DEPLOYMENT AND EVALUATION OF A SYSTEM FOR GROUND-BASED MEASUREMENT OFCLOUD LIQUID WATER TURBULENT FLUXES

Direct interception of windblown cloud water by forests has been dubbe d ''occult deposition'' because it represents a hydrological input tha t is hidden from rain gauges. Eddy correlation studies of this phenome non have estimated cloud water fluxes to vegetation yet have lacked es timates of error bounds. This paper presents an evaluation of instrume ntal and methodological errors for cloud liquid water fluxes to put su ch eddy correlation measurements in context. Procedures for data acqui sition, processing (including correction factors), and calibration tes ting of the particulate volume monitor (PVM) and forward-scattering sp ectrometer probe (FSSP) are detailed. Nearly 200 h of in-cloud data ar e analyzed for intercomparison of these instruments. Three methods of coordinate system rotation are investigated; the flux shows little sen sitivity to the method used, and the difference between fluxes at diff erent stations is even less sensitive to this choice. Side-by-side int ercomparison of two PVMs and one FSSP leads to error bounds of 0.01-0. 035 g m(-3) on half-hour mean cloud liquid water content (relative to typical values of 0.35 g m(-3)) and 2-3.5 mg m(-2) s(-1) on the surfac e-normal liquid water flux (typical magnitude of 7 mg m(-2) s(-1) for these data), depending on which instruments are compared.