Relationship between cloud radiative forcing, cloud fraction and cloud albedo, and new surface-based approach for determining cloud albedo (original) (raw)
Related papers
Journal of the Atmospheric Sciences, 1997
This study presents surface solar radiation flux and cloud radiative forcing results obtained by using a combination of satellite and surface observations interpreted by means of a simple plane-parallel radiative transfer model called 2001. This model, a revised version of a model initially introduced by Gautier et al., relates calibrated radiance observations from space to incoming surface solar flux. After a description of the model, an evaluation is presented by comparison with a more complex model that the authors have developed, the Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) based on the discrete ordinate model of Stamnes et al. This evaluation demonstrates this model's accuracy for instantaneous surface flux when used to retrieve daily (and monthly) surface solar flux. Limitations related to its lack of treatment of the bidirectional reflectance properties of clouds are also discussed and quantified by comparison with SBDART for instantaneous surface solar flux retrievals. The influence of satellite sensor calibration uncertainty is also examined in terms of surface solar flux. The model has been applied to hourly GOES data collected over the Atmospheric Radiation Measurement (ARM) program's central cloud and radiation testbed site in Oklahoma during a 14-month period to estimate hourly, daily, and monthly surface solar radiation flux. Comparisons of the model's results with surface measurements made from pyranometers located at the ARM site indicate good overall agreement. The best results are obtained for daily integrated clear skies with an rms error less than 10 W m Ϫ2 (or about 3% of the mean value) and a 2.8 W m Ϫ2 bias. These results indicate that the clear sky model is quite accurate and also that the threshold-based technique to detect cloudy conditions works well for the resolution of the satellite data used in this study. For partly cloudy conditions the comparisons show an rms error of about 20 W m Ϫ2 (or less than 7% of the mean) and a Ϫ2.5 W m Ϫ2 bias. The performance of the model degrades with cloud cover conditions with an rms error of 22 W m Ϫ2 (or 13% of the mean) and a bias of 13.9 W m Ϫ2 for overcast conditions. The results improve considerably for monthly average values with an rms error of about 11 W m Ϫ2 (or 4% of the mean) and a bias of 2.6 W m Ϫ2 for all conditions. The model has also been used to evaluate the cloud radiative forcing at the surface and results indicate large values of forcing for the spring and summer reaching daily values over 200 W m Ϫ2 in May.
Journal of Climate, 2006
Data collected at the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility (SCF) are analyzed to determine the monthly and hourly variations of cloud fraction and radiative forcing between January 1997 and December 2002. Cloud fractions are estimated for total cloud cover and for single-layered low (0-3 km), middle (3-6 km), and high clouds (Ͼ6 km) using ARM SCF ground-based paired lidar-radar measurements. Shortwave (SW) and longwave (LW) fluxes are derived from up-and down-looking standard precision spectral pyranometers and precision infrared radiometer measurements with uncertainties of ϳ10 W m Ϫ2. The annual averages of total and single-layered low-, middle-, and high-cloud fractions are 0.49, 0.11, 0.03, and 0.17, respectively. Both totaland low-cloud amounts peak during January and February and reach a minimum during July and August; high clouds occur more frequently than other types of clouds with a peak in summer. The average annual downwelling surface SW fluxes for total and low clouds (151 and 138 W m Ϫ2 , respectively) are less than those under middle and high clouds (188 and 201 W m Ϫ2 , respectively), but the downwelling LW fluxes (349 and 356 W m Ϫ2) underneath total and low clouds are greater than those from middle and high clouds (337 and 333 W m Ϫ2). Low clouds produce the largest LW warming (55 W m Ϫ2) and SW cooling (Ϫ91 W m Ϫ2) effects with maximum and minimum absolute values in spring and summer, respectively. High clouds have the smallest LW warming (17 W m Ϫ2) and SW cooling (Ϫ37 W m Ϫ2) effects at the surface. All-sky SW cloud radiative forcing (CRF) decreases and LW CRF increases with increasing cloud fraction with mean slopes of Ϫ0.984 and 0.616 W m Ϫ2 % Ϫ1 , respectively. Over the entire diurnal cycle, clouds deplete the amount of surface insolation more than they add to the downwelling LW flux. The calculated CRFs do not appear to be significantly affected by uncertainties in data sampling and clear-sky screening. Traditionally, cloud radiative forcing includes not only the radiative impact of the hydrometeors, but also the changes in the environment. Taken together over the ARM SCF, changes in humidity and surface albedo between clear and cloudy conditions offset ϳ20% of the NET radiative forcing caused by the cloud hydrometeors alone. Variations in water vapor, on average, account for 10% and 83% of the SW and LW CRFs, respectively, in total cloud cover conditions. The error analysis further reveals that the cloud hydrometeors dominate the SW CRF, while water vapor changes are most important for LW flux changes in cloudy skies. Similar studies over other locales are encouraged where water and surface albedo changes from clear to cloudy conditions may be much different than observed over the ARM SCF.
Impact of Ingesting Satellite-Derived Cloud Cover into the Regional Atmospheric Modeling System
Monthly Weather Review, 2002
This study investigates the extent to which assimilating high-resolution remotely sensed cloud cover into the Regional Atmospheric Modeling System (RAMS) provides an improved regional diagnosis of downward shortand longwave surface radiation fluxes and precipitation. An automatic procedure was developed to derive highresolution (4 km ϫ 4 km) fields of fractional cloud cover from visible band Geostationary Operational Environmental Satellite (GOES) data using a tracking procedure to determine the clear-sky composite image. Initial studies, in which RAMS surface shortwave radiation fluxes were replaced by estimates obtained by applying satellite-derived cloud cover in the University of Maryland Global Energy and Water Cycle Experiment's Surface Radiation Budget (UMD GEWEX/SRB) model, revealed problems associated with inconsistencies between the revised solar radiation fields and the RAMS-calculated incoming longwave radiation and precipitation fields. Consequently, in this study, the relationship between cloud albedo, optical depth, and water/ice content used in the UMD GEWEX/SRB model was applied instead to provide estimates of whole-column cloud water/ice that were ingested into RAMS. This potentially enhances the realism of the modeled short-and longwave radiation and precipitation. The ingested cloud image took the horizontal distribution of clouds from the satellite image but derives its vertical distribution from the fields simulated by RAMS in the time step immediately prior to assimilation. The resulting image was ingested every minute, with linear interpolation used to derive the 1-min cloud images between 15-min GOES samples. Comparisons were made between modeled and observed data taken from the Arizona Meteorological Network (AZMET) weather station network in southern Arizona for model runs with and without cloud ingestion. Cloud ingestion was found to substantially improve the ability of the RAMS model to capture temporal and spatial variations in surface fields associated with cloud cover. An initial test suggests that cloud ingestion enhanced RAMS short-term forecast ability.
Atmospheric Chemistry and Physics Discussions, 2014
Aerosols can alter the macro-and micro-physical properties of deep convective clouds (DCCs) and their radiative forcing (CRF). This study presents what is arguably the first long-term estimate of the aerosol-mediated changes in CRF (AMCRF) for deep cloud systems derived from decade-long continuous ground-based and satellite observations, model simulations, and reanalysis data. Measurements were made at the US Department of Energy's Atmospheric Radiation Measurement Program's Southern Great Plains (SGP) site. Satellite retrievals are from the Geostationary Operational Environmental Satellite. Increases in aerosol loading were accompanied by the thickening of DCC cores and the expansion and thinning of anvils, due presumably to the aerosol invigoration effect (AIV) and the aerosol microphysical effect. Meteorological variables dictating these cloud processes were investigated. Consistent with previous findings, the AIV is most significant when the atmosphere is moist and unstable with weak wind shear. Such aerosol-mediated systematic changes in DCC core thickness and anvil size alter CRF at the top of atmosphere (TOA) and at the surface. Using extensive observations, ∼ 300 DCC systems were identified over a 10 years period at the SGP site (2000-2011) and analyzed. Daily mean AMCRF at the TOA and at the surface are 29.3 W m −2 and 22.2 W m −2 , respectively. This net warming effect due to changes in DCC microphysics offsets the cooling resulting from the first aerosol indirect effect.
Journal of Geophysical Research, 1993
To understand the role of clouds in the atmospheric circulation and in the modulation of energy available at the surface, their effect on the the atmospheric and surface absorption should be determined. The C1 data of the Intemational Satellite Cloud Climatology Project, along with the Satellite Algorithm for Shortwave Radiation Budget, are used to estimate the shortwave cloud effects in terms of the cloud-radiative forcing at the top of the atmosphere (TOA), at the surface and of the atmospheric column on a global scale for the July months of 1983-1985. Global means of TOA cloud forcing range from -43.6 (1983) to -39.1 Wm -2 (1985). The cloud forcing for July 1985 is underestimated, by about 8 Wm -2, compared with that obtained from the Earth Radiation Budget Experiment. The cloud forcing at the surface is almost identical to that at the TOA, indicating that the effect of clouds on the shortwave energy budget of the surface-atmosphere system is such that most of the cooling is at the surface. Regression analysis of the computed fluxes shows a strong linear correlation between the TOA and surface cloud forcing. The monthly averaged regional values of the atmospheric cloud forcing are generally less than the estimated uncertainty of 20 Wm -2. Assuming that the uncerhainties cancel, the global mean of the atmospheric cloud forcing is between 1 and 2 Wm -2, suggesting a slight warming owing to the presence of clouds.
2002
Satellite cloud analyses can provide a long-term record of microphysical, macrophysical and radiative properties over a large area. However, to have confidence in these retrievals, the results must be compared with ground truth or reliable reference sources. The layered bispectral threshold method (LBTM; Minnis et al. 1995) has been used to derive cloud and radiative properties from the eighth geostationary operational environmental satellite (GOES-8) data over a 5-year period for the Southern Great Plains (SGP) domain. These properties include cloud OD, amount, height, temperature, and thickness, total and clear-sky longwave (LW) fluxes, clear-sky temperatures, and total and clear-sky albedos. In this paper, data from several ground instruments at the SGP central facility (CF) and from the Clouds and Earth’s Radiant Energy System (CERES; Wielicki et al. 1998) are used to examine the GOES-8 products.