A 10-yr Climatology of Arctic Cloud Fractions and Their Impact on Surface Radiation Budget at Barrow, Alaska

A 10-yr record of Arctic cloud fracitons and surface radiation budget has been generated using data collected from June 1998 to May 2008 at the Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) site and the nearby NOAA Barrow Observatory (BRW). The record includes the seasonal variations of cloud fraction (CF), cloud liquid water path (LWP), precipitable water vapor (PWV), surface albedo, shortwave (SW) and longwave (LW) fluxes and cloud radative forcings (CRFs), as well as their decadal variations. Values of CF derived from different instruments and methods agree well, having an annual average of ~0.74. Cloudiness increases from March to May, remains high (~0.8-0.9) from May to October, and then decreases over winter. More clouds and higher LWP and PWV occurred during the warm season (May-October) than the cold season (November-April). The minimum CF in March appears to be associated with a dipole pattern of atmospheric pressure with a high over the Beaufort Sea and low over the Aleutian Islands. A peak in CF during May is associated with predominant southerly airflow. The monthly means of estimated clear-sky and measured all-sky SW-down and LW-down fluxes at the two facilities are almost identical with the annual mean differences less than 1.6 Wm-2. The downwelling and upwelling LW fluxes remain almost constant from January to March, then increase from March and peak during July-August. The downwelling SW fluxes are primarily determined by seasonal changes in the intensity and duration of insolation over Northern Alaska, and are also strongly dependent on cloud fraction and optical depth, and surface albedo. The monthly variations of NET CRF follows the cycle of SW CRF, but values modified by LW CRF. Arctic clouds have a net warming effect on the surface throughout the year, with exception of the period from June to August when there is a cooling effect. The daily average surface albedos agree well at the two sites remaining high (>0.8) until late May, dropping below 0.2 after the snow melts around the middle of June and increasing during autumn once snow begins to accumulate. On the basis of long-term regression analyses CF has decreased by about 0.048 while temperature has increased by ≈ 1.1 oC over the 10-yr period, which can be characterized by tendencies of warming mainly during December and April. Regarding to the 2007 record minimum Arctic ice extent, this study has provided strong support. The 2007 summer cloud fractions at Barrow, Alaska, decrease 0.062, and downwelling SW and NET fluxes increase 28.6 Wm-2 and 11.3 Wm-2, respectively, which may partially contribute an increased surface temperature of 1.6 K compared to the 10-yr summer (June-August) means.