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Short-term Variability of Extinction by Broadband Stellar Photometry

Description: Aerosol optical depth variation over short-term time intervals is determined from broadband observations of stars with a whole sky imager. The main difficulty in such measurements consists of accurately separating the star flux value from the non-stellar diffuse skylight. Using correction method to overcome this difficulty, the monochromatic extinction at the ground due to aerosols is extracted from heterochromatic measurements. A form of closure is achieved by comparison with simultaneous or temporally close measurements with other instruments, and the total error of the method, as a combination of random error of measurements and systematic error of calibration and model, is assessed as being between 2.6 and 3% rms.
Date: March 18, 2005
Creator: Musat, I.C. & Ellingson, R.G.
Partner: UNT Libraries Government Documents Department

The Dependence of Cloud Particle Size on Non-Aerosol-Loading Related Variables

Description: An enhanced concentration of aerosol may increase the number of cloud drops by providing more cloud condensation nuclei (CCN), which in turn results in a higher cloud albedo at a constant cloud liquid water path. This process is often referred to as the aerosol indirect effect (AIE). Many in situ and remote sensing observations support this hypothesis (Ramanathan et al. 2001). However, satellite observed relations between aerosol concentration and cloud drop size are not always in agreement with the AIE. Based on global analysis of cloud effective radius (r{sub e}) and aerosol number concentration (N{sub a}) derived from satellite data, Sekiguchi et al. (2003) found that the correlations between the two variables can be either negative, or positive, or none, depending on the location of the clouds. They discovered that significantly negative r{sub e} - N{sub a} correlation can only be identified along coastal regions of the continents where abundant continental aerosols inflow from land, whereas Feingold et al. (2001) found that the response of r{sub e} to aerosol loading is the greatest in the region where aerosol optical depth ({tau}{sub a}) is the smallest. The reason for the discrepancy is likely due to the variations in cloud macroscopic properties such as geometrical thickness (Brenguier et al. 2003). Since r{sub e} is modified not only by aerosol but also by cloud geometrical thickness (H), the correlation between re and {tau}{sub a} actually reflects both the aerosol indirect effect and dependence of H. Therefore, discussing AIE based on the r{sub e}-{tau}{sub a} correlation without taking into account variations in cloud geometrical thickness may be misleading. This paper is motivated to extract aerosols' effect from overall effects using the independent measurements of cloud geometrical thickness, {tau}{sub a} and r{sub e}.
Date: March 18, 2005
Creator: Shao, H. & Liu, G.
Partner: UNT Libraries Government Documents Department

Retrieval of Cloud Ice Water Content Profiles from Advanced Microwave Sounding Unit-B Brightness Temperatures Near the Atmospheric Radiation Measurement Southern Great Plains Site

Description: One of the Atmospheric Radiation Measurement (ARM) Program important goals is to develop and test radiation and cloud parameterizations of climate models using single column modeling (SCMs) (Randall et al. 1996). As forcing terms, SCMs need advection tendency of cloud condensates besides the tendencies of temperature, moisture and momentum. To compute the tendency terms of cloud condensates, 3D distribution of cloud condensates over a scale much larger than the climate model's grid scale is needed. Since they can cover a large area within a short time period, satellite measurements are useful utilities to provide advection tendency of cloud condensates for SCMs. However, so far, most satellite retrieval algorithms only retrieve vertically integrated quantities, for example, in the case of cloud ice, ice water path (IWP). To fulfill the requirement of 3D ice water content field for computing ice water advection, in this study, we develop an ice water content profile retrieval algorithm by combining the vertical distribution characteristics obtained from long-term surface radar observations and satellite high-frequency microwave observations that cover a large area. The algorithm is based on the Bayesian theorem using a priori database derived from analyzing cloud radar observations at the Southern Great Plains (SGP) site. The end product of the algorithm is a 3D ice water content covering 10{sup o} x 10{sup o} surrounding the SGP site during the passage of the satellite. This 3D ice water content, together with wind field analysis, can be used to compute the advection tendency of ice water for SCMs.
Date: March 18, 2005
Creator: Seo, E-K. & Liu, G.
Partner: UNT Libraries Government Documents Department

Results from the Longwave Effective Cloud Fraction in the Cloudiness Intercomparison Introduction

Description: While it may seem to be a simple quantity, cloud amount is somewhat elusive. Different types of instruments placed next to each other can give different cloud amounts because they use different parts of the spectrum, have different fields of view, sampling rates, etc. Another consideration is that cloud amount depends on the physical scale under consideration. The cloud amount appropriate for comparison to a single pyrgeometer is not likely to be useful for a grid square with 100 km sides.
Date: March 18, 2005
Creator: Takara, E.E. & Ellingson, R.G.
Partner: UNT Libraries Government Documents Department