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  • New Cloud System Metrics to Assess Bulk Ice Cloud Schemes in a GCM
    Bulk microphysical properties of ice clouds, such as fall speed and ice crystal size distribution, strongly impact the life time and the radiative effects of these clouds. Three coherent bulk ice schemes, with fall speed and effective ice crystal diameter depending on both ice water content and temperature, have been constructed from published parameterizations. We present a novel upper tropospheric cloud system concept to study the impact of these schemes on the LMDZ climate simulations. For this evaluation, cloud data from hyperspectral infrared sounders Atmospheric InfraRed Sounder and IR Atmospheric Sounding Interferometer are used, because they include cirrus with visible optical depths as low as 0.2. The analogous satellite observation simulator, developed for this purpose, is also presented. The cloud system concept, applied to the data and to the simulator outputs, allows a process-oriented evaluation. In general, the new bulk ice schemes lead to a better agreement with the cloud data, in particular concerning the cloud system property distributions and the relation between cloud system properties and proxies mimicking the life stage and the convective depth. Sensitivity studies have demonstrated that both the introduction of the new schemes as well as the necessary adjustment of the relative width of the upper tropospheric subgrid water distribution lead to these improvements. Our studies also suggest to revise the formulation of the latter. more
  • Radiometric Stability Validation of 17 Years of AIRS Data Using Sea Surface Temperatures
    We evaluate the stability of the radiometric calibration of the Atmospheric Infrared Sounder (AIRS) by analyzing the trend in the time series of the difference between the brightness temperatures measured in the 1,231-cm(-1) atmospheric window channel, corrected for atmospheric transmission, relative to the Real-Time Global Sea Surface Temperature (RTGSST) for oceans between 30S and 30N. The observed bias relative to the RTGSST between 2002 and 2019 was less than 250 mK, with a 2-3-mK/yr trend. Establishing the stability of the 1,231-cm(-1) channel at tropical ocean temperatures at the 2-3-mK/yr level is a necessary but not sufficient condition of establishing the calibration stability of all AIRS channels over the full dynamic range at a comparable level. Our analysis indirectly establishes the stability of the RTGSST for the 2002-2017 time period and region at the 2-3-mK/yr level, with a degradation since 2017. more
  • Transport of the 2017 Canadian wildfire plume to the tropics via the Asian monsoon circulation
    We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of −0.05 W m−2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources. more
  • Variations in Water Vapor From AIRS and MODIS in Response to Arctic Sea Ice Change in December 2002-November 2016
    Water vapor plays a vital role in the Arctic sea ice and climate change. The Atmospheric Infrared Sounder (AIRS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) are useful tools for studying the water vapor variations over the entire Arctic. In this paper, we discuss precipitable water vapor (PWV) variations from the AIRS and MODIS Level-3 data products in response to Arctic sea ice change during December 2002-November 2016. The results indicate that both AIRS- and MODIS-derived PWV measurements tend to be underestimated, and this underestimation is generally greater for the latter than the former over both land and ocean. Additionally, both the AIRS and MODIS retrievals suggest that the atmospheres are drier in the solid sea ice pack and, to a lesser extent, in the marginal ice zone, than in the open ocean in each season. Moreover, statistically significant correlations between the PWV and sea ice concentration (SIC) anomalies are generally observed in all seasons under different ice conditions except in the open ocean, where variations in the PWV may be attributed to the poleward moisture transport from lower latitudes. The Arctic PWV and SIC variation patterns are also dominated by the atmospheric and ice conditions in the solid sea ice pack, respectively. The monthly mean PWV and SIC time series analysis further reveals that the Arctic PWV variations tend to be affected by a delayed effect of approximately 1.2-1.3 months from SIC changes from a climatological perspective. more