Precipitation at Hubbard Brook Experimental Forest and the roles of orography and wind in high precipitation events

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Cann, Matthew D.
Forty-five years of daily precipitation data at the Hubbard Brook Experimental Forest were studied to better understand climate scale changes and spatial precipitation patterns in high precipitation events. Mean annual precipitation has increased 11%, entirely as a result of an increased frequency of >30 mm day-1 events, and the highest precipitation events (>50 mm day-1) are 78% more common than they were 45 years ago suggesting a potential increase in flood frequency. In high precipitation events with a southerly, low-level wind, precipitation was, on average, higher in the center of the valley and the lowest near the mountain ridges. Easterly wind events demonstrated an increase in precipitation toward the west with no relationship to elevation. Five new precipitation gauges were installed within a ~2 km latitudinal gap in the existing ombrometer network in the lowest elevations of the Hubbard Brook valley. These gauges observed precipitation for six months between June and November 2015, and were used in addition to the existing network of gauges to better understand precipitation patterns. Stable water isotopes were used to investigate the location and impact of low-level orographic clouds in augmenting precipitation. Isotopic ratio measurements (δ18O and δD) of rainwater were taken in six high precipitation events with a southerly wind. Precipitation and δ18O data agree on the influence of low-level moisture directly impacting the precipitation amount, with an increase of heavy isotope content corresponding to higher precipitation amounts. Three mechanisms of orographic enhancement of precipitation are discussed. The lifting of stable, moist air over the southern ridge of HBEF is advected downstream some distance as condensation occurs and cloud water is washed out. The distance the orographic cloud drifts prior to becoming washed out depends on the horizontal wind speed and the pre-existing rain rate. Shear-induced turbulence may also be a common mechanism for enhanced precipitation in the White Mountains, as many high precipitation events exhibit blocked or retarded flow at the surface with rapidly increasing wind speeds above. Overturning cells are expected in the 750-1200 m ASL layer above the Hubbard Brook valley according to theory, enhancing condensation and augmenting precipitation through coalescence as it descends into the valley. This mechanism works in conjunction with the third mechanism, moisture flux convergence, which increases cloud water content over and upstream of the White Mountains.