey, Niger, in 2006. RADAGAST combined data from the ARM Program Mobile Facility (AMF) at Niamey airport with broadband satellite data from the GERB instrument on Meteosat‐8. The experiment was conducted in collaboration with the AMMA project. The focus in this paper is on the variations through the year of key surface and atmospheric variables. The seasonal advance and retreat of the Intertropical Front and the seasonal changes in near‐surface variables and precipitation in 2006 are discussed and contrasted with the behavior in 2005 and with long‐term averages. Observations from the AMF at Niamey airport are used to document the evolution of near‐surface variables and of the atmosphere above the site. There are large seasonal changes in these variables, from the arid and dusty conditions typical of the dry season to the much moister and more cloudy wet season accompanying the arrival and intensification of the West African monsoon. Back trajectories show the origin of the air sampled at Niamey and profiles for selected case studies from rawinsondes and from a micropulse lidar at the AMF site reveal details of typical atmospheric structures. Radiative fluxes and divergences are discussed in the second part of this overview, and the subsequent papers in this special section explore other aspects of the measurements and of the associated modeling.
Simultaneous observations of thermal radiative fluxes and radiances from the surface (Atmospheric Radiation Measurement Mobile Facility, Niamey) and top of atmosphere (Geostationary Earth Radiation ...Budget (GERB) instrument) during the Radiative Atmospheric Divergence using ARM Mobile Facility, GERB data, and AMMA Stations experiment are compared with results from a radiative transfer model (Edwards‐Slingo). Emphasis is placed on diagnosing the accuracy of the cloud‐free radiation measurements using multiple instruments at the surface. The surface forcing from aerosol is found to regularly exceed 20 Wm−2, and reached ∼100 Wm−2 during the March 2006 dust storm. Equivalent comparisons are made with top of atmosphere (TOA) measurements but here radiance closure is not achieved. A disagreement is found between the angular anisotropy derived from GERB products and that from radiative transfer (RT) calculations. A hybrid TOA radiative flux time series is created using RT‐calculated TOA anisotropy and GERB‐observed TOA radiance. At 1100 UT (local noon), this hybrid flux differs from the Edition 1 GERB product by a positive difference in the range ∼0–10 Wm−2. Three collections of fluxes exist to calculate column‐integrated atmospheric heating (divergence) from surface and TOA fluxes. The first two are fluxes from observations only or from RT calculations only. The third is a combination of RT calculation and observed fluxes that includes the hybrid flux. The resulting divergences are binned by sonde launch times and averaged over the year. The range of divergence during a day depends on the flux collection used (−200 to −111 Wm−2, −212 to −116 Wm−2, or −205 to −112 Wm−2) for observations only, for RT calculations only, or for observation‐calculation fluxes. All estimates agree as to the interday variation being larger than that of intraday variability.
The longwave radiation emitted by the atmosphere toward the surface downward longwave radiation (DLR) is a crucial factor in the exchange of energy between the earth surface and the atmosphere and in ...the context of radiation-induced climate change. Accurate modeling of this quantity is therefore a fundamental prerequisite for a reliable simulation and projection of the surface climate in coupled general circulation models (GCM).
DLR climatologies calculated in a number of GCMs and in a model in assimilation mode (reanalysis) are analyzed using newly available data from 45 worldwide distributed observation sites of the Global Energy Balance Archive (GEBA) and the Baseline Surface Radiation Network (BSRN). It is shown that substantial biases are present in the GCM-calculated DLR climatologies, with the GCMs typically underestimating the DLR (estimated here to be approximately 344 W m−2globally). The biases are, however, not geographically homogeneous, but depend systematically on the prevailing atmospheric conditions. The DLR is significantly underestimated particularly at observation sites with cold and dry climates and thus little DLR emission. This underestimation gradually diminishes toward sites with more moderate climates; at sites with warm or humid atmospheric conditions and strong DLR emission, the GCM-calculated DLR is in better agreement with the observations or even overestimates them. This is equivalent to creating an excessively strong meridional gradient of DLR in the GCMs.
The very same tendencies are independently found in stand-alone calculations with the GCM radiation codes in isolation, using observed atmospheric profiles of temperature and humidity for cloud-free conditions as input to the radiation schemes. A significant underestimation of DLR is calculated by the radiation schemes when driven with clear-sky atmospheric profiles of temperature and humidity representative for cold and dry climates, whereas the DLR is no longer underestimated by the radiation schemes with prescribed clear-sky profiles representative for a hot and humid atmosphere. This suggests that the biases in the GCM-calculated DLR climatologies are predominantly induced by problems in the simulated emission of the cloud-free atmosphere.
The same biases are also found in the DLR fluxes calculated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model in assimilation mode (reanalysis), in which the biases in the atmospheric thermal and humidity structure are minimized. This gives further support that the biases in the DLR are not primarily due to errors in the model-predicted atmospheric temperature and humidity profiles that enter the radiative transfer calculations, but rather are due to the radiation schemes themselves. A particular problem in these schemes is the accurate simulation of the thermal emission from the cold, dry, cloud-free atmosphere.
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