Retrieving Optical And Microphysical Properties Of Mixed-phase And Dusty Cloud Over Northwestern China From MFRSR

Clouds are an important modulator of the energy budget of the planet and play a critical role in the atmospheric energy cycle, water vapor cycle as well as earth climate system. The impact of clouds on the radiative fluxes, both at the surface and top of the atmosphere as well as the redistribution of the radiant energy in the atmosphere, depends on cloud cover, thermodynamic phase and optical depth. The northwestern China is a typical arid and semi-arid climatic characteristic representative area and there are some special cloud type, such as mixed-phase clouds and dusty clouds. The existence of these clouds increases the complexity and uncertainty of cloud optical properties retrieval. Thus it becomes more and more important to build up the retrieval method for mixed-phase cloud and dusty cloud. Based on the particular observation geometry of Multifilter Rotating Shadowband Radiometer (MFRSR) and accurate transmittance measurements, in this study, we have developed a method for retrieving optical properties of optically thin mixed-phase cloud and dusty cloud, respectively. Additionally, a ratio method for accurately estimating fractional sky cover from spectral transmittance measurements has also been developed. The theoretical sensitivity studies and validation and evaluation from measurements at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) site, Zhangye and Jingtai mobile facility during China-US joint dusty storms observation achieve satisfactory results.On the basis of the simultaneous measurements of direct and total radiation from MFRSR, we allow partitions of direct-beam and total radiation determined by different thermodynamic phase. Under the assumption of radiation closure, we further infer mixed ratio of cloud water and ice for optical thin clouds and identify cloud thermodynamic phase, and thus accurately estimate optical depths of optically thin mixed-phase cloud. For the new retrieval method, 1% measurement error will result in maximum uncertainties of 8.4% and 0.107 in retrieved total optical depths and mixed ratio, respectively. According to validation and evaluation using forward simulations and in situ measurements at SACOL, Zhangye and Jingtai sites, the new retrieval method achieves the high consistency of retrieved cloud optical depth from direct-beam and total radiation: the slope of 1.084 between the two with correlation coefficient of 0.96 and RMS of 0.679. The maximum biases (relative errors) of cloud optical depths within the range of effective radius of clouds are 0.21 (7.9%) and 0.30 (9.4%). Therefore, the retrieval method provides not only accuracy retrievals of cloud optical depths but also unique mixed ratio of cloud water and ice for optically thin clouds under overcast conditions.A ratio method for estimating fractional sky cover from spectral measurements has been developed. The spectral characteristics of clouds and clear-sky aerosols are utilized to partition sky fraction (sky cover). As illustrated in our sensitivity study and demonstrated in real measurements, the transmittance ratio at selected wavelengths is insensitive to solar zenith angle and major atmospheric gaseous absorption. With a localized baseline procedure, retrievals of this ratio method are independent of absolute calibration and weakly sensitive to changes in cloud and aerosol optical properties. Therefore this method substantially reduces the retrieval uncertainty. The uncertainty of this method, estimated through the sensitivity study and intercomparison, is less than 10%. With globally deployed narrowband radiometers, this simple ratio method can substantially enhance the current capability for monitoring fractional sky cover.Based on the scattering properties of nonspherical dust aerosol, a new approach is also developed for retrieving dust aerosol optical depths of dusty clouds. The new method is based on transmittance measurements from surface-based instruments multi-filter rotating shadowband radiometer (MFRSR) and cloud parameters from lidar measurements. It uses the difference of absorption between dust aerosols and water droplets for distinguishing and estimating the optical properties of dusts and clouds, respectively. As illustrated in sensitivity study, this new retrieval method is not sensitive to the retrieval error of cloud properties and the maximum absolute deviations of dust aerosol and total optical depths for thin dusty cloud retrieval algorithm are only 0.056 and 0.1, respectively, for given possible uncertainties. The retrieval error for thick dusty cloud mainly depends on lidar-based total dusty cloud properties. The retrieval method lays the foundation on accurately estimating dusty cloud radiative forcing and evaluating direct and indirect effect of dust aerosols on cloud.