This wind profiler–based study highlights key characteristics of the barrier jet along the windward slope of California’s Sierra Nevada. Between 2000 and 2007 roughly 10% of 100 000 hourly wind profiles, recorded at two sites, satisfied the sierra barrier jet (SBJ) threshold criteria described in the text. The mean magnitude of the terrain-parallel flow in the SBJ core (i.e., Vmax) was similar at both sites (∼17.5 m s−1) and at a comparable altitude, 500–1000 m above the surface. The cross-mountain wind speed was weak at the altitude of Vmax, consistent with blocked conditions. The seasonal cycle of SBJ occurrences showed a maximum during the cooler months and a minimum in summer. Additionally, the SBJ was stronger in winter than in summer. Because the warm-season (May–September) SBJs were different than their cool-season (October–April) counterparts and occurred during California’s dry season, they were not discussed in detail. An inventory of ∼250 cool-season SBJ cases from the two sites was generated (a case contains ≥8 consecutive SBJ profiles). Up to 60% of the nearby cool-season precipitation fell during SBJ cases, and these cases shifted the precipitation down the sierra’s windward slope and enhanced precipitation at the north end of the Central Valley (relative to non-SBJ conditions). The large number of cool-season SBJ cases was stratified by the mean strength and altitude of Vmax and by the case duration. Composite profiles of the along-barrier component for the top- and bottom-20 ranked cases in each of these three SBJ classes reveal stark differences in the magnitude and vertical positioning of the barrier jet. The three SBJ classes yielded uniquely different local precipitation characteristics in proximity to the wind profilers, with the strongest and longest-lived SBJs yielding the greatest precipitation. North American Regional Reanalysis plan-view composites were generated to explore the synoptic conditions responsible for, and to showcase the precipitation distributions associated with, the top- and bottom-20 ranked cases in each of the three classes of SBJs. The composite analyses yielded large contrasts between the SBJ classes that could prove useful in forecasting SBJs and their precipitation impacts. All SBJ classes occurred, on average, in the pre-cold-frontal environment of landfalling winter storms.
Mountains modulate stably stratified flows in the lower troposphere. In the Northern Hemisphere, an airstream approaching a mountain barrier slows down and is deflected leftward as a result of a weakened Coriolis force when the Froude number of that airstream is less than unity (Pierrehumbert and Wyman 1985; Smolarkiewcz and Rotunno 1990). The resulting corridor of low-level blocked flow resides upstream and below the top of that barrier and usually contains a barrier jet paralleling the long axis of the high terrain. Barrier jet flows, which are maintained by a statically stable pressure ridge dammed against the windward slope, have been documented across North America (and elsewhere), including Appalachia (e.g., Bell and Bosart 1988), the Rockies (e.g., Dunn 1992; Colle and Mass 1995; Cox et al. 2005), coastal Alaska and British Columbia (e.g., Loescher et al. 2006; Olson et al. 2007; Yu and Bond 2002; Overland and Bond 1995), the Pacific Northwest (e.g., Braun et al. 1997), coastal California (e.g., Doyle 1997; Yu and Smull 2000; Neiman et al. 2002, 2004), and along the western slope of California’s Sierra Nevada (e.g., Parish 1982; Marwitz 1983; Smutz 1986). Some of these same studies show that barrier jet–related flows can redistribute precipitation in physically plausible patterns. In addition, a new reanalysis-based study focusing on the mountainous coastal zone of southern California shows the impact of terrain blocking on the climatological distribution of precipitation in that semiarid region (Hughes et al. 2009).
California’s lofty Sierra Nevada (Fig. 1) plays a key role in the generation of orographically enhanced precipitation during the landfall of winter storms (e.g., Heggli and Rauber 1988; Pandey et al. 1999; Dettinger et al. 2004). The bulk of these storms arrive from the southwest quadrant, which favors the formation of the northward-directed sierra barrier jet (SBJ). Because California depends on precipitation runoff from the sierra for its water supply and power generation, coupled with the fact that Sacramento is now recognized as a city under significant risk of catastrophic flooding (e.g., Lund 2007) and that a changing climate may alter the characteristics of these landfalling storms (storm track, static stability, flow strength, moisture content, etc.), there has been a renewed interest in the SBJ because of its potential role in modulating the distribution of precipitation. In a mesoscale numerical modeling case study by Galewsky and Sobel (2005), the SBJ acted as a dynamic barrier along the windward slope of the northern sierra and thus contributed to flooding rains there. In another modeling case study, Reeves et al. (2008) found that an SBJ may have enhanced precipitation along a prominent westward jog in the northern sierra because the SBJ encountered the terrain slope at a more perpendicular angle at that locale. Kim and Kang (2007) utilized a regional climate model to investigate the influence of the sierra on the water cycle for a full winter season. They found that the SBJ transports a significant amount of moisture northward to the northern sierra, resulting in increased rainfall there. Finally, a recent modeling contribution by Smith et al. (2010) reports the occurrence of a strong SBJ at the base of the sierra that contributed to the northward water vapor flux and possible enhancement of precipitation in the northern sierra during the landfall of an atmospheric river (AR; e.g., Ralph et al. 2004; Neiman et al. 2008a).
Although revealing, these studies relied largely on either model detection of SBJs or on individual case studies to assess SBJs’ hydrometeorological impacts. In addition, unlike early SBJ case studies based on airborne observations (e.g., Parish 1982; Marwitz 1983) collected during the Sierra Cooperative Pilot Project (SCPP; Reynolds and Dennis 1986), the more recent body of work cited earlier did not focus on the structural characteristics of SBJs. The SCPP case studies show the SBJ centered at ∼1 km MSL near the windward base of the sierra and below the ∼3-km mountain ridge crest (Fig. 2a), with a core extending westward into the Central Valley and eastward up the sierra slope. To broaden these case study results, smutz (1986) analyzed serial rawinsonde profiles launched along the base of the Sierra at Sheridan, California (SHR; Fig. 1) during SCPP for seven cool seasons from 1976 to 1984. Benchmark statistical characterizations of the SBJ were provided (described in section 3a), although the synoptic to mesoα-scale (i.e., mesosynoptic) conditions responsible for specific SBJ attributes were not explored in detail, nor were the resulting hydrometeorological impacts, given the limited compositing capabilities of the day.
Using a unique multiyear dataset collected by 915-MHz wind profilers at Chico, California (CCO), and Grass Valley, California (GVY), in California’s Central Valley and along the windward slope of the sierra (Fig. 1), the current study strives to extend the SBJ statistical analysis of Smutz (1986), with the understanding that the wind profilers do not record thermodynamic information. The profilers yielded over 100 000 hourly wind profiles between 2000 and 2007, using this unattended remote sensing technology, compared with the 1849 SCPP rawinsondes launched with 3-h resolution solely in select cool-season SBJ conditions. In addition, the profilers recorded data every ∼100 m in the vertical—a threefold increase in vertical resolution compared to the rawinsonde data in Smutz (1986). Our expanded dataset allows us to perform a statistical analysis of the vertical-temporal character of SBJ cases and to explore the interseasonal variability of SBJs. Figure 2b shows an example of a strong SBJ event from CCO, during which time the core of strongest winds (>50 m s−1) lasted for 3 h at ∼1.2 km MSL. Because of the availability of two wind profilers, one in the foothills and the other on the valley floor, we are able to contrast key SBJ characteristics between these geographically disparate sites. Finally, we use the North American Regional Reanalysis (NARR; Mesinger et al. 2006) to establish the mesosynoptic dynamical context associated with SBJ events. The NARR and companion precipitation-gauge data are also used to assess hydrometeorological impacts of SBJs.
2. Key datasets
Research data were collected from 915-MHz radar wind profilers (e.g., Carter et al. 1995) at CCO and GVY. (Fig. 1; Table 1). These profilers were deployed by the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory (NOAA/ESRL) and represent the observational foundation of this study. The CCO profiler was sited in the northern Central Valley at 41 m MSL, whereas the GVY profiler was positioned farther southeast, on the western slope of the Sierra Nevada at 689 m MSL. The profilers provided hourly averaged vertical profiles of horizontal wind velocity from ∼0.1 to 4.0 km above ground, with ∼100-m vertical resolution and ∼1 m s−1 accuracy in all weather conditions. The profiler winds were objectively edited using the vertical–temporal continuity method of Weber et al. (1993) and an extra level of quality control was performed by visual inspection to flag the few remaining outliers. Data coverage exceeded 90% at both sites, up to at least 2.5 km MSL, far above the climatological altitude of the SBJ (section 3). Data collection spanned ∼7 yr at CCO and nearly 5 yr at GVY between 2000 and 2007 (Table 2). These deployments were supported largely by the NOAA’s Hydrometeorological Testbed program (HMT; Ralph et al. 2005) and by related studies prior to that. Infrequent wind-profiler outages occurred at both sites during their operational periods, lasting from several hours to a couple of weeks. A total of 60 468 (41 445) hourly profiles was recorded at CCO (GVY; Table 2). Both wind-profiler datasets were analyzed in detail, although greater emphasis is placed on the CCO dataset because of its longer and more complete time series.
Data from a transect of three precipitation gauges adjacent to each wind profiler (Fig. 1, Table 1) were analyzed to evaluate the impact of SBJs on finescale precipitation distributions along the western slope of the Sierra Nevada. Each transect is composed of a valley site [Chico (CHI) and Lincoln (LCN) at 70 and 61 m MSL, respectively], a midslope site [Cohasset (CST) and Rollins Reservoir (BRE) at 488 and 593 m MSL, respectively], and an upper-slope site [Four Trees (FOR) and Blue Canyon (BLC) at 1570 and 1609 m MSL, respectively]. In addition, data from a gauge at the northern terminus of the Central Valley [Lakeshore (LKS) at 335 m MSL) were analyzed to assess the impact of SBJs there. The gauge data were acquired from the California Data Exchange Center (available online at http://cdec.water.ca.gov) and then quality controlled via objective measures and with visual inspection for obvious outliers. The time interval between measurements ranged from 15 to 60 min, depending on the intensity of the precipitation, while the minimum measurement resolution varied from 0.01 to 0.04 in. (i.e., 0.254–1.016 mm).
To assess the mesosynoptic conditions responsible for the SBJs observed by the wind profilers, and to evaluate the regional precipitation distributions associated with these SBJs, we utilized the NARR (Mesinger et al. 2006). Covering all of North America and adjacent oceans, the NARR is available from 1979 to the present, with a horizontal grid spacing of 32 km and 45 vertical levels. The reanalysis blends observational and model first-guess data to estimate the state of the atmosphere at 3-h intervals. It ingests initial conditions from the operational Eta Data Assimilation System (Rogers 2005) and assimilates precipitation observations using derived latent heating profiles. For our region of interest across the western United States, the precipitation observations were scaled onto a grid based on monthly, orographically dominated climatological distributions, using the Parameter-Elevation Regressions on Independent Slopes Model (PRISM; Daly et al. 1994). A study by Bukovsky and Karoly (2007) reported that the NARR replicates continental U.S. precipitation well, although other regions of the domain are prone to enhanced inaccuracies. A related paper by West et al. (2007) cautioned against spurious grid-scale precipitation in the NARR, although the results we will present later show no evidence of such artifacts. Mo et al. (2005) demonstrated that NARR precipitation and moisture transports are most realistic during winter, which corresponds approximately to our cool-season focus. Reeves et al. (2008) showed good agreement during two heavy precipitation events in the sierra between observed fields and their NARR counterparts.
3. SBJ bulk characteristics
This section presents climatological characteristics of the SBJ. Initially, all wind profiles with predefined SBJ attributes are considered (section 3a), then coherent SBJ cases are examined (section 3b); a comparison between our SBJ profile analysis and that of Smutz (1986) is also provided (section 3a). All wind analyses described in this section are based on a terrain-relative coordinate system, whereby the axes are rotated 20° counterclockwise from the cardinal directions, as in the Smutz study. Hence, the positive V component of the flow aligns with the long axis of the Sierra Nevada (i.e., directed from 160° to 340°) and the cross-mountain wind (U) component points orthogonal to the sierra (positive values directed toward 70°).
a. Assessment based on individual wind profiles
To determine climatological SBJ characteristics, one must first design and apply objective SBJ criteria, as will be described later. The study by Smutz (1986) utilized 1849 rawinsonde soundings released between November and April during SCPP (1976–84) along the western (i.e., windward) base of California’s Sierra Nevada at SHR (60 m MSL; Fig. 1). Of their 1849 soundings, 85% were confined to the January–March window. A total of 1642 of those soundings contained SBJ attributes, defined in their study as having a relative maximum in the V-component profile greater than 0 m s−1 below 3 km MSL (i.e., below crest level). If more than one relative maximum was observed, the maximum with the greatest V was taken as the SBJ (defined as Vmax). The altitude of Vmax occurred most frequently in the 0.6-km MSL bin, whereas its mean altitude resided at 1.12 km MSL (Fig. 3a). The most commonly observed magnitude of Vmax ranged between 5 and 10 m s−1, whereas the mean value was 11.0 m s−1 (Fig. 3b). A skewed tail contains much stronger SBJ profiles, including four in the 40–45 m s−1 range.
More restrictive criteria are applied to our wind-profiler datasets for defining an SBJ profile. These added constraints (described hereinafter) are designed to ensure that only those profiles exhibiting an unambiguous SBJ signature are included in the subsequent analyses. In addition to employing the criteria of Smutz (1986), Vmax must exceed 12 m s−1. Also, the V component must decrease by more than 2 m s−1 with increasing height somewhere between the altitude of Vmax and 3 km MSL. Furthermore, Vmax must occur at the second radar range gate or higher (i.e., ≥∼200 m above ground), thus eliminating shallow surface-based flows. Finally, range gates adjacent in altitude to Vmax must contain data.
Using these more restrictive SBJ constraints, 6695 and 2563 profiles were tagged at CCO and GVY, respectively (Table 2). For comparison, histogram analyses from the profilers (Fig. 3c–f) are shown beneath those from SHR (Figs. 3a,b). The relative distributions of Vmax altitude at SHR and CCO are similar, as are their mean and modal Vmax altitudes (1.1 and 0.3–0.6 km, respectively), thus reflecting their comparable mountain-relative positions and station altitudes (e.g., Fig. 1). In contrast, the mean and modal Vmax altitudes at GVY are much higher at 1.82 and 1.5 km, respectively. From a ground-relative perspective, the mean Vmax altitude occurs ∼1 km above the surface at all three sites. These climatological results confirm the case-study depiction by Parish (1982) of the SBJ ascending the windward slope of the sierra (Fig. 2a). The mean magnitudes of Vmax at CCO and GVY are similar (17.9 and 17.4 m s−1, respectively), thus revealing that the strength of SBJs, on average, remains nearly constant up the windward slope. Both histograms depict a skewed tail with strong values >30 m s−1, although only CCO observed SBJ conditions in excess of 45 m s−1. It is unclear whether this difference arises because of a greater sample size at CCO or if the strongest SBJ conditions tend to occur over the flat terrain near the base of the sierra rather than along its windward slope. The main difference between the wind-profiler histograms of Vmax and their rawinsonde counterpart is explained by the 12 m s−1 Vmax minimum threshold applied only to the wind-profiler observations.
Histograms in Fig. 4 highlight the altitude-induced differences of wind direction and U-component wind speed observed in SBJ conditions at the Vmax altitude at CCO and GVY. That is, the mean and modal wind directions at CCO align nearly perfectly with the 160°–340° orientation of the sierra, as should be expected in shallow, terrain-blocked flow. In comparison, the mean and modal wind directions at GVY (positioned ∼700 m higher than at CCO) are rotated clockwise by 10°–12°, consistent with a pre-cold-frontal warm-advection environment, where SBJs typically occur (see section 5). The U-component wind speed at CCO shows stagnant mountain-normal flow that typifies strongly blocked conditions. In contrast, the cross-mountain component at GVY is also weak but distinctly positive (∼4–5 m s−1) at this higher altitude.
The year-round deployments at CCO and GVY provided the means to assess the annual cycle of SBJ occurrences (Fig. 5). The seasonal distribution of SBJ profiles was quite similar at these geographically distinct locations, with a well-defined maximum during the cooler months and a marked minimum during the summer. Not surprisingly, this cycle mirrors the landfalling baroclinic cyclone climatology and the associated seasonal cycle of precipitation, in California. By far, December has the greatest number of SBJ profiles at each site and may reflect a particularly active early-winter storm track, although it is unclear from the wind-profiler observations alone why this is the case and it is beyond the scope of the present study.
b. Assessment based on coherent cases
In an effort to tie SBJ conditions to coherent mesosynoptic disturbances (see sections 4 and 5), we used our multiyear profiler observations to generate inventories of SBJ cases, where an SBJ case is defined as a group of at least eight consecutive hourly wind profiles with SBJ attributes. The CCO site recorded 211 SBJ cases composed of 3527 profiles and the GVY site observed 84 cases with 1327 profiles (Table 2). More than half of all profiles initially tagged as SBJs were included in the inventory of SBJ cases.
Given the strong annual cycle of SBJ occurrences (Fig. 5), we stratified the cases by the season: winter (December–February; DJF), spring (March–May; MAM), summer (June–August; JJA), and autumn (September–November; SON). Composite V- and U-component wind profiles (Fig. 6) highlight the seasonally dependent vertical structures of the SBJ cases at CCO and GVY. The wintertime SBJ cases contain ∼60% of the total number of SBJ wind profiles, whereas the summer SBJ cases account for <4% of those profiles. At both sites, the SBJ is strongest in winter and weakest in summer, consistent with the seasonal variation in the strength of landfalling extratropical cyclones. The shoulder seasons exhibit intermediate magnitudes of flow, although the U-component profiles are more similar to their winter counterparts. The U-component profiles show stagnant cross-mountain flow in the lowest range gates. Other than summer, this flow increases sharply with height above the SBJ blocked flow, reflecting the mean large-scale, poleward-directed baroclinicity.
4. Analysis of cool-season SBJ cases
a. Defining the cool season and documenting local cool-season precipitation characteristics
SBJ cases have the greatest potential to modulate precipitation and associated hydrology during California’s cool season because this is when most of the precipitation falls in the state and SBJs are typically tied to transient storm systems during the cool season (e.g., Marwitz 1983, 1987). Hence, as the first step toward exploring the role of cool-season SBJs in modulating the precipitation in the Sierra Nevada, and to ascertain the mesosynoptic conditions responsible for cool-season SBJs, we examined the composite monthly profiles of wind direction and speed during SBJ cases at CCO and GVY (not shown). The composite profiles from October through April share similar characteristics, and are distinctly different than the composite profiles from the five warmer months. Hence, we are defining the cool season as October–April. By considering only the cool-season period, the total number of SBJ cases are reduced slightly from 211 to 172 (i.e., from 3527 to 3055 hourly profiles) at CCO and from 84 to 78 (i.e., from 1327 to 1240 profiles) at GVY (Table 2).
To estimate the impact of cool-season SBJ cases on regional precipitation, data from a baseline of three precipitation gauges were examined along the sierra’s windward slope near CCO and from a comparable, more southern baseline near GVY (Fig. 1, Table 1). The precipitation was stratified into two bins: the amount that fell during SBJ cases and the amount that fell in the absence of SBJ conditions (i.e., excluding all SBJ cases and profiles) when the respective wind profilers were operating. During the seven cool seasons of data collection at CCO, the gauges at CHI, CST, and FOR recorded 4612, 6628, and 14 894 mm of precipitation, respectively (Fig. 7a). Here, 47%–56% of precipitation fell during SBJ cases at CCO. A fourth gauge, located at the head of the Central Valley (LKS; Fig. 1, Table 1), measured 10 623 mm of precipitation, 60% of which fell during SBJ cases, thus revealing the dominant role of SBJs in contributing to the water supply in that part of the state. The five intermittent cool seasons of operations at GVY yielded 1510, 4049, and 4360 mm at LCN, BRE, and BLC, respectively (Fig. 7b). Only 28%–38% of this precipitation fell during SBJ cases at GVY. A local westward bend in the northern sierra near CCO quite likely intercepted the SBJs, thus contributing to enhanced orographic precipitation there (as depicted in a modeling case study by Reeves et al. 2008). In contrast, the orientation of the sierra is more linear near GVY, such that SBJs remained nearly parallel to the high terrain and less orographic enhancement occurred relative to the northern baseline. Nevertheless, the orographic precipitation enhancement was distinct along both baselines, although it was most prominent between the mid- and upper-slope sites of the northern baseline and between the valley and midslope sites of the southern baseline. This difference may have arisen, in part, because of transverse topographic variations along the sierra’s windward slope (as described above) and also because of shallow diabatically generated down-valley flows (e.g., Rangno et al. 1977; Steiner et al. 2003). Companion analyses of normalized precipitation accumulation (Figs. 7c,d) show that SBJ cases tend to redistribute the precipitation down the windward slope of the northern sierra (i.e., the precipitation shifts upstream) because the blocked flow acts as a dynamic barrier to the incoming ambient flow (Doyle 1997; Colle 2004).
b. Key cool-season SBJ attributes and their impact on local precipitation characteristics
The large number of cool-season SBJ cases at CCO were stratified by key SBJ attributes: the mean strength of Vmax, the mean altitude of Vmax, and the duration of the cases. Composite profiles of the V and U components for the top- and bottom-20 ranked cases (i.e., top, bottom ∼12%) in these three categorical distributions or classes are shown as the bright colors in Fig. 8, and the case listings are presented in Tables 3 –8.1 These tables reveal that the strong, high, and long-duration classes contain some overlapping cases, as do the weak, low, and short-duration classes. The composite V-component profiles of strong- and weak-SBJ cases reveal marked differences (Fig. 8a), including a jet magnitude of ∼26 m s−1 at ∼1.1 km MSL versus a much shallower jet of ∼13 m s−1 at ∼0.6 km MSL, and a 5–17 m s−1 offset through the 4-km depth of the composite profiles. The strong cases lasted twice as long as the weak ones (24.2 versus 11.8 h). Half of all the strong cases occurred in December alone. The composite V-component profile of the highest SBJ cases shows a jet positioned 4.5 times higher than in the low-SBJ composite (∼1.8 km versus ∼0.4 km MSL;2 Fig. 8c), although the magnitude of the jet is only modestly stronger (∼19 versus 16 m s−1). However, above ∼2 km MSL, the composite profile of the highest cases contains much stronger terrain-parallel flow (by 10–15 m s−1). The composites of the long- and short-duration V-component profiles (mean durations of 43 and 8 h, respectively) both contain a jet centered at ∼0.8–1.0 km MSL (Fig. 8e), although the Vmax in the long-lived composite profile is nearly 5 m s−1 stronger (19 versus 14.5 m s−1); December dominated the long-lived cases (i.e., 12 of 20). The corresponding composite U-component profiles (Figs. 8b,d,f) show very similar cross-mountain wind structure, based on strength and duration; however, the lowest SBJ cases exhibit the strongest cross-mountain winds and forward low-level shear, whereas the highest cases contain the weakest.
The three classes of SBJ cases at CCO produced different precipitation characteristics along the sierra’s nearby windward slope (i.e., the three-station transect: CHI, CST, and FOR) and at the head of the Central Valley (LKS), as is revealed in the histograms of case-averaged precipitation rate and accumulation (Fig. 9, Tables 3 –8; the southern baseline exhibited comparable traits but is not shown). Stratification by SBJ strength yields much larger precipitation rates (by a factor of ∼4–8) for the strong cases, although they both show a steady increase in precipitation rate with altitude up the sierra slope between CHI and FOR. The strong cases generate the greatest precipitation rates of the three classes of SBJs (from 1.37 mm h−1 in the valley to 3.02 mm h−1 at the upper-slope site to 3.37 mm h−1 at LKS), whereas the weak cases produce the least (0.18 to 0.75 mm h−1). Based on these results, and the fact that the strong cases persist, on average, for twice as long as the weak ones, the differences in case-averaged precipitation accumulation are even more extreme: the strong cases receive, on average, 33 mm in the valley, 73 mm at the upper-slope site, and 82 mm at LKS, roughly 9–16 times more than their weak counterparts. In short, strong-SBJ cases are associated with hydrologically significant storms, whereas the weak cases are not.
Stratification by SBJ altitude reveals dichotomous cross-mountain precipitation distributions for the high versus low cases along the three-station transect; namely, the low cases generate the largest increase in precipitation between the mid- and upper-slope sites of any shown in Fig. 9, whereas the high cases produce the weakest precipitation gradient across the three-station transect. The companion cross-mountain wind-profile composites in Fig. 8 show strong (weak) forward vertical shear for the low- (high-) SBJ cases. Based on numerical modeling results in Colle (2004), an increase in forward cross-mountain vertical shear decreases the magnitude of the upwind-tilted mountain wave and favors precipitation falling higher on the slope, whereas weaker forward shear yields a stronger mountain wave and an upwind shift in precipitation down the slope. Unlike the strong and long-duration cases, neither the high- nor low-SBJ cases generated copious precipitation, including at the head of the Central Valley.
The long- and short-duration SBJ cases both generate a steady increase in precipitation with increasing altitude along the three-station transect, similar to the results based on SBJ strength. However, the long-duration cases generate 2–3 times heavier precipitation rates than their short-lived counterparts. Most significantly, the mean duration of the long-lived cases is ∼5.5 times greater than the short events (Tables 7 and 8). Hence, the case-averaged precipitation accumulation is far greater (by a factor of 10 to 17) for the long cases, totaling 40 mm in the valley, 62 mm at the midslope site, 130 mm at the upper-slope site, and 110 mm at the head of the Central Valley. These values, which represent the largest for all classes of SBJs, should clearly yield major hydrologic consequences. In contrast, the small accumulations with the short-lived cases are hydrologically insignificant.
5. Composite NARR analyses based on key cool-season SBJ attributes
The NARR is used to explore the mesosynoptic conditions responsible for, and to showcase the precipitation distributions associated with, the top- and bottom-20 ranked cases in each of the three classes of SBJs. Plan-view NARR composites (Figs. 10 –15) are constructed from individual 3-h fields, whose dates and times fall within these SBJ cases, as are NARR composite geostrophic wind profiles at the grid point closest to CCO (Fig. 8) and NARR composite bulk dynamical parameters closest to CCO and at an offshore grid point closest to 37°N, 127°W (Table 9). The strong and weak NARR composites contain 165 and 88 individual fields, respectively. Likewise, the high and low composites include 142 and 105 fields each, and the long- and short-duration composites encompass 295 and 60 fields. A large-scale plan-view domain captures the mass and moisture fields, whereas an inset domain is used to depict the unrotated zonal and meridional wind components and precipitation. The rotation offset between the NARR and terrain-relative wind components is only 20°, so the unrotated components represent ∼94% of their rotated counterparts; unrotated NARR wind components are shown in plan view in Figs. 11, 13, and 15, whereas the profiles of geostrophic wind in Fig. 8 have been rotated to be directly comparable with the observed profiles. Because the inventories of SBJ cases at the CCO and GVY wind profilers yield similar NARR compositing results, we will show only those composites based on the longer and more continuous time series at CCO. The locations of CCO and the offshore site are shown on the NARR plan-view composites.
a. Mean strength of Vmax
Strong SBJs are tied to transient, high-amplitude waves at 500 and 900 hPa impacting California (Figs. 10a,c), whereas weak SBJs occur with low-amplitude waves in zonal flow making landfall in the Pacific Northwest (Figs. 10b,d). The geostrophic flow at CCO at the altitude of Vmax intersects the sierra obliquely in both composites (Figs. 8a,b). The range-perpendicular geostrophic wind is nearly twice as large for the strong composite, and is larger for the strong composite than for any of the other five classifications. The companion-integrated water vapor (IWV) fields (Figs. 10e,f) show a more distinct plume, with greater vapor content intersecting the California coast during strong SBJs. Composites of vertically integrated horizontal water vapor transport between the surface and 300 hPa (IVT; Neiman et al. 2008a) reveal markedly different attributes (see Figs. 10g,h): the strong composite is characterized by a corridor of large vapor fluxes (∼400 kg s−1 m−1) approaching California from the southwest, whereas the weak composite contains a weaker (∼300 kg s−1 m−1), zonally oriented plume at a more northern location. The strong IVT composite shows evidence of terrain modulation, both in California’s Central Valley and along its northern coastal zone. Specifically, enhanced IVT extends from the San Francisco Bay gap into the Central Valley, then northward up valley in the terrain-trapped flow, similar to that modeled by Smith et al. (2010) during a high-impact AR event. Similarly, a finger of enhanced, northward-deflected vapor fluxes parallels California’s northern coast. In contrast, the weak IVT composite exhibits no significant inland penetration of enhanced vapor fluxes and only marginally enhanced fluxes along California’s northern coastal zone. Despite the differing intensities, both IVT composites show a landfalling AR-like plume (as in Neiman et al. 2008a,b), which further highlights the fact that SBJs are typically tied to warm-sector flows in extratropical cyclones.
Figure 11 displays zoomed-in composite analyses of the 900-hPa wind components and 3-h precipitation rates. As expected, the meridional flow Vm is stronger in the northern Central Valley for the strong-SBJ cases (∼16 versus 8 m s−1). However, both composites portray weaker flow than the CCO wind-profiler observations at 1 km MSL (Fig. 8a). This discrepancy arises because of the coarse spatial resolution of the NARR and the lack of wind-profiler data assimilation into this gridded dataset, thus highlighting the value of observation-based climatology studies. A secondary corridor of terrain-enhanced meridional flow resides along the northern California coast in both composites but is more prominent during strong SBJs because this class of SBJs is associated with stronger landfalling storms. The zonal component of the 900-hPa flow Uz approaching California is greater in the strong-SBJ composite for the same reason. Both Uz composites show a band of flow reversal in the Central Valley (which, in part, reflects the stagnation zone in the cross-terrain component) and a band of enhanced westerly flow east of the Sierra Crest (representing leeside mountain wave activity, e.g., Durran 1990). The 3-h precipitation rates are much larger across the elevated terrain during strong SBJs. A local maximum of 6 mm (3 h)−1 resides over the northern Sierra Nevada where the range protrudes westward, whereas a larger maximum of 9 mm (3 h)−1 is situated farther north over the Shasta Mountain area. Strong-SBJ flow ascending these terrain features quite likely contributed to these maxima, as was first implied with surface observations (e.g., Reynolds and McPartland 1993) and shown more recently with modeling studies (Kim and Kang 2007; Reeves et al. 2008). In both SBJ composites, orographic ascent of maritime air contributed to precipitation in California’s coast ranges (e.g., Neiman et al. 2002). The more intense and widespread precipitation in the strong-SBJ composite also reflects the fact that the strong SBJs are tied to more intense storms, with stronger large-scale dynamics and orographic forcing.
b. Mean altitude of Vmax
The second set of NARR composites contrasts the background dynamics tied to high and low SBJ cases. The 500- and 900-hPa height fields for these disparate SBJ case composites (Figs. 12a–d) are qualitatively similar to those based on strength, with the following exceptions: the composite high-amplitude wave offshore of California at both levels is weaker for the high SBJs than the strong SBJs and the geostrophic zonal flow across California is stronger for the low SBJs than the weak ones. The sierra-perpendicular geostrophic flow at CCO is stronger during low SBJs, although the high SBJ composite has stronger sierra-parallel geostrophic flow (Figs. 8c,d). Composites of IWV (Figs. 12e,f) show a more organized plume, with greater water vapor content intersecting California for the high SBJs. However, corresponding IVT composites (Figs. 12g,h) contain a stronger vapor-flux plume directed at California during low SBJs because the layer mean flow over the eastern Pacific is stronger during low SBJs than high ones. Significantly, during high SBJs, the AR-like IVT plume originates from the southwest quadrant, whereas it is zonally oriented during the low SBJs, consistent with the differing height-field configurations. Neither composite shows a large influx of water vapor into the Central Valley.
Over the eastern Pacific, the composite 900-hPa wind fields (Figs. 13a–d) corroborate the geopotential height analyses—namely, stronger meridional flow and weaker zonal flow for the high SBJs. The magnitude of the 900-hPa meridional flow in the Central Valley is comparable for the high and low SBJs, as is also the case for the CCO wind-profiler observations just below 1 km MSL (Fig. 8a). However, as before, the NARR composites underestimate the magnitude of this flow. The zonal-wind composite for low SBJs contains a more intense signature of mountain wave activity to the lee of the sierra in response to a stronger zonal flow crossing this nearly north–south oriented range. The companion precipitation-rate composites (Figs. 13e,f) demonstrate that neither high nor low SBJs yield abundant precipitation. Nevertheless, the low SBJs generate larger precipitation rates over the high terrain, quite likely as a result of more favorable orographic forcing (i.e., stronger cross-mountain vapor fluxes intersecting the sierra) and a weaker mountain-wave response in a highly sheared environment (Colle 2004). The fact that the high SBJs reside near the Sierra Crest rather than ascend its lower windward slope may further explain the comparatively weak precipitation in the sierra during high SBJs.
c. Mean SBJ duration
Although the 500- and 900-hPa height composites for the long- and short-lived SBJs (Figs. 14a–d) both contain a low-amplitude trough offshore of California, only the composites for the long-lived cases are characterized by a lengthy fetch of west–southwesterly flow arcing across the eastern Pacific to California in a long wavelength synoptic pattern, thus making them unique among their counterparts for all three classes of SBJs. The geostrophic flow at CCO is similar in incidence angle for both composites but nearly twice as strong for the long composite (Figs. 8e,f). The corresponding IWV composites (Figs. 14e,f) show a highly organized plume of large vapor content intersecting California for the long cases but a nearly incoherent plume with comparatively dry conditions for the short cases. In fact, the composite IWV plume for the long (short) cases are the most (least) organized, with the greatest (least) vapor content of all six IWV composites. The IVT composites (Figs. 14g,h) also contrast markedly between the long and short cases. Specifically, the long cases are associated with classic AR conditions, whereby a narrow plume of strong vapor fluxes extends from the subtropical eastern Pacific to California. Vapor transports within this composite plume reach 425 kg s−1 m−1 and represent the largest value among the six IVT composites. In contrast, the short cases yield the least organized and weakest IVT (200 kg s−1 m−1) of the six composites. The long-duration composite possesses enhanced vapor fluxes into California’s Central Valley; these fluxes are then directed northward by the SBJ up the sierra’s windward slope and to the valley’s northern terminus. The Central Valley vapor fluxes in the long-duration SBJ composite (Fig. 14g) are not quite as intense as in the strong-SBJ composite (Fig. 10g), although both SBJ regimes contain the strongest valley fluxes directed obliquely toward the sierra, consistent with the fact that the heaviest mountain precipitation also falls during these conditions (e.g., Fig. 9). The long-duration IVT also shows a corridor of enhanced transport along the state’s northern coast. The IVT composite of short-duration cases has far weaker transports in the Central Valley and along the coast.
The 900-hPa wind and precipitation composites (Fig. 15) also show extreme differences between the long- and short-SBJ cases. First, blocked southerly flow is much stronger in the Central Valley (and along the coast) for the long-lived SBJs (Figs. 15a,b), in agreement with the corresponding SBJ composites observed by the CCO profiler (Fig. 8). Second, the incoming zonal flow from the Pacific is stronger during persistent SBJs, although the magnitude and aerial coverage of the Uz flow reversal in the Central Valley is also greater during these conditions (Figs. 15c,d), thus highlighting the fact that blocking is more robust with long-lived SBJs. Finally, mountain precipitation is more intense and widespread during lengthy SBJs, consistent with the fact that they are accompanied by the greatest vapor fluxes intersecting the high terrain.
d. Bulk dynamical characteristics of the SBJ classes
In an effort to explore dynamical linkages between incoming airstreams and opposing classes of SBJs, the NARR data are analyzed at the grid points closest to CCO and the offshore coordinate 37°N, 127°W (Table 9). Layer-mean dry Froude number (Fr) composites are calculated as the ratio of the cross-mountain wind U and the product of the barrier height (H = 2 km) and the Brunt–Väisälä frequency [N = (g/θ)(∂θ/∂z), where θ is the layer-mean potential temperature over which ∂θ/∂z is calculated] between 0 and 2 km MSL. The saturated Froude number (Frm) incorporates the saturated N (i.e., Nm; derivation as per Durran and Klemp 1982).
All SBJ composites at CCO are stably stratified below 2 km MSL, irrespective of dry or saturated conditions. The companion Fr and Frm values at CCO reveal a strongly blocked flow, given that U is small and the resulting Froude numbers range between 0 and 1 (e.g., Pierrehumbert and Wyman 1985; Smolarkiewicz and Rotunno 1990). Offshore, the layer-mean relative humidities are <80%, so the dry Froude number is the appropriate dynamical parameter to consider. All offshore dry Froude numbers are less than 1, despite the fact that U is much stronger here than at CCO; thus, revealing that the incoming airstreams will become blocked by the sierra. Given that all classes of SBJs occur during stably stratified conditions with small Froude numbers offshore, the SBJ characteristics may be related more to the strength of the incoming U than to the stability offshore. In fact, the strong- and long-duration SBJ case composites are associated with incoming values of U that are ∼50% greater than their weak and short-duration counterparts, although the high and low SBJ cases show no such contrast. The large values of incoming U for strong and long cases may contribute to the enhanced precipitation in the sierra because this flow ascends the SBJ and windward slope.
Another factor that can modulate SBJ characteristics is the altitude of the freezing level in precipitating conditions. Marwitz (1983, 1987) showed observationally that a layer of melting precipitation (typically situated 200–400 m below the freezing level, e.g., Stewart et al. 1984) intersecting the windward slope of the sierra can strengthen the shallow cold pool and windward high pressure in the SBJ regime via diabatic cooling, especially when precipitation increases in intensity in response to orographic lift. The net result can increase the strength of the blocked flow. If the melting level occurs above the Sierra Crest, then diabatic cooling will not preferentially enhance the cold pool above the windward slope. Comparison of the composite freezing levels for the strong- and weak-SBJ cases (1950 and 2250 m MSL, respectively; Table 9), suggests that the melting level intersected the sierra’s windward slope for the strong cases but skimmed the top of the Sierra Crest for the weak cases, thus strengthening the windward cold pool and SBJ flow only for the lower melting-level scenario. Surprisingly, the altitude of the SBJ does not appear sensitive to the height of the freezing level (nor does the SBJ duration).
This study documents composite characteristics of the SBJ based on multiple years of wind-profiler data gathered at two sites upstream of California’s northern Sierra Nevada Crest: CCO (in the Central Valley) and GVY (648 m higher on the sierra’s windward slope). Between 2000 and 2007, these sites recorded >9000 profiles that satisfied the SBJ criteria described in section 3. The mean magnitude of the terrain-parallel flow in the SBJ core (i.e., Vmax) was similar at both sites (∼17.5 m s−1) and at a comparable ground-relative position 500–1000 m above the surface, thus, revealing that the core of the SBJ ascends the windward slope of the sierra, as in case-study depictions during SCPP (e.g., Parish 1982). At the altitude of Vmax, the mean wind direction paralleled the sierra at CCO but was rotated clockwise by 10°–12° at GVY (Vmax was higher here by ∼700 m MSL), consistent with pre-cold-frontal warm-advection conditions where SBJs typically occur. At both sites, the cross-mountain wind speed was weak at the altitude of Vmax. The seasonal cycle of SBJ occurrences was distinct, with a well-defined maximum during the cooler months and a marked minimum in summer. Also, the SBJ was much stronger in winter than summer. These seasonally dependent results mirror California’s landfalling baroclinic cyclone climatology and associated seasonal cycle of precipitation.
In an effort to tie coherent mesosynoptic disturbances to SBJs, and to explore the role of SBJs in modulating precipitation in the sierra during the climatologically wet cool season, an inventory of SBJ cases from each site was generated for the October–April period (172 at CCO and 78 at GVY), where a case was defined as a group of at least eight consecutive hourly wind profiles with SBJ attributes. The cool-season orographic precipitation enhancement along the sierra’s windward slope was distinct during non-SBJ conditions; however, the presence of the SBJ redistributed the precipitation upwind, quite likely by acting as a dynamic barrier. About one-half (one-third) of the nearby cool-season precipitation fell during the SBJ cases at CCO (GVY). A local westward bend in the northern sierra near CCO quite likely intercepted the SBJs, thus contributing to enhanced orographic precipitation there (relative to GVY).
The cool-season SBJ cases at CCO were stratified based on three key attributes: strength, altitude, and case duration. The strong, high, and long-duration SBJ classes contain some overlapping cases, as do the weak, low, and short-duration classes. Composite profiles of the along-barrier component for the top- and bottom-20 ranked cases in each of these three SBJ classes revealed stark differences in the magnitude and vertical positioning of the barrier jet, although the cross-mountain composite profiles were generally less distinct. The three SBJ classes yielded different local precipitation characteristics; for example, the strong cases generated the greatest precipitation rates (1.37–3.02 mm h−1 from the valley to the upper slope), whereas the weak cases produced the least (0.18–0.71 mm h−1). The difference in case-averaged precipitation accumulation was even more extreme, given that the strong cases persisted for twice as long as the weak ones. The strong cases also produced copious precipitation at the northern end of the Central Valley because the SBJs were funneled up this orographically favored region. The precipitation rates and accumulations for the highest SBJs varied little along the windward slope, whereas the upper-slope site exhibited significant orographic enhancement relative to the lower sites for the lowest SBJs. In addition, the highest (lowest) SBJs were accompanied by weak (strong) cross-mountain vertical wind shear. A recent modeling study by Colle (2004) showed that increasing the cross-mountain shear decreases the mountain-wave response and shifts the precipitation up the windward slope. Long-lived SBJs yielded precipitation rates almost as large as the strong cases and generated the greatest case-averaged precipitation (40–130 mm) and hence the greatest likelihood of hydrologic impacts. In contrast, the short-lived cases were hydrologically insignificant.
NARR compositing was used to explore the mesosynoptic conditions responsible for, and to showcase the precipitation distributions associated with, the top- and bottom-20 ranked cases in each of the three classes of SBJs. Strong SBJs were tied to transient, high-amplitude troughs impacting California and strong vapor fluxes intersecting the state from the southwest, whereas weak SBJs occurred with low-amplitude troughs making landfall in the Pacific Northwest and weaker zonally oriented vapor-flux plumes. The NARR showed greater evidence of blocking during strong SBJs. The NARR precipitation rates were much larger in the northern sierra during strong-SBJ cases because the orographic forcing with strong SBJs was greater. Neither high nor low SBJs yielded abundant precipitation, in agreement with the precipitation gauge analyses. Of all the SBJ types, only the long-lived cases were characterized by a lengthy fetch of west–southwesterly flow across the eastern Pacific to California. The composite IWV plumes for the long- (short-) duration cases were the most (least) organized with the greatest (least) vapor content of all six IWV composites. The companion IVT composites contrasted similarly between the long and short cases. The long-duration and strong IVT composites possessed the strongest vapor fluxes into California’s Central Valley and up the valley toward the sierra, consistent with the heavy mountain precipitation that fell during those conditions.
Although an in-depth discussion of the SBJ dynamics is beyond the scope of this manuscript, several preliminary conclusions can be drawn with the aid of the NARR. First, the observed composite profiles at CCO differ significantly from their geostrophic counterparts throughout their 4-km depth, which suggests that the processes creating the SBJ and/or mountain-wave dynamics are impacting the flow well above the top of the topographical barrier. Second, the flow approaching the coast is dynamically conducive to blocking for all SBJ classes, although the strongest cross-barrier flows offshore create the strongest SBJs. And third, the altitude of the melting level may modulate the strength of the SBJ (via diabatic cooling).
SBJs typically occur during landfalling extratropical cyclones, regardless of the barrier jet’s strength, altitude, or duration, although the NARR composite analyses suggest that AR conditions are most robust during the strongest and longest-lived SBJs. An independent assessment of the AR–SBJ connection was made by inspecting twice-daily Special Sensor Microwave Imager (SSM/I) IWV satellite imagery during the October–April cool season for the 7-yr period when the CCO wind profiler recorded data. If the SSM/I imagery showed a long (>2000 km), narrow (<1000 km) plume of enhanced IWV (>2 cm) intersecting the California coast during the morning or afternoon on a given day, then that day was tagged an AR day [the same IWV thresholding was employed in Ralph et al. (2004) and Neiman et al. (2008a,b) to define an AR]. Table 10 shows the relationship between cool-season SBJ cases at CCO and ARs that made landfall in California on those SBJ days. Notably, 63% of the 172 SBJ cases coincided with an AR landfall. The strongest and longest-lived SBJs were almost always tied to landfalling ARs (85%–95%), whereas the relationship was least compelling for the weakest and shortest-lived SBJs (45%–55%). To more fully understand the role of SBJs in modulating precipitation and hydrology in the sierra, further study is required on the connection between ARs and SBJs, especially given that SBJs and ARs account for a large fraction of storm total rain and snow in the Sierra Nevada (e.g., Neiman et al. 2008a).
This study has been made possible by the dedicated engineering and technical team in NOAA/ESRL’s Physical Sciences Division (PSD); led by James Jordan and Clark King, this team built, deployed, and maintained the radars used in this study. Jessica Lundquist from the University of Washington provided insight into the connection between precipitation distributions and SBJs. Cathy Smith of PSD developed NCEP–NCAR reanalysis composite tools. Jim Adams electronically drafted a number of the figures. This research was supported by NOAA’s Hydrometeorological Test bed program and the Weather–Climate Connection Project. We appreciate the comments and suggestions by Dan Gottas and Allen White of PSD and from three anonymous reviewers. Their efforts improved the scope and quality of the manuscript.
Corresponding author address: Paul J. Neiman, NOAA/Earth System Research Laboratory, Physical Sciences Division, Mail Code R/PSD2, 325 Broadway, Boulder, CO 80305. Email: email@example.com
The summary wind-profiler statistics at the bottom of each table were calculated from their respective 20-case mean values rather than from the large number of wind-profiler soundings used to generate each composite profile in Fig. 8. Hence, the average Vmax magnitudes in the tables are larger than in the corresponding composite profiles.
Although the mean altitude of the high SBJs resided beneath the mean ∼2 km MSL altitude of the nearby Sierra Crest, the eight highest SBJ cases within this class were centered above the mean crest level. However, the altitudes of these highest SBJs were still less than the altitude of the Sierra Crest farther south where the SBJs likely formed.