RADIATIVE IMPACT OF CONTINENTAL STRATUS AT THE SOUTHERN GREAT PLAINS: A CLIMATOLOGICAL PERSPECTIVE
Open Access
- Author:
- Sengupta, Manajit
- Graduate Program:
- Meteorology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 07, 2002
- Committee Members:
- Thomas L Ackerman Jr., Committee Member
Johannes Verlinde, Committee Member
Dennis Lamb, Committee Member
Eugene Edmund Clothiaux, Committee Chair/Co-Chair
Timothy Joseph Kane, Committee Member - Keywords:
- clouds
boundary layer clouds
radiation
liquid water path
effective radius
climatology
stratus - Abstract:
- A four-year climatology of the radiatively important parameters of warm boundary layer clouds, including cloud liquid water path, cloud base height and normalized cloud forcing, was developed for the Southern Great Plains, Department of Energy Atmospheric Radiation Measurement Program Cloud and Radiation Testbed, site. The climatology shows that cloud liquid water paths were approximately exponentially distributed, ranging primarily from just above 0 to 0.2 mm. Effective radii from one year of retrievals were normally distributed, varying primarily between 5.5 microns and 9.5 microns. A one-year observational study of overcast boundary layer stratus illustrated that surface radiation was primarily sensitive to cloud liquid water path with effective radius having a secondary influence. Radiative transfer calculations using the previously determined natural ranges of effective radius and liquid water path explained the observed sensitivities. Overall, there was a 79% correlation between observed and computed surface fluxes when using a fixed effective radius of 7.5 microns in the calculations. The use of one-dimensional radiative transfer was demonstrated to be appropriate for the study, as the independent column approximation held for the overcast cases. A cloud liquid water path retrieval using infrared emission measurements was developed, as uncertainties in microwave radiometer retrieved liquid water paths were comparable to the mode of the observations. Independent verification of the results, which are unbiased on some days, but not others, with respect to the microwave radiometer derived values, is not straightforward and was not attempted in the study. Ultimately, this retrieval is valuable for estimating liquid water paths of clouds with low emissivities, thereby acting as a complement to microwave radiometer retrieved liquid water paths. A parameterization of normalized cloud forcing based on liquid water path and solar zenith angle was developed and tested against observations, yielding results comparable to the best set of explicit surface flux calculations. The parameterization provides a fast and accurate way of estimating either surface flux or liquid water path given the other. This parameterization is useful for research requiring solar energy estimates in the presence of clouds as well as evaluation of cloud-radiation interactions in weather prediction and climate models. It is also concluded from this study that measurement of the indirect aerosol effect will be problematic at the site, as variations in cloud liquid water path will most likely mask effects of variations in particle size. Furthermore, the absence of any bias relative to observations in the computed surface broadband fluxes based on effective radii derived from narrowband (415 nm) transmission indicates the absence of anomalous absorption in the clouds analyzed in this study.