Abstract

Deep convective clouds (DCCs) are an important player in the climate system. In this paper the authors use remote sensing data mainly from the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud product to investigate a few general cloud macro- and microphysical properties of DCCs. This investigation concentrates on the tallest convective clouds and associated thick anvils that are labeled “deep convective clouds.” General geographical patterns of DCCs from MODIS data are consistent with previous studies. By examining statistics of optical properties of DCCs over different locations of the globe, it is found that cloud optical depth distribution for DCCs shows little interannual variability for individual regions. These distributions, however, change with geographical regions. DCC ice particle size varies with surface elevation and cloud brightness temperature. DCCs that develop over elevated areas tend to have smaller ice particles at cloud top. There is a positive correlation between ice particle size and brightness temperature. The slope of this correlation has significant regional variations, which can be explained either with a simple thermodynamic consideration or with homogeneous freezing of aerosols. The findings have important implications in studying radiation budget, ice cloud microphysics parameterization, and troposphere–stratosphere water vapor exchange.

Abstract

Cloud overlapping has been a major issue in climate studies owing to a lack of reliable information available over both oceans and land. This study presents the first near-global retrieval and analysis of single-layer and overlapped cloud vertical structures and their optical properties retrieved by applying a new method to the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Taking full advantage of the MODIS multiple channels, the method can differentiate cirrus overlapping lower water clouds from single-layer clouds. Based on newly retrieved cloud products using daytime Terra/MODIS 5-km overcast measurements sampled in January, April, July, and October 2001, global statistics of the frequency of occurrence, cloud-top pressure/temperature (Pc/Tc), visible optical depth (τ _VIS), and infrared emissivity (ε) are presented and discussed. Of all overcast scenes identified over land (ocean), the MODIS data show 61% (52%) high clouds (Pc < 500 hPa), 39% (48%) lower clouds (Pc > 500 hPa), and an extremely low occurrence (<4%) of Pc between 500 and 600 hPa. A distinct bimodal distribution of Pc is found and peaks at ∼275 and ∼725 hPa for high and low clouds, thus leaving a minimum in cloud in the middle troposphere. Out of the 61% (52%) of high clouds identified by MODIS, retrievals reveal that 41% (35%) are thin cirrus clouds (ε < 0.85 and Pc < 500 hPa) and the remaining 20% (17%) are thick high clouds (ε ≥ 0.85). Out of the 41% (35%) of thin cirrus, 29% (27%) are found to overlap with lower water clouds and 12% (8%) are single-layer cirrus. Total low-cloud amount (single-layer plus overlapped) out of all overcast scenes is thus 68% (39% + 29%) over land and 75% (48% + 27%) over ocean, which is greater than the cloud amounts reported by the MODIS and the International Satellite Cloud Climatology Project (ISCCP). Both retrieved overlapping and nonoverlapping cirrus clouds show similar mean τ _VIS of ∼1.5 and mean ε of ∼0.5. The optical properties of single-layer cirrus and single-layer water clouds agree well with the MODIS standard retrievals. Because the MODIS retrievals do not differentiate between cirrus and lower water clouds in overlap situations, large discrepancies are found for emissivity, cloud-top height, and optical depth for high cirrus overlapping lower water clouds.

Abstract

The disposition of mean July clear-sky solar radiation in the Canadian Climate Centre second-generation general circulation model (CCC-GCMII) was analyzed by comparing top of the atmosphere (TOA) net fluxes with earth radiation budget experiment (ERBE) data and atmospheric and surface net fluxes with values inferred from Li's algorithm using ERBE data and European Centre for Medium-Range Weather Forecasts precipitable water data. GCMII tended to reflect ˜5 W m⁻² too much to space. Corresponding atmospheric and surface absorption, however, tended to be too low and high, respectively, by ˜30 W m⁻² over much of the Northern Hemisphere. These results were echoed when GCMII atmospheric absorption was compared to estimated results from Li's algorithm driven by GCMII TOA albedo and precipitable water.

The latest version of the CCC-GCM (GCMIII) has numerous upgrades to its clear-sky solar radiative transfer algorithm, the most important of which involve water vapor transmittances and aerosols that tend to enhance atmospheric absorptance. GCMIII's water vapor transmittance functions derive from Geophysical Fluid Dynamics Laboratory line-by-line results, whereas GCMII's were based on Air Force Geophysical Laboratory data. GCMIII includes crude distributions of background tropospheric aerosols, whereas GCMII neglected aerosols.

Li's algorithm was then driven by GCMIII data, and atmospheric absorption of solar radiation by GCMIII was assessed. Differences between GCMIII's and Li's atmospheric absorption over land were almost always within 5 W m⁻². Over oceans, differences were mostly between −5 W m⁻² and −15 W m⁻². This apparent underestimation over GCMIII's oceans probably stems from the algorithm's use of a thin, highly absorbing aerosol.

Abstract

An earlier parameterization that relates the outgoing solar flux at the top of the atmosphere to the flux absorbed at the surface is modified and extended to allow for variations in atmospheric properties that were not considered in the original parameterization. Changes to the parameterization have also been introduced as a result of better treatment of water vapor absorption in the detailed radiative transfer calculations. Corrections are introduced that account for the height of the surface (surface pressure), ozone amount, aerosol type and amount, and cloud height and cloud type, which is characterized by an effective cloud droplet radius. Finally, the results of applying the parameterization to Earth Radiation Budget Satellite measurements are compared with the measurements of the net solar flux measured from two instrumented towers.

Abstract

This study examines the consistency and inconsistency in shortwave (SW) top-of-atmosphere (TOA) reflectances and albedos obtained from satellite measurements of the Earth Radiation Budget Experiment (ERBE) and radiation modeling based on cloud properties retrieved from the Advanced Very High Resolution Radiometer (AVHRR). The examination focuses on completely overcast scenes covered by low-level, single-layered, maritime stratus with uniform cloud-top heights as determined from AVHRR measurements at near nadir. A radiation model was then applied to the retrieved cloud optical depths, droplet effective radii, and top temperatures to compute the SW TOA reflectances and albedos that are compared with coincident ERBE observations. ERBE-observed and AVHRR-based modeled reflectances show excellent agreement in terms of both trend and magnitude, but the two albedos exhibit significant differences that have a strong dependence on cloud optical properties and solar zenith angle (SZA). To unravel the differences, two major factors, that is, scene identification and angular dependence model (ADM), involved in converting reflectance to albedo are examined. It is found that the dependence is mainly caused by the use of a single ERBE–ADM for all overcast scenes, regardless of cloud optical properties. The mean difference in SW TOA flux is about 4–12 W m⁻², depending on SZA, but individual differences may reach up to 40–50 W m⁻² for persistent large or small cloud optical depths. Nearly all of the uniform low-level overcast scenes as determined by AVHRR are identified as mostly cloudy by ERBE, but the misidentification does not have any adverse effect on the albedo differences. In fact, replacing the ERBE mostly cloudy ADM with the overcast ADM exacerbates the albedo comparisons. The mean fluxes obtained with the two ADMs differ by ∼8 W m⁻² at SZA ≈ 33° and by 30 W m⁻² at SZA ≈ 60°.

Abstract

Global datasets of surface radiation budget (SRB) have been obtained from satellite programs. These satellite-based estimates need validation with ground-truth observations. This study validates the estimates of monthly mean surface insolation contained in two satellite-based SRB datasets with the surface measurements made at worldwide radiation stations from the Global Energy Balance Archive (GEBA). One dataset was developed from the Earth Radiation Budget Experiment (ERBE) using the algorithm of Li et al. (ERBE/SRB), and the other from the International Satellite Cloud Climatology Project (ISCCP) using the algorithm of Pinker and Laszlo and that of Staylor (GEWEX/SRB). Since the ERBE/SRB data contain the surface net solar radiation only, the values of surface insolation were derived by making use of the surface albedo data contained in the GEWEX/SRB product. The resulting surface insolation has a bias error near zero and a root-mean-square error (RMSE) between 8 and 28 W m⁻². The RMSE is mainly associated with poor representation of surface observations within a grid cell. When the number of surface observations are sufficient, the random error is estimated to be about 5 W m⁻² with present satellite-based estimates. In addition to demonstrating the strength of the retrieving method, the small random error demonstrates how well the ERBE derives the monthly mean fluxes at the top of the atmosphere (TOA). A larger scatter is found for the comparison of transmissivity than for that of insolation. Month to month comparison of insolation reveals a weak seasonal trend in bias error with an amplitude of about 3 W m⁻². As for the insolation data from the GEWEX/SRB, larger bias errors of 5–10 W m⁻² are evident with stronger seasonal trends and almost identical RMSEs.

Abstract

Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known.

Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends strongly on SZA and moderately on precipitable water and cloud type. The linear relationship provides a simple parameterization to estimate surface-absorbed flux from satellite-measured reflected flux at the TOA. Unlike other models, the present model makes explicit use of the SZA. Precipitable water is included as a secondary parameter. Surface-absorbed fluxes deduced from this simple parameterized model generally agree to within 10 W m⁻² with the absorbed fluxes determined from detailed radiative transfer calculations, without including information on the presence or absence of cloud, cloud type, optical thickness, or surface type.

Abstract

Several observational datasets were used to assess the quality of the radiative characteristics of the Canadian Climate Centre (CCC) second-generation GCM. The GCM data were obtained from the Atmospheric Model Intercomparison Project (AMIP) simulation. Data corresponding to the period January 1985 through December 1988 were examined since this period of the AMIP simulation overlaps with the Earth Radiation Budget Experiment (ERBE) and the International Satellite Cloud Climatology Project (ISCCP) datasets. Attention was given to mean January and July conditions. Optical properties of surfaces, clear skies, and cloudy skies were examined.

Ocean albedos are too high in the Tropics and too low in the polar regions relative to surface observations and theoretical estimates. Compared to a satellite-derived dataset, however, they are slightly underestimated. Throughout much of the Sahara and Saudi Deserts surface albedos are too low, while for much of Western Australia they are too high. Excessive amounts of snow in Southeast Asia seem to have been sustained by a localized snow albedo feedback related to inappropriate snow albedo specification and a weak masking effect by vegetation. Neglect of freshwater lakes in the Canadian Shield leads to negative and positive albodo anomalies in winter and summer, respectively.

Like many GCMS, the CCC model has too little atmospheric H₂0 vapor. This results in too much outgoing longwave radiation from clear skies, especially in the Tropics. Neglect of all trace gases except for C0₂ and weak H₂0 vapor absorption exacerbate this bias.

Assessment of the radiative properties of clouds was done very generally at this stage due to lack of confidence in available observational data. Total and high cloud fractions were compared to ISCCP estimates. Warm tropical oceans appear to have too much high cloud. Evaluation of low cloud fraction is less straightforward but it is clear that due to lack of a shallow convection scheme and coarse vertical resolution, the GCM is almost devoid of low clouds over cool oceans.

Cloud radiative forcing CRF from the GCM was compared to CRF obtained from ERBE data. Globally averaged, net CRF is in excellent accord with observations but shortwave and longwave CRFs are too strong. Zonal averages, however, reveal biases in which clouds act to cool the Tropics too much and cool the high latitudes too little during summer, yet they warm polar regions too much during winter. Regional examination shows that these biases are confined largely to oceans. Tropical oceans have excessive shortwave CRF despite good total cloud amounts. This may be due to neglect of cloud geometry effects on solar radiative transfer.

Abstract

A parameterization that relates the reflected solar flux at the top of the atmosphere to the net solar flux at the surface in terms of only the column water vapor amount and the solar zenith angle was tested against surface observations. Net surface fluxes deduced from coincidental collocated satellite-measured radiances and from measurements from towers in Boulder during summer and near Saskatoon in winter have mean differences of about 2 W m⁻², regardless of whether the sky is clear or cloudy. Furthermore, comparisons between the net

Abstract

The present study investigates the interdecadal variation of precipitation over the Hengduan Mountains (HM) during rainy seasons from various reanalysis and observational datasets. Based on a moving t test and Lepage test, an obvious rainfall decrease is identified around 2004/05. The spatial distribution of the rainfall changes exhibits large and significant precipitation deficits over the southern HM, with notable anomalous lower-level easterly divergent winds along the southern foothills of the Himalayas (SFH). The anomalous easterlies are located at the northern edge of two cyclones, with two centers of positive rainfall anomalies over the west coast of India (WCI) and the Bay of Bengal (BOB). Observational evidence and numerical experiments demonstrate that the decadal changes of SST over the WP and WIO suppress rainfall over the eastern Indian Ocean (EIO) through large-scale circulation adjustment. The EIO dry anomalies trigger the cross-equatorial anticyclonic wind anomalies as a Rossby wave response, and further cause anomalous meridional circulation and moisture transport over the WCI and BOB, favoring the rainfall increase there. The anomalous easterlies at the northern edge of two cyclones induced by the wet anomalies–related heating cause the divergence anomalies along the SFH, resulting in the reduction of precipitation in the HM. In turn, the two anomalous cyclones and dry anomalies have positive feedback on the wet and easterly wind anomalies, respectively, emphasizing the importance of the circulation–heating interaction.