A Microphysics Guide to Cirrus - Part II: Climatologies of Clouds and Humidity from Observations

Abstract:

This study presents airborne in-situ and satellite remote sensing climatologies of cirrus clouds and humidity. The climatologies serve as a guide to the properties of cirrus clouds, with the new in-situ database providing detailed insights into boreal mid-latitudes and the tropics, while the satellite-borne data set offers a global overview.

To this end, the quality checked Cirrus Guide II in-situ database is created from airborne in-situ measurements during 150 flights in 24 campaigns. The archive contains IWC, N_ice, R_ice , RH_ice and H₂O¹ for each of the flights. Depending on the specific parameter, the database has extended by about a factor of 5-10 compared to previous studies (Schiller et al. 2008, JGR, and Krämer et al. 2009, ACP).

Selected results from the in-situ measurement database are: (i) the medians of N_ice, R_ice and RH_ice have distinct patterns in the IWC-T parameter space. Lookup tables of these variables can be used to improve global model cirrus representation and remote sensing retrieval methods. (ii) Across all latitudes, the thicker liquid-origin cirrus predominate at lower altitudes, while at higher altitudes the thinner in situ-origin cirrus prevail. (iii) Exemplary investigations of the radiative characteristics of in situ-origin and liquid-origin cirrus show that the in situ-origin cirrus only slightly warm the atmosphere, while liquid-origin cirrus have a strong cooling effect.

An important step is the provision of a global cirrus N_ice climatology derived from ten years of satellite remote sensing observations. The in-situ measurement database has been used to evaluate and adjust the satellite observations. From the satellite database we found that (i) N_ice medians of the most frequently occuring cirrus, sorted by geographical regions, are highest in the tropics, followed by austral/boreal mid-latitudes, Antarctica and the Arctic. Since the satellite climatologies enclose the entire spatial and temporal N_ice occurrence, we could deduce that (ii) half of the cirrus are located in the lowest, warmest cirrus layer (224-242K) and contain a significant amount of liquid origin cirrus.

1 IWC: ice water content, N_ice: number concentration of ice crystals, R_ice : ice crystal mean mass radius, RH_ice: relative humidity with respect to ice, H₂O: water vapor mixing ratio.