The Statistical Relationship between Upslope Flow and Rainfall in California's Coastal Mountains: Observations during CALJET

Abstract

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.

The relation between airflow and orographic precipitation on the southern side of the Alps as revealed by weather radar

“…In the unblocked cases (high Froude number), a great enhancement of precipitation was observed over the lower windward alpine slopes; in the blocked cases (low Froude number), precipitation was also enhanced upstream of the barrier. Similar findings were also obtained by Neiman et al (2002) and James and Houze (2005), who studied orographic precipitation enhancement mechanisms over California's coastal mountains. A conceptual model for the orographic precipitation mechanisms in stable blocked and unstable unblocked flows was proposed by Medina and Houze (2003).…”

“…This change from easterly to southerly winds with height at low levels is typically observed during heavy orographic precipitation (Kappenberger and Kerkmann, 1997;Ferretti et al, 2000;Rotunno and Ferretti, 2001;Ralph et al, 2005) and is associated with an upper-level trough which, approaching the Alps from the west, provides the moist large-scale flow responsible for the heavy rain (Grazzini, 2007). The radiosounding profile of Figure 4 does not show the presence of a strong low-level jet, although this is an element that is commonly observed when extratropical cyclones impinge upon mountain chains (Browning et al, 1975;Neiman et al, 2002;Ralph et al, 2005). The reason for this might be that Figure 4 shows a mean profile that includes different stages of the precipitation events and not only the most intense phases of orographic precipitation, in which the low-level jet is more likely to be present (Neiman et al, 2002).…”

“…The radiosounding profile of Figure 4 does not show the presence of a strong low-level jet, although this is an element that is commonly observed when extratropical cyclones impinge upon mountain chains (Browning et al, 1975;Neiman et al, 2002;Ralph et al, 2005). The reason for this might be that Figure 4 shows a mean profile that includes different stages of the precipitation events and not only the most intense phases of orographic precipitation, in which the low-level jet is more likely to be present (Neiman et al, 2002). The vertical profile of the wind above the radar site is given by the thin lines in Figure 4.…”

The neglected Indo‐Gangetic Plains low‐level jet and its importance for moisture transport and precipitation during the peak summer monsoon

“…In contrast, a strongly positive Fr m represents a stable layer which then caps the near surface flow from rising over the terrain (i.e., blocked) [Galewsky and Sobel, 2005;Galewsky, 2008]. A typical outcome of a blocked flow is a terrainparallel barrier jet [Neiman et al, 2002].…”

Quantitative precipitation forecasting in the Alps: The advances achieved by the Mesoscale Alpine Programme

“…High-resolution (4 km) real-time forecasts using MM5 were initiated in 1998 (Mass and Kuo, 1998) and the results were reported in Colle et al (2001) and Mass et al (2002). Meanwhile, a number of field projects were designed and conducted, either immediately before or after MAP, to better understand precipitation over complex terrain, such as CALJET (California Land-falling Jets experiment, Ralph et al, 1999), PACJET (Pacific Land-falling Jets experiment, Neiman et al, 2002), IPEX (Intermountain Precipitation Experiment, Schultz et al, 2002) and IMPROVE (Improvement of Microphysical Parameterization through Observational Verification Experiment, Stoelinga et al, 2003). In CAL-JET, orographically modified precipitation was studied in connection with the 'landfall' of oceanic cyclones and associated fronts (Kingsmill et al, 2006).…”