The California Landfalling Jets Experiment (CALJET) was carried out during the winter of 1997/98, in part to study orographic rainfall in California's coastal mountains using coastal wind profilers. This observational study statistically links hourly rainfall rates observed by tipping-bucket rain gauges in California's quasi-linear coastal mountains to the hourly averaged upslope component of the flow measured by coastal wind profilers immediately upstream. Vertical profiles of the linear correlation coefficient of upslope flow versus rain rate are calculated on a case-by-case basis, for all cases containing a low-level jet (LLJ), and for the winter season of 1997/98. These correlation coefficient profiles show a direct relationship between the magnitude of the upslope flow impacting the coast and the magnitude of the rain rate in the downstream coastal mountains. Maximum correlation coefficients are as large as 0.94 in some individual cases, 0.75 for a composite of LLJ cases, and 0.70 for the winter season.Using three locations with differing coastal terrain characteristics, it is found that the layer of upslope flow that optimally modulates orographic rainfall is near mountaintop, that is, about 1 km above mean sea level for California's coastal ranges. This height also corresponds to the mean altitude of landfalling LLJs observed by the coastal profilers. The correlation coefficient in this layer is largest when the rain rates are used from the coastal mountain sites rather than from the coastal sites, thus further highlighting the physical connection between upslope flow and orographic rainfall in the coastal mountains. The presence of shallow, terrain-blocked flow modulates the correlation coefficient profiles below mountaintop, such that the low-level flow at the coast is poorly correlated with rain rates observed in the coastal mountains. However, cases without significant blocking retain relatively large correlation coefficient values below mountaintop.Landfalling LLJs produce the largest enhancement of upslope flow at the altitude of the LLJ, despite the existence of terrain-modified flows below mountaintop during some LLJ events. The steepest increase in rain rate for a given increase in upslope flow also occurs at jet level, as does the largest correlation coefficient of upslope flow versus rain rate. Therefore, the upslope-induced orographic rain-rate response associated with landfalling LLJs is largest (2.55 mm h Ϫ1 ) and statistically most robust near the altitude of those LLJs.

Ground-based measurements of particle size and fall speed distributions using a Particle Size and Velocity (PARSIVEL) disdrometer are compared among samples obtained in mixed precipitation (rain and wet snow) and rain in the Oregon Cascade Mountains and in dry snow in the Rocky Mountains of Colorado. Coexisting rain and snow particles are distinguished using a classification method based on their size and fall speed properties. The bimodal distribution of the particles' joint fall speed-size characteristics at air temperatures from 0.5°to 0°C suggests that wet-snow particles quickly make a transition to rain once melting has progressed sufficiently. As air temperatures increase to 1.5°C, the reduction in the number of very large aggregates with a diameter Ͼ 10 mm coincides with the appearance of rain particles larger than 6 mm. In this setting, very large raindrops appear to be the result of aggregrates melting with minimal breakup rather than formation by coalescence. In contrast to dry snow and rain, the fall speed for wet snow has a much weaker correlation between increasing size and increasing fall speed. Wet snow has a larger standard deviation of fall speed (120%-230% relative to dry snow) for a given particle size. The average fall speed for observed wet-snow particles with a diameter Ն 2.4 mm is 2 m s Ϫ1 with a standard deviation of 0.8 m s Ϫ1 . The large standard deviation is likely related to the coexistence of particles of similar physical size with different percentages of melting. These results suggest that different particle sizes are not required for aggregation since wet-snow particles of the same size can have different fall speeds. Given the large standard deviation of fall speeds in wet snow, the collision efficiency for wet snow is likely larger than that of dry snow. For particle sizes between 1 and 10 mm in diameter within mixed precipitation, rain constituted 1% of the particles by volume within the isothermal layer at 0°C and 4% of the particles by volume for the region just below the isothermal layer where air temperatures rise from 0°to 0.5°C. As air temperatures increased above 0.5°C, the relative proportions of rain versus snow particles shift dramatically and raindrops become dominant. The value of 0.5°C for the sharp transition in volume fraction from snow to rain is slightly lower than the range from 1.1°to 1.7°C often used in hydrological models.

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