On the properties of cirrus clouds over the tropical West Pacific

Abstract

Understanding Earth's climate is a complex undertaking, and requires the development of accurate simulations that combine all the natural elements that can affect climate. The data that goes into these models needs to be as precise as possible. and collecting reliable data is an important and sometimes difficult step in this research. Clouds, in particular tropical thin cirrus clouds, exert a major influence on climate through cloud radiative forcing (CRF). These clouds, with their variable height, optical depth and particle distribution, make collecting accurate data difficult. Investigation by remote sensing techniques, in particular by LIDARs alone or in combination with other instruments, has become a popular way of retrieving cirrus cloud physical and microphysical properties such as particle concentration (N), characteristic diameter (D), ice water content ( IWC) and ice water path (IWP).

The aim of this study is to document the properties of tropical thin cirrus using combinations of data collected from various remote sensing systems. A new method for determining cloud optical depths along with a new parameterization that treats multiple scattering effects in the LIDAR equation is introduced. A novel "inverse” model applied to lidar measurements produces profiles for IWC and N. A new, simple analytical method developed for the combination of RADAR and LIDAR system is also presented. This combination of data provides vertical profiles for D, N, and IWC. One key finding derived from the measured data is the observed relationship between optical depth and IWP. Two different methods that combine active and passive techniques are explored. The first is the LIRAD technique, which combines LIDAR and IR radiometric data. A new approach towards the LIRAD method is introduced and from this approach we are able to identify three distinct cloud backscatter regimes correlated with the mid-cloud temperature. Second, the combination between the NASA Cloud Physics Lidar and MODIS Airborne Sensor data is examined. A new method, developed to analyze MODIS data, alleviates the complex problem of asymmetric scattering induced by ice crystals through the use of similarity and scaling. What is new is the way the ambiguities in defining key scaling parameters are overcome.

There are a number of limitations that have to be applied to the results of this study. First, the new lidar analysis tools developed are only applicable to thin cirrus clouds through which the lidar can penetrate. This corresponds to optical depth of approximately 1.5 and IWP of 27 g m2, assuming a 30 µm effective radius particle. Second, the cloud physical information derived from lidar alone, is subject to the imposed assumption of particle size. This assumption is relaxed in the radar-lidar method which is used to determine the credibility of the value of particle size used.

The results obtained using these new analyses tools are used to study the radiative budget of tropical thin cirrus. It is shown that thin cirrus radiatively heat the atmosphere both within the layer of cloudiness as well as within the atmosphere below. A convenient parameterization of the LW and SW CRF as a function of the IWP is tested against measured data.