Qilong Min

Cloud properties derived from surface MFRSR measurements and comparison with GOES results at the ARM SGP Site

We describe a family of inversion methods to infer the optical depth, τ, of warm clouds from surface measurements of spectral irradiance. Our most complex retrieval also uses the total liquid water path measured by a microwave radiometer to obtain the effective radius, re, of the cloud droplets. We apply these retrievals to data from the Atmospheric Radiation Measurement (ARM) Program, and compare our results to those produced by the GOES satellite for episodes where total overcast was observed. Our surface-based estimates of τ agree with those from GOES when the optical depths are <10, but are consistently larger by as much as a factor of 2 when optical depths are greater. We show that the uncertainties associated with the surface-based retrievals are less than those done from a satellite, and argue from the time series of the observations and the statistics of the measurements that the disagreement is not merely a consequence of the larger spatial average sampled by the satellite.

Thin Liquid Water Clouds: Their Importance and Our Challenge

Abstract Many of the clouds important to the Earths energy balance, from the Tropics to the Arctic, contain small amounts of liquid water. Longwave and shortwave radiative fluxes are very sensitive to small perturbations of the cloud liquid water path (LWP), when the LWP is small (i.e., < 100 g m−2; clouds with LWP less than this threshold will be referred to as “thin”). Thus, the radiative properties of these thin liquid water clouds must be well understood to capture them correctly in climate models. We review the importance of these thin clouds to the Earths energy balance, and explain the difficulties in observing them. In particular, because these clouds are thin, potentially mixed phase, and often broken (i.e., have large 3D variability), it is challenging to retrieve their microphysical properties accurately. We describe a retrieval algorithm intercomparison that was conducted to evaluate the issues involved. The intercomparison used data collected at the Atmospheric Radiation Measurement (ARM) S...

The rotating shadowband spectroradiometer (RSS) at SGP

The RSS provides continuous spectral measurements of total-horizontal, diffuse-horizontal, and direct-normal irradiance from 360 to 1100 nm using the automated shadowband technique. We show instrument performance, calibration method and accuracies, case data from the first-generation instrument operated at the Southern Great Plains (SGP) site, and comparisons with the improved design now starting operation.

Aerosol indirect effect studies at Southern Great Plains during the May 2003 Intensive Operations Period

During May 2003 the Department of Energys Atmospheric Radiation Measurement Program conducted an Intensive Operations Period (IOP) to measure the radiative effects of aerosol and clouds. A suite of both in situ and remote sensing measurements were available to measure aerosol and cloud parameters. This paper has three main goals: First, it focuses on comparison between in situ retrievals of the radiatively important drop effective radius r e and various satellite, airborne, and surface remote sensing retrievals of the same parameter. On 17 May 2003, there was a fortuitous, near-simultaneous sampling of a stratus cloud by five different methods. The retrievals of r e agree with one another to within ∼20%, which is approximately the error estimate for most methods. Second, a methodology for deriving a best estimate of r e from these different instruments, with their different physical properties and sampling volumes, is proposed and applied to the 17 May event. Third, the paper examines the response of r e to changes in aerosol on 3 days during the experiment and examines the consistency of remote sensing and in situ measurements of the effect of aerosol on r e . It is shown that in spite of the generally good agreement in derived r e , the magnitude of the response of r e to changes in aerosol is quite sensitive to the method of retrieving r e and to the aerosol proxy for cloud condensation nuclei. Nonphysical responses are sometimes noted, and it is suggested that further work needs to be done to refine these techniques.

Joint statistics of photon path length and cloud optical depth: Case studies

We show the joint statistics of photon path length and cloud optical depth for cloudy sky cases observed at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site between September and December 1997. The photon path lengths are retrieved from moderate resolution oxygen A-band observations taken by a rotating shadow band spectroradiometer (RSS). For high optical depth cloud cases, two different populations in the scattergram of the path length versus cloud optical depth are apparent. One population is a result of single-layer cloud cases that exhibit a small variation of path length enhancement over a large optical depth range, together with a strong correlation between the radiation field and the cloud liquid water path, while the second population is attributed to multiple-layer cloud cases with large variability of enhanced photon path lengths. When the optical depth is less than 5, the population of cases appears to bifurcate as the solar air mass increases, with the lower branch exhibiting pressure-weighted path lengths shorter than the direct beam path lengths at these larger solar zenith angles. Using information from a millimeter-wave cloud radar, together with lidar and balloon-borne sonde data to further analyze these cases demonstrates that this bifurcation is caused by the altitude of the scattering; thin clouds aloft produce the lower branch and low-level aerosols produce the upper branch.

Joint statistics of photon pathlength and cloud optical depth

A mean pressure- and temperature-weighted photon pathlength in the atmosphere can be inferred from moderate resolution measurements in the O2 A-band. We show a pathlength retrieval method and calibration results for measurements from a Rotating Shadowband Spectroradiometer (RSS), and present the joint statistics of pathlength and cloud optical depth for cloudy skies observed at the Southern Great Plains (SGP) site from September 30 to December 22, 1997. Two different population branches are apparent in the scattergram of the pathlength versus cloud optical depth; we attribute these to 1) singlelayer cases exhibiting small variations of pathlength enhancement over large optical depth ranges; and 2) multiple layer cases with large variances of enhanced photon pathlengths.

Clouds, Aerosol, and Precipitation in the Marine Boundary Layer: An ARM Mobile Facility Deployment

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Importance of Accurate Liquid Water Path for Estimation of Solar Radiation in Warm Boundary Layer Clouds: An Observational Study

A 1-yr observational study of overcast boundary layer stratus at the U.S. Department of Energy Atmospheric Radiation Measurement Program Southern Great Plains site illustrates that surface radiation has a higher sensitivity to cloud liquid water path variations when compared to cloud drop effective radius variations. The mean, median, and standard deviation of observed cloud liquid water path and cloud drop effective radius are 0.120, 0.101, 0.108 mm and 7.38, 7.13, 2.39 mm, respectively. Liquid water path variations can therefore cause 3 times the variation in optical depth as effective radius—a direct consequence of the comparative variability displayed by the statistics of the two parameters. Radiative transfer calculations demonstrate that, over and above the impact of higher liquid water path variability on optical depth, normalized cloud forcing is 2 times as sensitive to liquid water path variations as it is to effective radius variations. Consequently, radiative transfer calculations of surface flux using observed liquid water paths and a fixed effective radius of 7.5 mm have a 79% correlation with observed values. This higher sensitivity of solar flux to liquid water path is a result of the regimes of natural occurrence of cloud liquid water paths and cloud drop effective radii.

Critical Evaluation of the ISCCP Simulator Using Ground-Based Remote Sensing Data

Abstract Given the known shortcomings in representing clouds in global climate models (GCMs), comparisons with observations are critical. The International Satellite Cloud Climatology Project (ISCCP) diagnostic products provide global descriptions of cloud-top pressure and column optical depth that extend over multiple decades. Given the characteristics of the ISCCP product, the model output must be converted into what the ISCCP algorithm would diagnose from an atmospheric column with similar physical characteristics. This study evaluates one component of this so-called ISCCP simulator by comparing ISCCP results with simulated ISCCP diagnostics that are derived from data collected at the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility. It is shown that if a model were to simulate the cloud radiative profile with the same accuracy as can be derived from the ARM data, the likelihood of that occurrence being classified with similar cloud-top pressure and ...

An adjoint formulation of the radiative transfer method

Radiative transfer problems may be solved “adjointly” from an observed excident radiance flux distribution backward to the incident fluxes. We describe an adjoint formulation based on the discrete ordinate radiative transfer method, with application to atmospheric radiative transfer, including effects of the surface albedo. We compare this adjoint approach with forward radiative transfer solutions for a set of synthetic cases and also with observed surface irradiance data from a multifilter rotating shadow band radiometer (MFRSR). We compute the irradiances and mean intensities at arbitrary altitudes for fixed sky conditions but varying solar zenith angles. For these cases the adjoint method is comparably accurate and markedly faster.