Ground-Based Cirrus Observation

[摘要] A ground-based observation system has been developed at the Meteorological Research Institute (MRI) for simultaneous measurements of cloud structure and radiative properties of high-level ice clouds. In this observation system, the cloud microphysical quantities and spectral radiances in the region of the 10μm window could be simultaneously observed by a Hydrometeor Video Sonde (HYVIS) and a Fourier Transform Infrared (FTIR) spectro-radiometer. On the basis of the results measured by HYVIS, the size distribution of ice particles was approximated by the power law distribution, and the simulation calculations of observed radiances were made for the some combination of the lower limit of power law distribution (γ1) and the optical thickness at 10.5μm(τ10.5). Using these data, the minimum error point is searched in the error map. By this method, we retrieve optical thickness and particle size information.The optically thin, medium and thick cases were analyzed in order to investigate the characteristic of our retrieval method. It is found that the optical thickness determined the magnitude of observed radiance, and size distributions determined the slope of the spectrum in the 860 to 980cm-1 region. The simulated and observed radiances agree within ±2mW/(m2 sr cm-1), except the 9.6μm ozone band and the root mean squares error 1.2mW/(m2 sr cm-1). The error analysis showed that though 5% systematic error is permitted to optical thickness retrieval in some cases, 1% systematic error causes large errors in the size information retrieval, and that a ±500m height error corresponds to 1 to 3% radiance error. Furthermore, the values estimated from the radiance in the region of wavenumber 860 to 980cm-1 were almost the same as those from radiance in the whole region, and the optical thickness may be determined from data in the region 1080 to 1200cm-1 without being affected by the size distribution. We also retrieved τ10.5 and effective radius (γeff) using the log-normal size distribution within the same degree of error as obtained by the power law model.Furthermore, we investigated the relationship between the parameters retrieved. The ratio of the visible optical thickness at 0.5μm(τ0.5) to the infrared one at 10.5μm(τ10.5) on June 22 is 0.4 to 1.0, and that on June 30 is 1.0 to 1.8. There is a tendency that the ratio of τ10.5 to τ0.5 becomes large as the effective radius increases. We also investigated the relation between τ10.5 and the effective radius (γeff) of log-normal size distribution. The relation between them changes case by case. There is a negative correlation between them in the case of June 22. In the case of June 30, the effective radius is scattered between 20 and 85μm.Concurrent NOAA-11 satellite data over the Tsukuba area were also analyzed. The results show that observation from the satellite is consistent with the ground based observation, and that the large difference of the brightness temperature observed by the different window channels is closely related to the existence of small ice particles of order 10μm.

[学科分类] 大气科学