During the spring and summer growing season, the weather in the northeastern U.S. is frequently punctuated by the passage of cold (or "dewpoint") fronts. Each frontal passage is followed by several days during which the boundary layer experiences a gradual warming, and then moistening until the next frontal event. Direct visible evidence of this post-frontal air mass modification is the enhanced occurrence of boundary layer ("fair weather") cumulus clouds. What impact these clouds have on forest physiology, specifically evapotranspiration and carbon uptake, are questions that remain unanswered. Does the presence of these clouds have a tempering effect on the forest-climate system, both by modifying the light environment and reducing vegetative stresses such as vapor pressure deficit and leaf temperature? An equally important issue is the lack of a comprehensive fine scale cloud climatology for global climate and operational forecasting models, where smaller-scale cumulus clouds are poorly parameterized.To resolve these questions it is necessary to:
1) produce a cloud climatology with a specific subset of fair weather cumulus clouds, and
2) quantify the surface-atmosphere exchange processes for particular subsets of the cloud climatology.
Using data from hundreds of Automated Surface Observing System (ASOS) stations (all of which report cloud ceiling and sky conditions in addition to the usual meteorological parameters), including several dozen in the northeast U.S., we present a large-scale as well as regional cloud climatology. With radiation, heat, moisture and trace gas flux measurements from a tower site in central Massachusetts, an analysis of the heat, moisture and carbon budgets during different sky regimes is described.
For the past three years, the ASOS cloud climatology for the northeastern U.S. indicates that daytime fair weather cumulus frequency during the spring and summer growing season is nearly double that of the non-growing season. For 1995, the impact of drought conditions and the corresponding decrease in the incidence of frontal passages resulted in a reduction of fair weather cumulus frequency. During 1996, a relatively wet spring and summer resulted in a steady appearance of fair weather cumulus throughout the growing season. Finally, although the growing season in 1997 was relatively dry, the regularity of frontal passages and absence of extended periods of drought or high temperatures resulted in the highest occurrence of fair weather cumulus for the three years analyzed.
Given the state of the sky during the growing season, a key question is whether partly cloudy skies are a favored regime for the forest ecosystem. We present evidence that evapotranspiration and carbon uptake by the forest approach optimum values during quiescent weather regimes dominated by fair weather cumulus clouds.