Effects of Arctic haze on low-level stratus properties and the surface radiation budget.

摘要

This study is aimed at improving understanding of the indirect effects of long-range transport of mid-latitude aerosol pollution on Arctic climate. Retrieval methods are developed to obtain the properties of single-layer stratus clouds and precipitation be low clouds using the Atmospheric Radiation Measurement program (ARM) data from the North Slope of Alaska - Adjacent Arctic Ocean (NSA-AAO) site at Barrow, Alaska. With the retrieved properties of clouds and precipitation, and ground-based aerosol measurements from the adjacent Global Monitoring Division (GMD) laboratory, the effects of aerosol on cloud properties and surface radiation balance are studied.; The precipitation retrieval algorithm is based on a relationship between precipitation characteristic size and mean fall speed, where mean fall speed is obtained from a time-averaged Doppler velocity. From retrieved precipitation particle size and ARM radar reflectivity measurements, profiles of precipitation rate and number concentration are calculated. The cloud retrieval technique is based on surface infrared spectral radiation measurements. By comparing the spectral emissivity and ozone-band transmissivity from measurements with those in a model-calculated look-up table, cloud effective radius and optical depth are obtained.; Arctic aerosol can be classified as belonging to either a nucleation mode or haze mode based on aerosol particle size. No measurements were obtained of particle size at NSA-AAO, so haze mode aerosol concentrations are inferred based on measurements of aerosol light scattering. It is shown that haze mode aerosol can increase cloud thermal emissivity by increasing cloud particle concentration and decreasing cloud particle effective radius. By increasing the cloud thermal emissivity under cloudy skies, haze pollutants from mid-latitudes are associated with an increase in surface longwave cloud radiative forcing of 5 to 10 W m-2, and an increase in net cloud radiative forcing of 5.2 W m-2 (or ∼1.3 K). Assuming the frequency of Arctic low-level cloud occurrence is 60% and half of clouds are polluted, haze is associated with a yearly increase in cloud longwave radiative forcing of 1.5 to 3 W m-2 and an increase in cloud net radiative forcing of 1.6 W m-2 (or ∼0.4 K) on average. It is also shown that the seasonal cycle in haze is controlled primarily by precipitation scavenging below or within clouds. No relationship was found between aerosol light scattering and ice cloud properties, perhaps due to the differing compositions of haze aerosol and ice crystal nuclei.