Arctic stratus cloud properties and their effect on the surface radiation budget: Selected cases from FIRE ACE

To study Arctic stratus cloud properties and their effect on the surface ra diation balance during the spring transition season, analyses are performed using data taken during three cloud and two clear days in May 1998 as part of the First ISCCP Regional Experiment (FIRE) Arctic Cloud Experiment (ACE ). Radiative transfer models are used in conjunction with surface- and sate llite-based measurements to retrieve the layer-averaged microphysical and s hortwave radiative properties. The surface-retrieved cloud properties in Ca ses 1 and 2 agree well with the in situ and satellite retrievals. Discrepan cies in Case 3 are due to spatial mismatches between the aircraft and the s urface measurements in a highly variable cloud field. Also, the vertical st ructure in the cloud layer is not fully characterized by the aircraft measu rements. Satellite data are critical for understanding some of the observed discrepancies. The satellite- derived particle sizes agree well with the c oincident surface retrievals and with the aircraft data when they were coll ocated. Optical depths derived from visible-channel data over snow backgrou nds were overestimated in all three cases, suggesting that methods currentl y used in satellite cloud climatologies derive optical depths that are too large. Use of a near-infrared channel with a solar infrared channel to simu ltaneously derive optical depth and particle size appears to alleviate this overestimation problem. Further study of the optical depth retrieval is ne eded. The surface-based radiometer data reveal that the Arctic stratus clou ds produce a net warming of 20 W m(-2) in the surface layer during the tran sition season suggesting that these clouds may accelerate the spring time m elting of the ice pack. This surface warming contrasts with the net cooling at the top of the atmosphere (TOA) during the same period. An analysis of the complete FIRE ACE data sets will be valuable for understanding the role of clouds during the entire melting and refreezing process that occurs ann ually in the Arctic.