Impact of clouds on microwave remote sensing

Abstract

The thesis presents a new radiative transfer model that can take into account the impact of cirrus clouds on microwave satellite remote sensing. The ice particles in cirrus clouds interact with microwave radiation mostly through scattering. This can also lead to polarization ofthe radiation. Thus, a radiative transfer model that can solve the vector radiative transfer equation for a multiple scattering medium is essential. This has led to the development of the scattering version of the AtmosphericRadiative Transfer Simulator (ARTS), which was previously a clear sky radiative transfer model. The model can be used for up, limb- and down-looking sensors, although this thesis focuses only on the effect of clouds on down-looking sensors. In order to understand the scattering signal produced by cirrus clouds, it is essential to know the ranges of cirrus microphysical properties and how they are influencing the single scattering properties of the ice particles in the cirrus clouds. The work presented here shows that single scattering properties are highly dependent on the size parameter. The radiative transfer simulations are done for various cloud scenarios at some example frequencies, which correspond to the center frequencies of the upper side band of the channels of the Advanced Microwave Sounding Unit-B (AMSU-B) instrument. The sensitivity to cirrus cloud parameters like the ice water content, the particle size andshape, and the cloud altitude at these frequencies are examined. It is shown that the scattering signal is very much dependent on the ice water content and the particle size. The scattering signal is also dependent on the sensitivity of these channels to water vapor. The water vapor channels are sensitive to changes in cirrus cloud properties only when the cloud altitude is above the sounding altitude of the channel. This implies that channel 18 has the minimum effect from cirrus clouds and channel 20 has the maximum effect from the same cirrus cloud. Although the effect of cirrus clouds is to decrease the brightness temperature compared to the clear sky case, for a surface sensitive channel, the presence of cirrus clouds at low altitudes can lead to a brightness temperature enhancement due to cloud emission against a cold surface background. This is also true in the case of liquid clouds, which have higher absorption than scattering and which are occurring at lower altitudes. For the water vapor sensitive channels, the presence of liquid clouds decreases the brightness temperature if the cloud is located above the sensing altitude of the channel. Finally, a comparison of the brightness temperatures simulated by the model to collocated AMSU-B observations is presented. The results presented show that the model is able to capture the brightness temperature depression seen in the observation that are due to cirrus clouds. The reasons for the discrepancies in the comparison are presented in detail.