资源环境科技发展态势分析平台

Abstract. The radiative energy budget in the Arctic undergoes a rapid transformation compared to global mean changes. Understanding the role of cirrus in this system is vital, as they interact with short- and long-wave radiation and the presence of cirrus can be decisive as to a net gain or loss of radiative energy in the polar atmosphere.

In an effort to derive radiative properties of cirrus in a real scenario in this sensitive region, we use in-situ measurements of ice water content (IWC) performed during the POLSTRACC aircraft campaign in the boreal winter and spring 2015/2016 employing the German research aircraft HALO. A large dataset of IWC measurements of mostly thin cirrus at high northern latitudes was collected in the upper troposphere and also frequently in the lowermost stratosphere. From this dataset we selected vertical profiles that sampled the complete vertical extent of cirrus cloud layers. These profiles exhibit a vertical IWC structure that will be shown to control the instantaneous radiative effect both in the long and short wavelength regimes in the polar winter.

We perform radiative transfer calculations with the UVSPEC model from the libRadtran program package in a one-dimensional column between the surface and the top of the atmosphere (TOA), taking as input the IWC profiles, as well as the state of the atmospheric column at the time of measurement, as given by weather forecast products. In parameter studies, we vary the surface albedo and solar zenith angle in ranges typical for the Arctic region. We find the strongest (positive) radiative forcing of cirrus over bright snow, whereas the forcing is mostly weaker and even ambiguous over the open ocean in winter and spring. The IWC structure over several kilometres in the vertical affects the irradiance at the TOA through the distribution of optical thickness. A strong heating rate profile within the cloud drives dynamical processes and contributes to the thermal stratification at the tropopause.

Our case studies highlight the importance of a detailed resolution of cirrus clouds and consideration of surface albedo for estimations of the radiative energy budget in the Arctic.