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In this case study, measured solar irradiances above and below Arctic cirrus are compared to simulations of the European Centre for Medium-Range Weather Forecasts' Integrated Forecasting System (IFS) making use of offline runs of the operational ecRad radiation scheme. Independent of the solar irradiances, cirrus properties are derived from active remote sensing and used to evaluate the optical and microphysical parameterizations in ecRad. The data set was collected in the central Arctic over sea ice (81° - 90° North) with the High Altitude LOng range research aircraft (HALO) during a campaign in March and April 2022. HALO was equipped with broadband radiation and remote sensing instrumentation, including upward and downward-looking pyranometers (solar irradiance), a cloud radar, and a multi-wavelength water vapor differential absorption lidar. Flight legs above andbelow single-layer cirrus were performed. Measurements of solar irradiance are used to evaluate the ecRad radiation scheme in two case studies of optically thin and thick cirrus. The optically thin cirrus had amean transmissivity of 0.9, while the optically thick cirrus had a transmissivity of about 0.6. Different ice optics parameterizations optionally available within ecRad are tested to improve the match between simulation and measurements. Furthermore, the IFS predicted ice water content and ice effective radius were replaced by values retrieved with the radar and lidar. The choice of ice optics parameterizations does not significantly improve the model-measurement agreement. However, introducing the retrieved ice microphysical properties brings measured and modelled irradiances into closer agreement for the optical thin cirrus, while the optically thick cirrus caseis now simulated as too thick. From the comparison of the different ice optics parametrizations in the original and in the forced setup, it can be concluded that the ice water content forcasted by the IFS is realistic. The missmatch between observed and simulated irradiances mostly originates from the assumed or parameterized ice effective radius. [ABSTRACT FROM AUTHOR]

The clouds in the Atlantic trade-wind region are known to have an important role in the global climate system. Acquiring a comprehensive characterization of these clouds based on observations is a challenge, but it is a necessary piece of information for the evaluation of their representation in models. An exploration of how the macrophysical and microphysical cloud properties and organization of the cloud field impact the large-scale cloud radiative forcing is presented here. Direct measurements of the cloud radiative effects from the Broadband AirCrAft RaDiometer Instrumentation (BACARDI) on board the High Altitude and LOng Range Research Aircraft (HALO) and cloud observations from the GOES-16 satellite during the Elucidating the role of clouds-circulation coupling in climate (EUREC4A) campaign provide evidence to demonstrate what drives the cloud radiative effects in shallow trade-wind clouds. We find that the solar and terrestrial radiative effects of these clouds are largely driven by their macrophysical properties (cloud fraction and a scene-averaged liquid water path). However, we also conclude that the microphysical properties, cloud top height and the organization of the cloud field demonstrate an increasing relevance in determining the cloud radiative effects as the cloud fraction increases. [ABSTRACT FROM AUTHOR]

The radiative role of ice clouds in the atmosphere is known to be important, but uncertainties remain concerning the magnitude and net effects. However, through measurements of the microphysical properties of cirrus clouds, we can better characterize them, which can ultimately allow for their radiative properties to be more accurately ascertained. It has recently been proposed that there are two types of cirrus clouds - in situ and liquid origin. In this study, we present observational evidence to show that two distinct types of cirrus do exist. Airborne, in situ measurements of cloud ice water content (IWC), ice crystal concentration (Nice), and ice crystal size from the 2014 ML-CIRRUS campaign provide cloud samples that have been divided according to their origin type. The key features that set liquid origin cirrus apart from the in situ origin cirrus are a higher frequency of high IWC (> 100 ppmv), higher Nice values, and larger ice crystals. A vertical distribution of Nice shows that the in situ origin cirrus clouds exhibit a median value of around 0.1 cm-3, while the liquid origin concentrations are slightly, but notably higher. The median sizes of the crystals contributing the most mass are less than 200 μm for in situ origin cirrus, with some of the largest crystals reaching 550 μm in size. The liquid origin cirrus, on the other hand, were observed to have median diameters greater than 200 μm, and crystals that were up to 750 μm. An examination of these characteristics in relation to each other and their relationship to temperature provides strong evidence that these differences arise from the dynamics and conditions in which the ice crystals formed. Additionally, the existence of these two groups in cirrus cloud populations may explain why a bimodal distribution in the IWC-temperature relationship has been observed. We hypothesize that the low IWC mode is the result of in situ origin cirrus and the high IWC mode is the result of liquid origin cirrus. [ABSTRACT FROM AUTHOR]

Ice clouds are known to be major contributors to radiative forcing in the Earth's atmosphere, yet describing their microphysical properties in climate models remains challenging. Among these properties, the ice water content (IWC) of cirrus clouds is of 5 particular interest both because it is measurable and because it can be directly related to a number of other radiatively important variables such as extinction and effective radius. This study expands upon the work of Schiller et al. (2008), extending a climatology of IWC by combining datasets from several European and US airborne campaigns and ground-based lidar measurements over Julich, Germany. The relationship between 10 IWC and temperature is further investigated using the new merged dataset and probability distribution functions (PDFs). A PDF-based formulation allows for representation of not only the mean values of IWC, but also the variability of IWC within a temperature band. The IWC-PDFs are found to be bimodal over the whole cirrus temperature range, which might be attributed to different cirrus formation mechanisms such as het15 erogeneous and homogeneous freezing. The PDFs of IWC are further compared to distributions of cirrus ice crystal number and mass mean radius, which show that the general relationship between IWC and temperature appears to be influenced much more by particle number than by particle size. [ABSTRACT FROM AUTHOR]