Cirrus clouds in the extratropical tropopause and lowermost stratosphere region
Final Report Abstract
The aim of the CiTroS study was to gain a deeper understanding of cirrus macrophysical properties (cloud top and bottom height, vertical extent, the cloud position with respect to the tropopause) and cirrus microphysical properties (ice water content (IWC), particle size). For this purpose, limb infrared emission spectra measured by GLORIA (Gimballed Limb Observation for Radiance Imaging of the Atmosphere) during the WISE (Wave-driven ISentropic Exchange) campaign were analyzed. A new cloud detection based on a cloud extinction retrieval has been developed and characterised. It allows a more accurate description of vertical extent of the cloud (cloud base and top height) compared to a well-established two colour methods. The method is applicable to all GLORIA campaign data, not only WISE, and can better quantify potential effects of particles on the operational trace gas retrievals. Both, old and new detection methods delivered a surprisingly large amount of cirrus in the northern extratropical tropopause region, with occurrence frequencies up to 30%. Comparison with one of the currently best reanalysis datasets (ERA5) show a significant underestimation for these high altitude clouds in the reanalyses. This fact points to two potential weaknesses in the reanalysis models around the tropopause: (1) the cloud parametrisation may lack a significant cloud formation process; and/or (2) the meteorological conditions (e.g. temperature, updraft velocity, total water content) are not adequately presented to trigger the cloud formation process in the models. However, model studies with CLaMS-Ice, the in-house trajectory-based two moment microphysical bulk model, showed for selected flight segments of the WISE campaign that the microphysical model is able to reproduce cirrus clouds above the tropopause observed by GLORIA. The reason is most likely that in the simulations smalls scale temperature fluctuations, seemingly important to initiate ice formation, are superimposed to the ERA5 data. The combination of the well resolved GLORIA measurements for cirrus clouds in the vicinity of the tropopause region together with a microphysical model like CLaMS-Ice might now help to improve the cloud parameterisations in global models. The controversial discussion in the literature on the amount of cirrus clouds above the tropopause has now an additional argument for the significance of these clouds in respect to their frequency of occurrence. The results of CiTroS show that the excellent vertical resolution together with an improved diagnostic for an exact as possible location of the tropopause supports the higher occurrence frequencies reported in literature. Using conservative criteria like in the literature for the uncertainty of the tropopause determination (typically 500 m above the tropopause) the frequency of occurrence is in the range of 4% – 7 %. However, since the used dataset is ERA5, which has a higher vertical resolution, when considering 250 m above the tropopause, the fraction increases to 13% – 17 %. These numbers represent a so far not expected amount of cloudiness and may have an imprint on various physical processes in this part of the atmosphere (e.g. radiative effect of cirrus, water vapor transport to the stratosphere). First ice water content (IWC) retrievals - based on the GLORIA extinction retrieval, look-up tables from a Mie-scattering model and measurement-based parameterisation of the ice particle size dependency on IWC and temperature from the literature - show consistent results in comparison to the reanalysis data for cirrus clouds, when the analysis is restricted to the range of the typical IWC sensitivity of GLORIA and altitudes below the tropopause. Finally, for the first time a 3D cloud shape retrieval for IR limb measurements has been developed and successfully applied to the GLORIA data of the WISE campaign. The retrieved information of the location and horizontal extent of the cloud along the line of sight of the instrument is an important step forward to reduce uncertainties in the IWC retrievals of limb sounders.
Publications
-
Cirrus cloud shape detection by tomographic extinction retrievals from infrared limb emission sounder measurements, Atmos. Meas. Tech., 13, 7025–7045
Ungermann, J., Bartolome, I., Griessbach, S., Spang, R., Rolf, C., Krämer, M., Höpfner, M., and Riese, M.
-
Dissertation: ‘Cirrus clouds in the extratropical tropopause and lowermost stratosphere region’ zur Erlangung des Grades Doktor der Naturwissenschaften (Dr. rer. nat.), University of Wuppertal, September 2021
Bartolome Garcia, Irene
-
Observation of cirrus clouds with GLORIA during the WISE campaign: detection methods and cirrus characterization, Atmos. Meas. Tech., 14, 3153–3168
Bartolome Garcia, I., Spang, R., Ungermann, J., Griessbach, S., Krämer, M., Höpfner, M., and Riese, M.
