This dissertation consists of three parts, each devoted to a particular issue of significant importance for satellite-based remote sensing of cirrus clouds. In the first part, we develop and present a fast infrared radiative transfer model on the basis of the adding-doubling principle. The model aims to facilitate the radiative transfer computations involved in hyperspectral remote sensing applications. The model is applicable to a variety of cloud conditions, including vertically inhomogeneous or multilayered clouds. It is shown that for hyperspectral applications the model is two order-of-magnitude faster than the well-known discrete ordinate transfer (DISORT) model, while maintains a similar accuracy. The second part is devoted to the investigation of uncertainties in the FSSP (Forward Scattering Spectrometer Probe) measurement of cloud extinction by small ice particles. First, the single-scattering properties of small ice particles in cirrus clouds are derived and compared to those of equivalent spheres according to various definitions. It is found that, although small ice particles in cirrus clouds are often “quasi-spherical”, their scattering phase functions and asymmetry factors are significant different from those of ice spheres. Such differences may lead to substantial underestimation of cloud extinction in FSSP measurement, if small ice particles are assumed to be spheres. In the third part, we present a comparison of cirrus cloud optical thickness retrievals from two important instruments, MODIS (Moderate Resolution Imaging Spectrometer) and POLDER (Polarization and Directionality of Earth’s Reflection), on board NASA’s A-train satellite constellation. The comparison reveals a large difference. Several possible reasons are discussed. It is found that much of the difference is attributable to the difference between the MODIS and POLDER retrieval algorithm in the assumption of cirrus cloud bulk scattering properties. Potential implications of the difference for climate studies are investigated. An important finding is that the use of an unrealistic cirrus bulk scattering model might introduce artificial seasonal variation of cirrus optical thickness and shortwave radiative forcing into the retrieval.

The dissertation first investigates the single-scattering properties of inhomogeneous ice crystals containing air bubbles. Specifically, a combination of the ray-tracing technique and the Monte Carlo method is used to simulate the scattering of light by randomly oriented large hexagonal ice crystals containing spherical or spheroidal air bubbles. The effect of the air bubbles within ice crystals is to smooth the phase functions, diminish the 22° and 46° halo peaks, and reduce the backscatter in comparison with the case of bubble-free ice crystals. Cloud reflectance look-up tables were generated at the wavelengths of 0.65 μm and 2.13 μm to examine the impact of accounting for air bubbles in ice crystal morphology on the retrieval of ice cloud optical thickness and effective particle size. To investigate the effect of the representation of aggregates on electromagnetic scattering calculations, an algorithm is developed to efficiently specify the geometries of aggregates and to compute some of their geometric parameters such as the projected area. Based on in situ observations, aggregates are defined as clusters of hexagonal plates with a chain-like overall shape. An aggregate model is developed with 10 ensemble members, each consisting of between 4-12 hexagonal plates. The scattering properties of an individual aggregate ice particle are computed using the discrete dipole approximation or an Improved Geometric Optics Method, depending upon the size parameter. The aggregate model provides an accurate and computationally efficient way to represent all aggregates occurring within ice clouds. We developed an algorithm to determine an appropriate ice cloud model for application to satellite-based retrieval of ice cloud properties. Collocated Moderate Resolution Imaging Spectroradiometer and Multi-angle Imaging SpectroRadiometer (MISR) data are used to retrieve the optical thicknesses of ice clouds as a function of scattering angle in the nine MISR viewing directions. The difference between cloud optical thickness and its averaged value over the nine viewing angles can be used to validate the ice cloud models. Using the data obtained on 2 July 2009, an appropriate ice cloud model is determined. With the presence of all the uncertainties in the current operational satellite-based retrievals of ice cloud properties, this ice cloud model has excellent performance in terms of consistency in cloud property retrievals with the nine MISR viewing angles.

This dissertation focuses on the global investigation of optically thin cirrus cloud optical thickness (tau) and microphysical properties, such as, effective particle size (D_(eff)) and ice crystal habits (shapes), based on the global satellite observations and fast radiative transfer models (RTMs). In the first part, we develop two computationally efficient RTMs simulating satellite observations under cloudy-sky conditions in the visible/shortwave infrared (VIS/SWIR) and thermal inferred (IR) spectral regions, respectively. To mitigate the computational burden associated with absorption, thermal emission and multiple scattering, we generate pre-computed lookup tables (LUTs) using two rigorous models, i.e., the line-by-line radiative transfer model (LBLRTM) and the discrete ordinates radiative transfer model (DISORT). The second part introduces two methods (i.e., VIS/SWIR- and IR-based methods) to retrieve tau and D_(eff) from satellite observations in corresponding spectral regions of the two RTMs. We discuss the advantages and weakness of the two methods by estimating the impacts from different error sources on the retrievals through sensitivity studies. Finally, we develop a new method to infer the scattering phase functions of optically thin cirrus clouds in a water vapor absorption channel (1.38-µm). We estimate the ice crystal habits and surface structures by comparing the inferred scattering phase functions and numerically simulated phase functions calculated using idealized habits.

From June to July 2021, the Leipzig Institute for Meteorology (LIM) participated in the Cirrus in High Latitudes (CIRRUS-HL) campaign. Utilizing the German High Altitude Long Range Research Aircraft (HALO), 24 research flights were conducted out of Oberpfaffenhofen, Germany. The initial goal of the campaign was to sample high-latitude cirrus clouds with a combination of in-situ and remote sensing instrumentation. However, due to the global coronavirus pandemic, the flights had to be carried out from southern Germany instead of northern Sweden. Thus, the flight time in Arctic latitudes was limited. Therefore, more objectives concerning midlatitude cirrus were included in the campaign goals. LIM contributed to CIRRUS-HL with measurements by the Broadband AirCrAft RaDiometer Instrumentation (BACARDI) and the Spectral Modular Airborne Radiation measurement sysTem (SMART). While BACARDI measured broadband solar and terrestrial upward and downward irradiance, SMART measured spectrally resolved solar upward radiance as well as upward and downward irradiance. / Von Juni bis Juli 2021 nahmen einige Mitarbeitende des LIM an der CIRRUS-HL Kampagne teil. Mit dem deutschen Forschungsflugzeug HALO (High Altitude Long Range Research Aircraft) wurden 24 Forschungsflüge von Oberpfaffenhofen, Deutschland, aus durchgeführt. Ursprüngliches Ziel der Kampagne war es, Zirruswolken in hohen Breitengraden mit einer Kombination aus In-situ- und Fernerkundungsinstrumenten zu untersuchen. Aufgrund der weltweiten Corona-Pandemie mussten die Flüge jedoch von Süddeutschland statt von Nordschweden aus durchgeführt werden. Daher wurden weitere Ziele in Bezug auf Zirruswolken in mittleren Breiten in die Ziele der Kampagne aufgenommen. Das LIM-Team betrieb die breitbandigen und spektralen Strahlungssensoren BACARDI (Broadband AirCrAft RaDiometer Instrumentation) und SMART (Spectral Modular Airborne Radiation measurement sysTem), wobeiBACARDI die breitbandige solare und terrestrische Auf- und Abwärtsstrahlung und SMART die spektral aufgelöste solareAuf- undAbwärtsstrahlung sowie dieAufwärtsstrahlungsdichte maß.