Fu-Lung Chang

Estimating the vertical variation of cloud droplet effective radius using multispectral near‐infrared satellite measurements

[1] This paper presents a satellite-based retrieval method for inferring the vertical variation of cloud droplet effective radius (DER) by utilizing multispectral near-infrared (NIR) measurements at 1.25, 1.65, 2.15, and 3.75 μm, available from the Moderate Resolution Imaging Spectrometer (MODIS) satellite observations. The method is based on the principle that these multispectral NIR measurements convey DER information from different heights within a cloud, which is sufficient to allow for the retrieval of a linear DER vertical profile. The method is applicable to low-level, nonprecipitating, stratiform clouds as their DER often increases monotonically from cloud bottom to cloud top. As such, an optimum linear DER profile can be derived by comparing multispectral NIR measurements to corresponding model values generated for a large set of linear DER profiles. The retrieval method was evaluated and compared to the conventional 3.7-μm retrieval method by applying both methods to some marine stratocumulus clouds with in situ observations of microphysical profiles. Capable of capturing the DER variation trend, the retrieved linear DER profiles showed large improvement over the conventional 3.75-μm retrievals. Mean differences between the linear DER retrievals and observed profiles were generally small for both cloud top and bottom (<1.0 μm), whereas the conventional retrievals are prone to systematic overestimation near cloud bottom. The sensitivities of the linear DER retrieval to various parameters, as well as the error analyses, were also investigated extensively.

A Near-Global Climatology of Single-Layer and Overlapped Clouds and Their Optical Properties Retrieved from Terra/MODIS Data Using a New Algorithm

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 (VIS), and infrared emissivity () 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 ( 0.85 and Pc 500 hPa) and the remaining 20% (17%) are thick high clouds ( 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 VIS of 1.5 and mean 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.

A New Method for Detection of Cirrus Overlapping Water Clouds and Determination of Their Optical Properties

The frequent occurrence of high cirrus overlapping low water cloud poses a major challenge in retrieving their optical properties from spaceborne sensors. This paper presents a novel retrieval method that takes full advantage of the satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS). The main objectives are identification of overlapped high cirrus and low water clouds and determination of their individual optical depths, top heights, and emissivities. The overlapped high cloud top is determined from the MODIS CO2-slicing retrieval and the underlying low cloud top is determined from the neighboring MODIS pixels that are identified as single-layer low clouds. The algorithm applies a dual-layer cloud radiative transfer model using initial cloud properties derived from the MODIS CO2-slicing channels and the visible (0.65 m) and infrared (11 m) window channels. An automated iterative procedure follows by adjusting the high cirrus and low water cloud optical depths until computed radiances from the dual-layer model match with observed radiances from both the visible and infrared channels. The algorithm is valid for both single-layer and dual-layer clouds with the cirrus optical depth 4 (emissivity 0.85). For more than two-layer clouds, its validity depends on the thickness of the upper-layer cloud. A preliminary validation is conducted by comparing against ground-based active remote sensing data. Pixel-by-pixel retrievals and error analyses are presented. It is demonstrated that retrievals based on a single-layer assumption can result in systematic biases in the retrieved cloud top and optical properties for overlapped clouds. Such biases can be removed or lessened considerably by applying the new algorithm.

Impact of the Vertical Variation of Cloud Droplet Size on the Estimation of Cloud Liquid Water Path and Rain Detection

Abstract Cloud droplet effective radius (DER) and liquid water path (LWP) are two key parameters for the quantitative assessment of cloud effects on the exchange of energy and water. Chang and Li presented an algorithm using multichannel measurements made at 3.7, 2.1, and 1.6 μm to retrieve a cloud DER vertical profile for improved cloud LWP estimation. This study applies the multichannel algorithm to the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) data on the Aqua satellite, which also carries the Advanced Microwave Scanning Radiometer (AMSR-E) for measuring cloud LWP and precipitation. By analyzing one day of coincident MODIS and AMSR-E observations over the tropical oceans between 40°S and 40°N for overcast warm clouds (>273 K) having optical depths between 3.6 and 23, the effects of DER vertical variation on the MODIS-derived LWP are reported. It is shown that the LWP tends to be overestimated if the DER increases with height within the cloud and underestimated if the DER decreases with...

Cloud optical depths and TOA fluxes: Comparison between satellite and surface retrievals from multiple platforms

Performances of two cloud property retrieval schemes are assessed by comparison with each other. The study is limited to liquid phase clouds, Two parameters are assessed: cloud optical depth in the visible band and broadband shortwave (SW) flux at the top-of-atmosphere (TOA). Retrievals are based on look-up tables for a variety of conditions using an adding-doubling code coupled with LOWTRAN-7 transmittance models. Comparisons of cloud optical depths retrieved from ground measurements with those from ISCCP DX data agree better than previously reported comparisons with the original ISCCP CX data. Likewise, good agreement is obtained between retrieved and inferred TOA SW fluxes. Differences fall within uncertainties of input parameters, as well as shortcomings in the use of a plane-parallel radiative transfer model and in the inversion schemes themselves.

Examining the Relationship between Cloud and Radiation Quantities Derived from Satellite Observations and Model Calculations

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. ERBEobserved 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 m22, depending on SZA, but individual differences may reach up to 40‐50 W m22 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 ; 8Wm 22 at SZA 338 and by 30 W m22 at SZA 608.

The Dependence of TOA Reflectance Anisotropy on Cloud Properties Inferred from ScaRaB Satellite Data

Abstract An angular dependence model (ADM) describes the anisotropy in the reflectance field. ADMs are a key element in determining the top-of-the-atmosphere (TOA) albedos and radiative fluxes. This study utilizes 1-yr satellite data from the Scanner for Radiation Budget (ScaRaB) for overcast scenes to examine the variation of ADMs with cloud properties. Using ScaRaB shortwave (SW) overcast radiance measurements, an SW mean overcast ADM, similar to the Earth Radiation Budget Experiment (ERBE) ADM, was generated. Differences between the ScaRaB and ERBE overcast ADMs lead to biases of ∼0.01–0.04 in mean albedos inferred from specific angular bins. The largest biases are in the backward scattering direction. Overcast ADMs for the visible (VIS) wavelength were also generated using ScaRaB VIS measurements. They are very similar to, but a little smaller at large viewing angles and a little larger at nadir, than the SW overcast ADMs. To evaluate the effect of cloud properties on ADMs, ScaRaB overcast observation...