Extended Abstract (2.5M)2.2
The diurnal cycle of the summer cloud-capped Arctic boundary layer
Michael Tjernström, CIRES/Univ. of Colorado, Boulder, CO
In this paper, we investigate the diurnal cycle of the summer Arctic cloud-capped boundary layer, which in a sense is a marine cloud-capped PBL but differs in several aspects from the prototypical marine stratocumulus-capped PBL that has previously been extensively studied. Through much of summer, the surface net radiation remains positive through the whole day, but the available radiative energy mostly goes into latent heat of melting ice and snow; thus there is an efficient control of the surface temperature. The sun remains above the horizon through the whole diurnal cycle, but insolation remains relatively weak and the zenith angle high. The cloud-capping inversion is often accompanied with an increase in moisture, rather than a drying as in subtropical stratocumulus.
Our observations indicate that there is in fact a substantial diurnal cycle in this boundary layer. In terms of low-level temperature, the magnitude is small but significant with slightly higher temperatures during the day. Depending only on standard near-surface weather stations it is easily missed. The top of the boundary –layer, however, is slightly cooler in the middle of the day; the result is a destabilisation during the day with, more intense mixing manifested in, among other things, lower wind speeds during the day. In contrast to other stratocumulus-capped PBL's, the cloud layer is the deepest in the middle of the day and into the afternoon; the maximum incoming radiation is therefore biased to before noon.
Session 2, Cloudy Boundary Layers 1
Tuesday, 23 May 2006, 8:00 AM-10:00 AM, Rousseau Suite
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