Cirrus Clouds; Study of Microphyscial Properties and Their Seasonal & Temperature Dependence in the Tropics and Mid-latitudes
Abstract: Cirrus clouds are high-level ice clouds covering ≈ 30% of Earth’s surface. The cirrus clouds ice crystals can be formed by heterogeneous or homogeneous nucleation, where temperatures for heterogeneous nucleation was set to -38 - -30 °C and for homogeneous nucleation below -42 °C. The study will focus on microphysical properties of cirrus clouds in the tropics, 0-30°, and the mid-latitudes, 30-60°, since diﬀerent cirrus types dominate each respective region. The report is based on cirrus measurements from the satellites Cloudsat and CALIPSO. The dataset DARDAR (liDAR/raDAR) processes the satellite data, giving access to the microphysical parameters ice water content (IWC), ice crystal ICER (ICER) and ice crystal number concentration (ICNC). A cirrus ﬁlter, was applied to ensure solely cirrus clouds being studied and the data was averaged to monthly values. The data ranged from year 2008 to 2016. The data revealed clear seasonal patterns for the vast majority of the microphysical parameters. In the tropics, ICER, ICNC 100 and IWC co-vary with temperature. ICNC 5 and 25 have opposite variations compared to temperature. An unexpected result in the Tropics was that the northern hemisphere (NH) and southern hemisphere (SH) had a similar seasonal variation in temperature, both with peaks during the NH summer. The midlatitudes corresponded to lower values and warmer temperatures, compared to the tropics. In addition, the correlations with temperature were lower for the mid-latitudes. An unexpected ﬁnding in the mid-latitudes was that an increase in IWC did not correspond to a larger ICER. For ICNC 5 and 25 the values were higher for clouds classiﬁed as homogeneous than for those classiﬁed as heterogeneous, in contrast to the other parameters. Higher concentrations of smaller particles in homogeneous clouds is anticipated, since large crystals are expected in heterogeneous clouds. Overall, ICER showed the strongest temperature dependence followed by ICNC 100, particularly during boreal autumn season. IWC did not seem particularly temperature dependent. This study will aid the understanding of cirrus clouds, their structure and their climate eﬀect. The study is part of a larger project investigating the eﬀect of volcanic aerosols on cirrus clouds.
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