Session 1.4 High resolution measurement of precipitation in the Sierra

Monday, 30 August 2010: 10:15 AM
Alpine Ballroom A (Resort at Squaw Creek)
John Hallett, DRI, Reno, NV

Presentation PDF (1.1 MB)

Measurment of precipitation, particularly in any highly irregular terrain as the Sierra, requires simultaneous and collocated measurement of both the vertical flux as a rain/snow rate together with a measure of the local wind speed. High resolution measurement further require fast response sensors for sufficent representivity,(better than 1 minute, at least) and size greater than hand size, having capability of measuring both rain or snow or any mix thereof with an uncertainty better than 1/100 inch/hr and a few m/s. The hotplate precipitation gauge, comprising a sandwich construction of two identical ridged plates,thermally and electrically separated by 2cm, is deployed horizontally and maintained at about 100C by intrinsic heaters.Controlled power(watts, measured as an absolute quantity) closely maintains temperature and provides a measure of precipitation by rapid evaporation latent heat. This dominates the heat economy, requiring selection of liquid or solid latent heat depending on air temperature. Wind convective heat loss ocurs for the top plate and wind alone for the shielded bottom plate. Long term measurements may be combined to demonstrate a changing moisture climate through a frequency analysis of precipitation rate,as how much prcepitation fell at differnt rates for selected periods.Choice may be as a frontal passage or a storm or a decade. The frequency analysis has advantage that local changes of terrain (grass, trees, buldings, parking lots, snow terrain) growing or decaying are excluded. A further advantage of the frequency analysis lies in providing insight into the physical processes and serves as a guide to design the hotplate system itself through the range needed to be covered. For example, in the high Sierra, most winter precipitation falls at about 4cm/hr with not much beyond twice that amount,useful to know for choice of power supplies. Similarly, in some sheltered lee regions, winds do not exceed 20 mph compared with crests where 100mph is more likely.A further advantage of the hotplate system is that birds, having tried to settle on it once, learn not to return. The system is designed to keep data during a power outage and to revive itself when power returns. Data may be transfered and plotted remotely as desired.In the high Sierra choice of site is paramount as it is inconvenient to dig out the system in mid season. Uncertainties arise on occasion by hail bounce or raindrop splash,estimated to be minimal considering other features of measurement of precipitation. Dual systems (one hotplate above the other) provide information on blowing snow and lead to characterization by a 1 minute Richardson number by incorporating a vertical profile of temperature and horizontal windspeed. Support by NSF Acknowledged.
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