The Cloud and Aerosol Chemistry Experiment (CACHE) was designed to probe the remote marine atmosphere, and investigate its interaction with a montane, coastal forest on the Olympic Peninsula of Washington State.The prime objective of this experiment was to make in situ measurements of cloudwater fluxes to the forest canopy, and to identify the governing processes for cloud water deposition. Other goals included analysis of the relationship between cloud microphysics and precursor aerosol, and quantification of carbon dioxide exchange between forest and atmosphere.
Researchers believe cloud water deposition to be important both hydrologically and chemically for montane ecosystems where it occurs frequently, and it has been sited as a potential cause of forest injury in the Appalachian Mountains of the Eastern U.S. However, very few direct measurements of this process have been made; more often, researchers have resorted to the use of inferential measurements and models which have not been validated. In 1993 and 1994, Oregon State University (OSU) researchers conducted eddy correlation measurements (CACHE 1 and 2) of cloud water deposition at Cheeka Peak, Washington.They measured substantial liquid water fluxes of ~1mm per 24 hours of surface cloud, about twice the magnitude of those reported by researchers in the United Kingdom. The greatest uncertainty in the reported measurements is the degree to which the flux measured at a single point over the canopy is representative of fluxes over a broader area of complex terrain.
During the third measurement campaign (dubbed CACHE-3 by OSU, and CACHE94 by NCAR), ASTER was deployed along with existing instrumentation from OSU. The addition of ASTER to the experimental design allowed for measurements of cloud water fluxes at three stations, with 5m vertical and 28m horizontal separations, to address representativeness of the measured flux. Many operational periods were defined, to allow for variable instrument configurations including side-by-side intercomparisons for the OSU cloud microphysical instruments. The calibration of these instruments was tested on a near-daily basis. The standard ASTER deployment provided for a variety of auxiliary measurements (especially meteorological profiles and radiation measurements), many of which may be helpful in describing and defining the process of cloud water deposition. Further efforts were made to characterize the structure of the forest canopy at Cheeka Peak. The data set described herein is certainly the best to date for analyzing cloud water deposition and its governing processes.
As an add-on to the CACHE-3 deployment, a fast response carbon dioxide sensor was operated at one of the flux stations to measure forest respiration and photosynthetic rates. Canopy measurements were made over a 100m transect through the forest (upwind of the sampling towers), to determine speciation, leaf areas, avnd height of the conifers. The CO2 sensor was also subjected to periodic calibration and automatic zero response
The ASTER facility is maintained by the Surface and Sounding Systems Facility of the Atmospheric Technology Division of the National Center for Atmospheric Research .