The calculation of cloud parameters from AVHRR data

Abstract

This thesis deals with the electromagnetic radiation that is reflected from and emitted by a clear or cloudy terrestrial atmosphere. Calculations are performed in order to estimate the radiation exitant to space in the visible, the near visible, and the two infrared windows at approximately 4 μm and 11 μm. The NOAA-9 satellite Advanced Very High Resolution Radiometer (AVHRR/2) is used as the data acquisition system. The satellite AVHRR system is described, as well as future TIROSN satellites. The pre-launch calibration of the radiometer is described in some detail. Post-launch calibration is examined using a reflectance difference method. The radiometer gains are shown to have changed by more than 20% from their pre-launch values. A LOWTRAN based atmospheric model suitable for New Zealand is developed to provide cloud free transmission values. Atmospheric scattering is shown to contribute negligibly to visible cloud measurements (for 8-bit radiometer resolution). The Channels 1 to 3 terrestrial reflectance is averaged from more than two years of satellite data. Sea-surface state is examined as a function of windspeed. A method of extracting surface temperature in rugged terrain is developed. Dual temperature measurements at 10.8 and 11.8 μm are used to estimate surface air temperature. The transfer of radiation in a single layer cloud is calculated as a function of cloud thickness, observation/illumination geometry and the effective radius of the cloud drop-distribution. The variation in drop-size is contained in an ensemble of modified-Gamma distributions whose effective radius varies from 1 to 30 μm with a fixed dispersion of 0.25. A technique is established which allows effective cloud radius to be estimated from 3.7 μm AVHRR/2 imagery. The radiation values at 0.65 μm and 11.8 μm are used to correct the 3.7 μm reflectance for variation in cloud thickness and temperature respectively. The procedure is limited to warm clouds of moderate optical depth. In addition, the inversion process generates values of cloud top height and temperature, cloud depth and column liquid water content. The method is used to observe the development of orographic cloud formed over the southern half of the North Island of New Zealand. The inversion results for this type of cloud are in agreement with aircraft measurements carried out by the New Zealand Meteorological Service and are well fitted by a simple drop-growth model.