Sensitivity analysis of terahertz wave passive remote sensing of cirrus microphysical parameters

Abstract

Cirrus clouds play an important role in the energy budget and the hydrological cycle of the atmosphere. It is still one of the largest uncertainties in the global climate change studies. This is mainly attributable to the measurement discrepancies of cirrus parameters, especially the microphysical parameters, which are constrained by the existing methods. With THz wavelengths on the order of the size of typical cirrus cloud particles and therefore being sensitive to cirrus clouds, THz region is expected to have a promising prospect concerning measuring cirrus microphysical parameters (ice water path and effective particle size). In order to evaluate the effects of cirrus microphysical parameters on THz transmission characteristics and the sensitivity of cirrus in THz region, the THz radiation spectra at the top of atmosphere in the clear sky and the cloudy situations are simulated and calculated based on the atmospheric radiative transfer simulator. The effects of cirrus particle shape, particle size and ice water path on THz transmission characteristics are obtained by analyzing the brightness temperature difference between the two situations, and the sensitivity parameters that quantitatively describ the effects. The results indicate that cirrus particle shape, particle size and ice water path have different effects on the THz wave propagation. The cirrus effect varies also with channel frequency. Overall, in the low frequency channels, cirrus effects are enhanced with the increases of particle size and ice water path; in the high frequency channels, cirrus effects are more complicated and vary with particle size and ice water path. The effects are first enhanced and then turned into saturation. The THz wave is sensitive to cirrus cloud ice water path and effective particle size, and THz wave may be the best waveband for remote sensing of cirrus microphysical parameters in theory. For thin clouds, the sensitivity parameters are approximately constant, indicating that the spectral brightness temperature at the top of the atmosphere almost shows linear relationship with ice water path, and the sensitivity parameters increase with frequency increasing. For thick clouds, the sensitivity of cirrus to ice water path decreases and gradually becomes saturated, and the higher the frequency, the more quickly it tends to saturation level. Compared with the microwave and infrared, THz wave can provide many detailed information about cirrus. The two-channel look-up table indicates that THz wave passive remote sensing of cirrus may be a stable and effective method. The results will be conducible to developing the technology of THz wave remote sensing of cirrus microphysical parameters. Moreover, it is also beneficial to improving the cirrus detection precision.