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    (a) WRF nested modeling domain setup. The parent (outermost) domain horizontal grid dimensions are 301 × 301 grid points in west–east and south–north directions (15-km grid spacing in the horizontal direction). Two levels of nesting occur inside the parent domain. First, there is a domain with 151 × 151 grid points (5-km grid spacing). Nested in that is a domain with 201 × 201 grid points (1-km grid spacing) centered on the study area. Overlaid are the vertical cross sections A–B and C–D referenced in this study. (b) Representation of model topography (shaded; units in m) in the innermost domain (1-km grid) (source: United States Geological Survey; source: http://ned.usgs.gov). Overlaid are the west–east and north–south vertical cross sections E–F and G–H employed in this nest, and the stations BLU, TRK, TVL, CSN, and REV and the river basins referenced in this study. The Sierra Nevada and Carson mountain ranges are also indicated in the figure. State line (California–Nevada) is shown by the thick line. Dashed lines indicate the latitudes and longitudes. Also shown are the cross sections A–B and C–D within this domain.

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    Schematic of (a) polar jet streak depicting ageostrophic motions in the entrance and exit regions, (b) meso-β-scale ridging with total and ageostrophic full wind barbs at each end of the ridge, and (c) upper-level and midlevel isentropic planes over the Lake Tahoe region. The mesolow is located to the lee of the Carson Range. Splitting motions at the meso-β-scale ridge aloft indicate ageostrophic motions. (d) Location of mid- and upper-level jet streaks relative to California coastal range, Sierra Nevada Crest, and Carson Range (left to right).

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    Schematic of shallow water analog of Fig. 2 for the leeside wave structures.

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    Observed soundings at REV (air temperature = solid line; dewpoint temperature = dashed line; full barb = 5 m s−1) valid at (a) 0000 UTC 2 Jan 1997, (b) 0000 UTC 3 Jan 1997, (c) 0000 UTC 31 Dec 2005, and (d) 1200 UTC 31 Dec 2005 (source: http://weather.uwyo.edu/upperair/sounding.html). Solid horizontal line indicates 500-hPa pressure level on the diagram.

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    NARR-diagnosed 250-hPa geopotential height (contour interval = 120 m), isotach (shaded; m s−1), air temperature (dashed; contour interval = 5°C), and ageostrophic winds (full barb = 5 m s−1) valid at 1500 UTC and 1800 UTC (a),(b) 2 Jan 1997 and (c),(d) 31 Dec 2005 (source: http://nomads.ncdc.noaa.gov/). Locations of TRK, TVL, and REV are shown.

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    Time series of hourly precipitation amounts observed (solid) and simulated on 1-km grid (dashed) for REV during (top) 1200 UTC 30 Dec 2005–1200 UTC 1 Jan 2006 and (bottom) 0000 UTC 1 Jan–0000 UTC 3 Jan 1997.

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    WRF (5-km grid)-simulated vertical velocity (shaded; light gray = downward; dark gray = upward; arrows indicate rising and sinking motions—magnitudes in cm s−1 are indicated inside the arrows), ageostrophic wind vectors (normalized vector length), and equivalent potential temperature (long dashed; contour interval = 2.5 K) valid at 1500 UTC 31 Dec 2005 for (top) CTRL experiment and (bottom) NLH experiment along the cross section A–B (see Fig. 1). Location of REV is indicated.

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    (a) 1-km WRF-simulated spatial distribution of 36-h precipitation accumulations (mm; also shown in boxes) ending at 0000 UTC 1 Jan 2006 from (left) CTRL and (right) NLH experiments. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada). (Note: see appendix for the observed values at these stations.) (b) WRF-simulated spatial distribution at 1-km resolution of 48-h precipitation accumulations (mm; 1 in. = 25.4 mm; also shown in boxes) ending at 0000 UTC 3 Jan 1997 from (left) CTRL and (right) NLH experiments. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

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    WRF-simulated total hydrometeor mixing ratio (g kg−1) at 5-km resolution along the cross section C–D (shown in Fig. 1a) from (a) CTRL and (b) NLH simulations. Vertical motion is indicated by white arrows (magnitudes in cm s−1 are indicated inside the arrows), relative humidity (solid contours; contour interval = 10%), equivalent potential temperature (dashed contours; contour interval = 2 K), and horizontal winds (full barb = 5 m s−1) valid at 1500 UTC 31 Dec 2005. Location of REV is indicated in the figure.

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    WRF-diagnosed 250-hPa horizontal velocity divergence on 1-km grid (shading units in 10−5 s−1; positive and negative magnitudes are indicated in boxes), isolines of height (solid; contour interval = 120 m), and horizontal winds (full barb = 5 m s−1) at 1500 UTC 31 Dec 2005 for (a) CTRL simulation and (b) NLH simulation. Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

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    (a) WRF-simulated vertical velocity at 1-km resolution (shaded; cm s−1) along the cross section E–F shown in Fig. 1b (arrows indicate rising and sinking motions; high and low magnitudes are shown in italics), relative humidity (solid lines; contour interval = 5%), and equivalent potential temperature (dashed lines; contour interval = 1 K) for the CTRL simulation valid at 1500 UTC 31 Dec 2005. Locations of REV and the surface lows and highs are indicated in the figure. (b) As in (a), but for the NLH simulation.

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    WRF-simulated vertical velocity at 1-km resolution (cm s−1) along the cross section G–H (shown in Fig. 1b); arrows indicate rising and sinking motions; local maximum is indicated inside the arrows), relative humidity (>50%; solid lines; contour interval = 5%), and horizontal winds (full barb = 5 m s−1) valid at 1500 UTC 31 Dec 2005 for the (a) CTRL and (b) NLH experiments. Locations of REV and CSN are indicated.

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    WRF-diagnosed wind shear at 1-km resolution (full barb = 5 m s−1) and static stability (shaded; −K km−1; magnitudes are indicated in boxes) in the 700–800-hPa layer at 1500 UTC 31 Dec 2005 from (a) CTRL and (b) NLH simulations. Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

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    Lagrangian air parcel trajectories arriving at 500 hPa on 31 Dec 2005 over the stations REV (squares) and CSN (circles) diagnosed from the WRF 15-km grid (left) CTRL and (right) NLH simulations.

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    Time series of mean sea level pressure (hPa), air temperature (°C), relative humidity (RH; %), parcel acceleration (103 m s−2) along the Lagrangian air parcel back trajectory arriving at 500 hPa on 31 Dec 2005 1500 UTC over REV for (left) CTRL and (right) NLH simulations.

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    WRF-diagnosed absolute magnitudes of moisture fluxes at 1-km resolution (|ρVq|, where ρ is the density air, V is the horizontal velocity vector, and q is the water vapor mixing ratio) at 800 hPa (shaded; 102 kg m−2 s−1; local maxima are shown in boxes) and vectorial representation at 500 hPa (normalized vector lengths), valid at 1500 UTC 31 Dec 2005 from the (a) CTRL and (b) NLH simulations. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

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    WRF-simulated spatial distribution of 36-h precipitation accumulations at 1-km resolution (mm; 1 in. = 25.4 mm; also shown in boxes) ending at 0000 UTC 1 Jan 2006 using (a) Purdue Lin, (b) Morrison, (c) WSM6, and (d) GSFC microphysical schemes. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

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    Observed and diagnosed from 5-km WRF grid ending at 1600 UTC 24-h precipitation accumulations (mm) for (a) 30 Dec 2005 and (b) 2 Jan 1997 for different river basins (see also Fig. 1b) in Nevada and California.

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The Role of Windward-Side Diabatic Heating in Sierra Nevada Spillover Precipitation

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  • 1 Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada
  • | 2 Center for Climate Change Research, Indian Institute of Tropical Meteorology, Pune, India
  • | 3 Raytheon Polar Services Company, Centennial, Colorado
  • | 4 Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada, and NOAA/National Severe Storms Laboratory, Norman, Oklahoma
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Abstract

This study focuses on the meso-α- and meso-β-scale manifestations of the latent-heat-induced reduction of windward-side blocking to two flood-producing precipitation events on the leeside of the Sierra Nevada. Two simulations were performed—one employing full microphysics [control (CTRL)] and a second in which the latent heating terms are turned off in the microphysics [no latent heating (NLH)]. The differences between the CTRL and NLH are consistent with upstream latent heating—the moist, divergent, and ascending flow dominates the leeside of the mountain range in the CTRL producing copious spillover precipitation while dry high-momentum/downslope-descending flow dominates the NLH simulation on the leeside. A comprehensive sequence of events for spillover precipitation is formulated as follows: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet’s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow.

Corresponding author address: Dr. Michael L. Kaplan, Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512. E-mail: mike.kaplan@dri.edu

Abstract

This study focuses on the meso-α- and meso-β-scale manifestations of the latent-heat-induced reduction of windward-side blocking to two flood-producing precipitation events on the leeside of the Sierra Nevada. Two simulations were performed—one employing full microphysics [control (CTRL)] and a second in which the latent heating terms are turned off in the microphysics [no latent heating (NLH)]. The differences between the CTRL and NLH are consistent with upstream latent heating—the moist, divergent, and ascending flow dominates the leeside of the mountain range in the CTRL producing copious spillover precipitation while dry high-momentum/downslope-descending flow dominates the NLH simulation on the leeside. A comprehensive sequence of events for spillover precipitation is formulated as follows: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet’s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow.

Corresponding author address: Dr. Michael L. Kaplan, Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512. E-mail: mike.kaplan@dri.edu

1. Introduction

Earlier studies on the subject of extreme spillover precipitation events on the leeside of major mountain ranges broadly focused on three key physical processes—namely, 1) latent heating on the windward-side airflow over complex topography (Doyle and Shapiro 2000; Doyle and Smith 2003; Jiang 2003; Colle 2004; Galewsky and Sobel 2005; Zängl 2007), 2) barrier jets (Medina et al. 2005; Kim and Kang 2007; Reeves et al. 2008; Lundquist et al. 2010; Nieman et al. 2010), and 3) upstream static instability (Sinclair 1994, 1997; Chater and Sturman 1998; Dettinger et al. 2004; Wallmann and Milne 2007; Marzette 2008; Underwood et al. 2009; Kaplan et al. 2009; Milne et al. 2010). For brevity we will focus on elements 1 and 3 in this study because of their unambiguous mutual dependence.

A brief review of the aforementioned literature is presented here as follows. Two-dimensional idealized simulations carried out by Jiang (2003) showed that the latent heat release on the windward slope can facilitate the ascent of low-level air to greater height over mountains—that is, nearly twice the height of the dry air ascent. Further, the simulations indicated that warmer upstream surface air delayed flow splitting, stagnation, and gravity wave breaking. Similarly, the 2D idealized studies of Colle (2004) showed a very strong dependence of ambient wind speed with the mountain-wave structure (upstream stability) and spillover precipitation. Galewsky and Sobel (2005) and Zängl (2007) showed that the upstream latent heating had a profound effect on orographic blocking and downstream moisture/momentum transport and precipitation using a fully consistent initial dataset based on observations. Wallmann and Milne (2007) and Milne et al. (2010) used operational products to show that destabilization of the mid- and upper-level upstream flow fields favor spillover precipitation.

Several studies (e.g., Sinclair 1994; Sinclair et al. 1997; Chater and Sturman 1998) have identified the nondimensional mountain height number (M) as an objective measure of the likelihood of precipitation in and around a mountain barrier. It is given by
e1
where Nm (the moist Brunt–Väisälä frequency) is a measure of stability (Durran and Klemp 1982; Barcilon and Fitzjarald 1985), H is the height of the mountain barrier, and is the mean upstream wind speed. These studies demonstrated significant correlations between upstream static stability, ambient high wind velocities, available moisture, and leeside precipitation. Marzette (2008) provided evidence that significant reduction of M results in heavy leeside spillover precipitation in the Sierra Nevada (location shown in Fig. 1). Underwood et al. (2009) and Kaplan et al. (2009) (collectively referred to here as UNK) highlighted the pan-Pacific synoptic-scale processes over a multiday period that precedes the spillover precipitation events on the leeside of the Sierra Nevada. Through climatological composites and numerical simulations they showed that unstable and high-momentum flows upstream and over the Sierra Nevada enhance the likelihood of flood-producing precipitation on the leeside.
Fig. 1.
Fig. 1.

(a) WRF nested modeling domain setup. The parent (outermost) domain horizontal grid dimensions are 301 × 301 grid points in west–east and south–north directions (15-km grid spacing in the horizontal direction). Two levels of nesting occur inside the parent domain. First, there is a domain with 151 × 151 grid points (5-km grid spacing). Nested in that is a domain with 201 × 201 grid points (1-km grid spacing) centered on the study area. Overlaid are the vertical cross sections A–B and C–D referenced in this study. (b) Representation of model topography (shaded; units in m) in the innermost domain (1-km grid) (source: United States Geological Survey; source: http://ned.usgs.gov). Overlaid are the west–east and north–south vertical cross sections E–F and G–H employed in this nest, and the stations BLU, TRK, TVL, CSN, and REV and the river basins referenced in this study. The Sierra Nevada and Carson mountain ranges are also indicated in the figure. State line (California–Nevada) is shown by the thick line. Dashed lines indicate the latitudes and longitudes. Also shown are the cross sections A–B and C–D within this domain.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Given the host of studies, one could anticipate that a strong coupling between the distribution of moist convection over the mountain barrier and mesoscale adjustments can facilitate strong leeside vertical motions and cross-mountain moisture fluxes. However, none of these aforementioned studies addressed the finescale perturbations in pressure and moisture on the leeside due to the effect of complex terrain-induced circulations as well as the mesoscale coupling of the mass, momentum, and diabatic heating. We intend to provide a more detailed understanding of the multiscale links among these phenomena. This is carried out by examining the hypothesis that the interactions between the large-scale upper-level jet and windward-side latent heating produce an important mountain-wave structure that facilitates leeside spillover precipitation. The hypothesis is focused on the evolving mass flux profile across the mountain barrier that favors enhanced downstream moisture fluxes with the spillover precipitation episodes.

In particular we will investigate the linkages between large-scale jet dynamics, mesoscale jets, and pressure perturbations. An example of this can be seen in the conceptual diagrams in Figs. 2 and 3. Shown in Fig. 2 are the hypothesized fundamental jet streak adjustments starting with the indirect circulation and ending with the convectively induced midlevel jet streak. Shown in Fig. 3 are the conceptualized cross-mountain jet structures with leeside wave dynamics, including the variation of a hypothetical free surface over the mountain barrier and its implication for precipitation. As will be discussed later, the simulated upstream diabatic heating is seen to have a radical effect on the structure of the free surface flowing over the complex terrain in accordance with shallow water theory. The mesoscale details in this study are gleaned from high-resolution numerical simulations with the Weather Research and Forecasting Model (WRF; Skamarock et al. 2008).

Fig. 2.
Fig. 2.

Schematic of (a) polar jet streak depicting ageostrophic motions in the entrance and exit regions, (b) meso-β-scale ridging with total and ageostrophic full wind barbs at each end of the ridge, and (c) upper-level and midlevel isentropic planes over the Lake Tahoe region. The mesolow is located to the lee of the Carson Range. Splitting motions at the meso-β-scale ridge aloft indicate ageostrophic motions. (d) Location of mid- and upper-level jet streaks relative to California coastal range, Sierra Nevada Crest, and Carson Range (left to right).

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Fig. 3.
Fig. 3.

Schematic of shallow water analog of Fig. 2 for the leeside wave structures.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Although there have been only 14 intense leeside precipitation events documented over the Sierra Nevada between 1950 and 2007, it is noteworthy to mention that all of them uniquely occurred in conjunction with strong jet streaks and baroclinic zones (Underwood et al. 2009). This study expands upon the analysis of UNK with a focus on the finer-scale circulations during two episodes of heavy precipitation within the Truckee River basin located on the leeside of the Sierra Nevada (Fig. 1). The two extreme precipitation events we examine occurred during 1–3 January 1997 and 30 December 2005–1 January 2006 with emphasis on the latter event because of availability of a more complete set of observations for this event.

Section 2 presents an observational analysis of the events. Section 3 provides an overview of the numerical model setup and experiments. The similarities and differences in model results for the two events including a forecast paradigm are presented in section 4. Summary and conclusions are found in section 5.

2. Observational analyses

a. Case studies

We focus on two heavy precipitation events (1–3 January 1997 and 30 December 2005–1 January 2006). These events brought copious amounts of rain that led to flood situations on the leeside of the Sierra Nevada in northwestern Nevada (Rigby et al. 1998; O’Hara et al. 2007). In both of these case studies, the most extreme precipitation occurred over the leeside of the Sierra Nevada when the final trough in a sequence of 4–5 cyclone-scale waves crossed the entire Pacific Ocean basin (cf. Kaplan et al. 2009, their Figs. 5 and 6). The trough and ridge patterns of each wave took approximately 4–5 days to cross the Pacific Ocean. The heaviest precipitation occurred when the final trough became embedded in the exit region of a zonal polar jet streak that spanned most of the Pacific Ocean basin. As the exit region of this polar jet approached the Pacific coast, plumes of midlevel moisture, generated by mesoscale convective systems within the jet exit region northeast of the Hawaiian Islands, arrived at the upslope of the Sierra Nevada. These plumes (midlevel rivers) remained reasonably intact as they rose above the coastal mountain ranges of northern California.

b. Precipitation

Annual precipitation in the Truckee River drainage basin on the leeside of the Carson Range (shown in Fig. 1b) is 150–200 mm. On the windward side it is 1500–1800 mm (Pratt 1997). For both events, precipitation accumulations for a period of 36–48 h were 200–350 mm (25–150 mm) on the immediate windward (lee) slope of the Sierra Nevada. Thus, a large fraction of the annual precipitation at stations such as the one at Reno, Nevada (REV), fell in less than 2 days. The flooding in the REV area caused damages in excess of $10 million (U.S. dollars) for each event and presented major public safety hazards, as might have been expected (NCDC 1997, 2005).

The 1997 event is a historic flood event associated with widespread precipitation over the Sierras and much of the U.S. West Coast. The 6-day precipitation accumulation during 29 December 1996–3 January 1997 at Blue Canyon (BLU; location shown in Fig. 1b), a windward-side station in the northern Sierra Nevada, was 845 mm. More than 50% of annual precipitation at BLU was recorded during the 0000 UTC 1 January–0000 UTC 3 January 1997 period. Truckee (TRK) and Tahoe City, California, in the Truckee River basin each recorded about 300 mm on the lee side of the Sierra Nevada. During this time there were three occurrences of heavier precipitation rates at REV: 1) 1500 UTC 1 January, 2) 0300 UTC 2 January, and 3) 1500 UTC 2 January 1997. The study will focus on the last time period since it is associated with the largest precipitation event. The hourly measurements showed maximum rain gauge readings of 1.3 mm for REV and 7.9 mm for BLU at 1500 UTC 2 January 1997. The 48-h rain gauge totals for REV, TRK, and BLU ending at 0000 UTC 3 January 1997 were 30.7, 118.1, and 455.2 mm, respectively.

Based on rain gauge data, REV had two occurrences of intense precipitation rates during the 2005 event. The first was short lived, extending over a 3-h period beginning at 0500 UTC 31 December 2005. It was associated with the arrival of a very moist midlevel plume. This was followed by a second long-lived event during 1000–2000 UTC 31 December 2005 with the arrival of the polar jet’s exit region and trough. On the lee slope of the Sierra Nevada and the Carson Range (see Fig. 1b for the geographical location), the maximum hourly precipitation amounts at 1500 UTC 31 December 2005 were REV 6.6 mm and south of REV 13.8 mm, and Carson City (CSN) 9 mm. Significant amounts occurred in the Truckee River basin, particularly over the lee slope of the Carson Range; that is, 48-h precipitation accumulations ending at 1200 UTC 1 January 2006 were 99.3 mm for TRK, 54.6 mm for a location north of REV (39.6°N, 119.8°W), 98.4 mm for a location south of REV (39.4°N, 119.8°W), 142.8 mm at CSN, and 111.2 mm at South Lake Tahoe (TVL). It should be emphasized that about 20% of annual precipitation fell at these locations during this period. On the windward slope of the Sierra Nevada, BLU recorded 214 mm for the same period. These precipitation amounts for both case studies indicate that the extreme precipitation was not isolated but widespread in space and time.

c. Soundings

Figure 4 shows the observed REV soundings at 0000 UTC 2–3 January 1997 and at 0000 UTC and 1200 UTC 31 December 2005. One can see the following key features from the soundings for the 1997 event: 1) the development of the low-level southerly (mountain parallel) flow below 800 hPa under the midlevel west–southwesterly jet, 2) substantial (2°–4°C) cooling in the 700–500-hPa layer between 2 and 3 January, 3) moistening within that same layer sustaining a near-moist-neutral lapse rate, and 4) a nearly uniform west–southwesterly jet between 800 and 450 hPa. The thermodynamic structure of the 2005 event has signatures similar to the 1997 event. The sharp cooling in time in the sounding at 1200 UTC 31 December 2005 follows a surge a warm air at midlevels that had arrived at 0000 UTC 31 December 2005.

Fig. 4.
Fig. 4.

Observed soundings at REV (air temperature = solid line; dewpoint temperature = dashed line; full barb = 5 m s−1) valid at (a) 0000 UTC 2 Jan 1997, (b) 0000 UTC 3 Jan 1997, (c) 0000 UTC 31 Dec 2005, and (d) 1200 UTC 31 Dec 2005 (source: http://weather.uwyo.edu/upperair/sounding.html). Solid horizontal line indicates 500-hPa pressure level on the diagram.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

d. Synoptic analyses

In the 1997 case, the North American Regional Reanalysis (NARR; Mesinger et al. 2006) analyzed upper-level jet at 250 hPa is oriented northeast–southwest with the exit region over the Sierra Nevada and an offshore trough at 1500 UTC 2 January 1997 (Figs. 5a,b). A weak secondary jet is located to the northeast of the Sierra Nevada over the northern Great Plains. The ageostrophic wind vectors are directed across the jet toward the right side of the jet, looking downstream over most of California and Nevada. By 1800 UTC 2 January 1997 the pattern of ageostrophy has not changed much but the exit region has continued eastward into the Great Basin. Figures 5c,d show the NARR analyzed mass and momentum fields on 250 hPa for the 2005 case at 1500 and 1800 UTC 31 December 2005. The upper-level jet exit region approaches the California coast at 1500 UTC 31 December 2005 with a west–northwest to east–southeast orientation. A secondary wind maximum is located northeast of REV and to the east of the trough centered over southeastern Oregon. Ageostrophic wind vectors extending across the exit region of the main jet streak (Fig. 5c) over the southeast Pacific and part of the central California–Nevada region have a strong southeastward cross-isoheight orientation. By 1800 UTC 31 December 2005 (Fig. 5d), the rightward-directed and northerly ageostrophic flow strengthens over the region south and east of REV.

Fig. 5.
Fig. 5.

NARR-diagnosed 250-hPa geopotential height (contour interval = 120 m), isotach (shaded; m s−1), air temperature (dashed; contour interval = 5°C), and ageostrophic winds (full barb = 5 m s−1) valid at 1500 UTC and 1800 UTC (a),(b) 2 Jan 1997 and (c),(d) 31 Dec 2005 (source: http://nomads.ncdc.noaa.gov/). Locations of TRK, TVL, and REV are shown.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

In both case studies, one can notice that the 250-hPa jet exhibits differing flow regimes in its left and right exit regions particularly over Oregon (north of California and Nevada). The ageostrophic flow in the left exit region is subgeostrophic and has a component directed both upstream and to the left of the geostrophic wind. This enhances the meso-α-scale velocity divergence aloft between the leftward-directed ageostrophic flow to the north and rightward-directed flow to the south. There is an indirect circulation associated with the polar jet’s exit region. The mean sea level pressure trough beneath this indirect circulation is stronger in the 2005 case than the 1997 case. The 2005 case exhibits an embedded subsynoptic low during the period 1200–1800 UTC 31 December 2005 (Figures not shown). During this period, the surface observations at TVL and REV exhibited the largest drop in mean sea level pressure at any time and any location for the case study. These drops exceeded 7 hPa in 6 h (not shown). Based on these observations, the subjectively analyzed subsynoptic or mesoscale trough would be located within a triangular region bounded by the locations of TVL, REV, and TRK (see Fig. 1b for the geographical locations). Also apparent in both case studies are the west–southwesterly 700-hPa wind flow regimes perpendicular to the mountain barrier extending across the Sierra Nevada.

To summarize, both case studies have similarities in the hydrological, thermodynamical, and hydrodynamical features seen from the soundings, NARR analyses, and surface observations. They include 1) a broad mesoscale convective system extending from the windward side to the leeside of the Sierra Nevada during the period of heaviest leeside precipitation, 2) a strong primary jet streak whose exit region is approaching the Sierra Nevada in proximity to an upstream trough with an indication of a secondary downstream wind maximum (i.e., another jet streak downstream separate from the main jet streak), 3) significant rightward-directed cross-jet ageostrophic wind flow aloft on the south side of the jet exit region indicating a transverse thermally indirect circulation in the exit region looking downstream, 4) a leeside trough and midtropospheric cross-mountain flow just above it, 5) a near-surface flow parallel to the mountain barrier, and 6) nearly saturated moist neutral midtropospheric lapse rates with deep west–southwesterly flow extending to the leeside of the Sierra Nevada.

3. WRF setup and validation

a. WRF setup

High-resolution numerical simulations are conducted using the mass core version of the WRF (Skamarock et al. 2008). The WRF is built over a parent domain (15-km horizontal grid spacing) and two levels of nesting (grid sizes of 5 and 1 km in the horizontal) occur inside the parent domain as shown in Fig. 1. The interactive strategy between the model domains is one way, in which the larger-scale grid provides the lateral boundary conditions for the smaller-scale grid. The Truckee River and Carson River basins (Fig. 1b) are the focal target area in this study, which encompasses the surface and upper-air stations REV and CSN on the Nevada side and TRK and TVL on the California side, respectively. Four cross sections (A–B and C–D on the 5-km grid and E–F and G–H on the 1-km grid), shown in Fig. 1, are employed in this study to diagnose the vertical structure of the flow fields along and across the mountain ranges. The model configuration has 47 levels in the vertical extending up to 15 km AGL; 18 vertical levels were below 1.5 km AGL with the lowest model level set at 10 m AGL. A time step of 30 s was used for the parent domain with a reduction in the time steps for the finer-scale domains in accordance with the parent-to-nest gridsize ratio. Two numerical experiments are conducted for each case study: 1) control (CTRL; full microphysics) and 2) no latent heating (NLH) simulation (microphysics in which the latent heating terms are turned off).

The model physics in the CTRL and NLH experiments include the following: 1) momentum and heat fluxes at the surface are computed using the Eta surface layer scheme (Janjić 1996, 2001) following the Monin–Obukhov similarity theory; 2) turbulence parameterization is formulated following the Mellor–Yamada–Janjić 1.5 order closure model (Mellor and Yamada 1974, 1982; Janjić 2001); 3) convective processes are represented by the Betts–Miller–Janjić cumulus scheme (only on the parent domain; Betts 1986; Betts and Miller 1986; Janjić 1994); 4) the land surface processes are represented following the Noah land surface model (Noah LSM), which provides the surface sensible, latent heat fluxes, and upward longwave and shortwave fluxes to the atmospheric model (Chen and Dudhia 2001; Ek et al. 2003); and 5) radiative processes are parameterized using the Rapid Radiative Transfer Model for longwave radiation (Mlawer et al. 1997) and Dudhia’s shortwave scheme (Dudhia 1989).

Cloud microphysical processes are parameterized using the Thompson scheme (in CTRL; Thompson et al. 2004, 2008), while the heating and cooling effect of condensation and evaporation as well as freezing and melting in the Thompson scheme is turned off for the NLH experiment. That is, with the exception of these microphysical processes, all other physical parameterizations in the NLH experiment are the same as used in the CTRL simulation.

In addition, four other WRF microphysical schemes are tested for sensitivity to precipitation amounts on the leeside of the Sierra Nevada. They are Purdue Lin microphysics (Lin et al. 1983), WRF Single-Moment 6-Class microphysics (WSM6; Hong and Lim 2006), National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) microphysics (Tao et al. 1989, 2003; Shi and Tao 2006), and the Morrison scheme (Morrison et al. 2005, 2009). The basic representation of mixed phase microphysics in WRF microphysical schemes follows the earlier work of Lin et al. (1983), Rutledge and Hobbs (1983, 1984), and Reisner et al. (1998).

Three-hourly NARR datasets were used for initialization and lateral boundary conditions in the simulations. The WRF is initialized at 0000 UTC 1 January 1997 (1200 UTC 30 December 2005) for the first (second) case.

b. Precipitation verification

Figure 6 shows the observed, CTRL-, and NLH-simulated time series of hourly precipitation amounts at REV for both cases. The simulated timing of the spillover precipitation burst is in good agreement with the observations within about 3 h, while the leeside spillover precipitation is absent in the NLH experiment. The sensitivity examination of precipitation accumulations using different WRF microphysical schemes is briefly highlighted in the appendix.

Fig. 6.
Fig. 6.

Time series of hourly precipitation amounts observed (solid) and simulated on 1-km grid (dashed) for REV during (top) 1200 UTC 30 Dec 2005–1200 UTC 1 Jan 2006 and (bottom) 0000 UTC 1 Jan–0000 UTC 3 Jan 1997.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

4. Mesoscale diagnoses: CTRL and NLH simulations

In this section we present the diagnosis from the WRF simulations addressing the meso-α-, meso-β-, and meso-γ-scale interactions resulting in spillover precipitation episodes on the leeside of the Sierra Nevada. The preponderance of the analyses is intercomparing the CTRL (microphysics with latent heating) and NLH (microphysics with no latent heating) simulations for the 2005 case study, while the findings from the 1997 event are briefly addressed in this section.

a. Background indirect jet streak circulations

Figure 7 shows the vertical motion, ageostrophic winds, and equivalent potential temperature (θe) from the CTRL and NLH experiments along a vertical cross section A–B (oriented northwest–southeast shown in Fig. 1) at 1500 UTC 31 December 2005. This is a period of significant spillover precipitation on the Sierra Nevada leeside. The NLH simulation showed two strong signals of sinking, one south–southeast and the other north–northwest of REV. There is rightward (south–southeastward) ageostrophic flow closely aligned with both of these sinking cells. The large scale and location of these features relative to the jet’s exit region indicate a circulation that is thermally indirect, approximately 700 km across, and mountain-parallel consistent with a background quasigeostrophic (QG) signal. The CTRL simulation showed a strong ascending cell over REV and the rightward-directed ageostrophic vectors are largely absent. This clearly indicates that the effect of latent heating modulates the sense of the rightward-directed ageostrophy and mass convergence near REV to upstream/leftward-directed and divergence, respectively. Near-neutral moist stability near REV is most evident accompanying the cold front between 600 and 800 hPa in the NLH simulation in both cases. Additionally, the NLH is much drier with lower θe values as compared to the CTRL; the location of the mountain wave is shifted downstream with a stronger descending cell and low-level (near surface) wind flow.

Fig. 7.
Fig. 7.

WRF (5-km grid)-simulated vertical velocity (shaded; light gray = downward; dark gray = upward; arrows indicate rising and sinking motions—magnitudes in cm s−1 are indicated inside the arrows), ageostrophic wind vectors (normalized vector length), and equivalent potential temperature (long dashed; contour interval = 2.5 K) valid at 1500 UTC 31 Dec 2005 for (top) CTRL experiment and (bottom) NLH experiment along the cross section A–B (see Fig. 1). Location of REV is indicated.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

b. Windward slope diabatic forcing

Figures 8a,b show the 36–48-h precipitation accumulations from the CTRL and NLH simulations for the 1997 and 2005 events. The CTRL simulation showed a strip of spillover precipitation that matched the observations (cf. Kaplan et al. 2009, their Fig. 1) in contrast with the NLH-simulated precipitation amounts. The CTRL spillover regime and precipitation amounts at REV and CSN were larger for the 2005 event (left panel in Fig. 8a) compared to the 1997 event (Fig. 8b). The upslope maxima in the precipitation distribution for the 1997 case in the CTRL (Fig. 8b) is more organized with larger amounts than the filamentary structure seen for the 2005 event on the windward side (Fig. 8a).

Fig. 8.
Fig. 8.

(a) 1-km WRF-simulated spatial distribution of 36-h precipitation accumulations (mm; also shown in boxes) ending at 0000 UTC 1 Jan 2006 from (left) CTRL and (right) NLH experiments. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada). (Note: see appendix for the observed values at these stations.) (b) WRF-simulated spatial distribution at 1-km resolution of 48-h precipitation accumulations (mm; 1 in. = 25.4 mm; also shown in boxes) ending at 0000 UTC 3 Jan 1997 from (left) CTRL and (right) NLH experiments. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Figure 9 sheds light on the windward slope adjustment processes and their interaction with the jet exit region for the 2005 case study. Shown in this figure is the total hydrometeor mixing ratio, relative humidity, and θe from the CTRL and NLH simulations along the west–east-oriented cross section C–D (Fig. 1). The CTRL simulation distinctly indicates the formation of a strong plume of hydrometeors with markedly different moist stability on the windward slope of the Sierra Nevada relative to the NLH simulation. The plume in the CTRL is much larger in magnitude on the upslope side of the mountains and it extends farther over the leeside than the plume simulated in the NLH. It propagates downstream in time and weakens as it exits the leeside ascent plume.

Fig. 9.
Fig. 9.

WRF-simulated total hydrometeor mixing ratio (g kg−1) at 5-km resolution along the cross section C–D (shown in Fig. 1a) from (a) CTRL and (b) NLH simulations. Vertical motion is indicated by white arrows (magnitudes in cm s−1 are indicated inside the arrows), relative humidity (solid contours; contour interval = 10%), equivalent potential temperature (dashed contours; contour interval = 2 K), and horizontal winds (full barb = 5 m s−1) valid at 1500 UTC 31 Dec 2005. Location of REV is indicated in the figure.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Clearly condensation is much larger in magnitude in the CTRL and is focused in the upslope region. Approximately 125 km west of REV, the lapse rate within the 650–400-hPa layer is 1.2 × 10−4 K Pa−1 (2.4 × 10−4 K Pa−1) for the CTRL (NLH) simulation. The ratio is much larger in the NLH simulation when these lapse rates are combined with the weaker cross-mountain flow () in the midtroposphere. This is more favorable for blocking. As will be shown later, however, relying on the blocking calculation alone does not fully describe the mesoscale details that separate the CTRL from the NLH simulations. The effect of upstream latent heating is to modify the downstream mass fluxes due to convection, which produce results consistent with the blocking effects. A diabatic outflow jet in proximity to the reduced static stability forces mass downstream. This process is, however, consistent with the reduced blocking signal accompanying reduced static stability. Thus, the mesoscale details of spillover precipitation are clearly much more complicated than the implications from the nondimensional mountain height [Eq. (1)] alone.

Figure 10 shows horizontal winds, divergence, and isoheight structure on the 250-hPa surface over the region of interest at 1500 UTC 31 December 2005 from the CTRL and NLH simulations. One can notice that the difference between the divergence, wind flow, and isoheights in the CTRL and NLH simulations is substantial. The 250-hPa winds in the jet exit region vary little in magnitude from the windward to the leeside locations in the CTRL. In the NLH the flow decelerates across the mountains, inducing weak convergence over the leeside. The divergence fields reflect this in the NLH simulation for both case studies as they exhibit significant perpendicular wave structures to the polar jet core oriented northwest–southeast (approximately 40-km horizontal wavelength). This indicates a dominance of gravity waves normal to the jet in the NLH simulations relative to the CTRL simulations. The difference in divergence and height patterns likely reflects the dominance of convective outflow in the CTRL and gravity waves in the NLH. That is, the divergence is more widespread over the leeside in the CTRL while gravity waves are more dominant in the NLH. As will be seen later, this contention of gravity waves in the NLH is supported with quadrature signals in the vertical motion fields (note Fig. 11b). Thus, the CTRL simulation produced a broad area of divergence aloft between TRK and REV northward and convergence between TRK and CSN aligned with the exit region core, while the NLH generates a predominantly gravity wave train signal transverse to the jet with weaker downstream divergence and more convergence at scales of motion larger than gravity waves. To summarize, a more dominant jet transverse circulation in the CTRL favors a persistent area of precipitation over and east of the region near REV while a dominant blocking regime seen in the NLH is not favorable for prolonged local ascent.

Fig. 10.
Fig. 10.

WRF-diagnosed 250-hPa horizontal velocity divergence on 1-km grid (shading units in 10−5 s−1; positive and negative magnitudes are indicated in boxes), isolines of height (solid; contour interval = 120 m), and horizontal winds (full barb = 5 m s−1) at 1500 UTC 31 Dec 2005 for (a) CTRL simulation and (b) NLH simulation. Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

The diagnosed 500–250-hPa layer thickness field during 1200–1800 UTC 31 December 2005 (not shown) in the CTRL and NLH simulations showed that the latent heating in the CTRL results in a substantial increase in thickness upstream of the Sierra Nevada. It can be contrasted in this region with a ridge (trough) aloft in the CTRL (NLH) simulation upstream of the Carson Range (see Fig. 1 for the location)—that is, a ridge in the CTRL and trough in the NLH west of the Carson Range but over the windward side of the Sierra Nevada. This favors outflowing convectively perturbed airflow at 250 hPa as well as midlevel flow around the mountains in the CTRL relative to the NLH simulation that showed rectilinear flow aloft, weaker cross-mountain flow, and gravity wave features. The CTRL wind patterns are consistent with a convectively generated outflow jet (e.g., Hamilton et al. 1998).

c. Leeside circulations

Figures 11a,b show the vertical motion, relative humidity, and θe along the cross section E–F (see Fig. 1b) at 1500 UTC 31 December 2005 from the CTRL and NLH simulations. One can clearly see that the ascending motion in the CTRL exceeds 150 cm s−1 within the moist isentropic trough near REV. Notice, in particular, the substantial differences in leeside relative humidity, as the CTRL is much wetter near REV than seen in the NLH—that is, a substantially deeper moist layer consistent with larger θe values can be seen along most of the cross section in the CTRL as compared to the NLH. The entire mountain-wave structure in the CTRL is substantially shifted relative to the NLH with dominant descent in the NLH near REV. The NLH simulation showed much larger-scale structure in the θe perturbation over the high terrain.

Fig. 11.
Fig. 11.

(a) WRF-simulated vertical velocity at 1-km resolution (shaded; cm s−1) along the cross section E–F shown in Fig. 1b (arrows indicate rising and sinking motions; high and low magnitudes are shown in italics), relative humidity (solid lines; contour interval = 5%), and equivalent potential temperature (dashed lines; contour interval = 1 K) for the CTRL simulation valid at 1500 UTC 31 Dec 2005. Locations of REV and the surface lows and highs are indicated in the figure. (b) As in (a), but for the NLH simulation.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Figure 12 shows the horizontal winds, relative humidity, and vertical motion along the south–north cross section G–H (Fig. 1b) oriented parallel to the Carson Range. Consistent with Fig. 9, notice that the near-surface winds in the NLH are stronger while the winds above 700 hPa in the CTRL are stronger and more zonal. Figures 13a,b show the vertical shear and static stability at 1500 UTC 31 December 2005 for the CTRL and NLH simulations. The CTRL simulation clearly showed stronger wind shear and lower static stability in the lower troposphere on the leeside than seen in the NLH as if there is stronger downward momentum flux in the NLH simulation. The dominant leeside moist isentropic trough in the CTRL is located near the 50-km location west–southwest of REV within the region of the lowest mean sea level pressure and the strongest westerly wind shear in the boundary layer (Fig. 11a). Also one can see a stronger ascent plume in the CTRL above this leeside trough and the turning of the flow as well as a high relative humidity plume above the leeside trough (Figs. 11a and 12a). This indicates a coupling among 250-hPa divergence, westerly midlevel flow, south–southwesterly near-surface flow, low static stability, saturation, and low mean sea level pressure is evidently seen in the CTRL as opposed to the NLH. It may also reflect an entirely different mountain-wave structure in which the flow in the CTRL is more accurately defined by a subcritical flow regime as opposed to supercritical flow regime in the NLH (e.g. Lin 2007, his Fig. 3).

Fig. 12.
Fig. 12.

WRF-simulated vertical velocity at 1-km resolution (cm s−1) along the cross section G–H (shown in Fig. 1b); arrows indicate rising and sinking motions; local maximum is indicated inside the arrows), relative humidity (>50%; solid lines; contour interval = 5%), and horizontal winds (full barb = 5 m s−1) valid at 1500 UTC 31 Dec 2005 for the (a) CTRL and (b) NLH experiments. Locations of REV and CSN are indicated.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Fig. 13.
Fig. 13.

WRF-diagnosed wind shear at 1-km resolution (full barb = 5 m s−1) and static stability (shaded; −K km−1; magnitudes are indicated in boxes) in the 700–800-hPa layer at 1500 UTC 31 Dec 2005 from (a) CTRL and (b) NLH simulations. Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Consistent with these signals are the back-trajectory calculations shown in Figs. 14 and 15. The Lagrangian back trajectories arriving at REV at 500 hPa in the CTRL simulation exhibit more intense lifting on the leeside than seen in the NLH during the period 1300–1500 UTC 31 December 2005 with parcel height rises (falls) in the CTRL (NLH). The vertical parcel motion in the CTRL at REV is much stronger (40–65 hPa h−1) on the windward side of the Sierra Nevada during the period 1000–1300 UTC 31 December 2005 with the greatest three-dimensional parcel acceleration seen for REV at about 1200 UTC and during 1400–1500 UTC. Further, the sea level pressure falls at the parcel locations were substantial during this period. This indicates the strength of the ascending circulations on the warm leeside of the system accompanying the net latent heating in the column as can be inferred from Fig. 15. This increased ascent is analogous to strong diabatic isallobaric ageostrophic flow with a midlevel convective jet in the CTRL as opposed to the NLH, where the parcel traces out an anticyclonic arc accompanying the mass displacement (Wolf and Johnson 1995; Hamilton et al. 1998; Kaplan et al. 1998).

Fig. 14.
Fig. 14.

Lagrangian air parcel trajectories arriving at 500 hPa on 31 Dec 2005 over the stations REV (squares) and CSN (circles) diagnosed from the WRF 15-km grid (left) CTRL and (right) NLH simulations.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Fig. 15.
Fig. 15.

Time series of mean sea level pressure (hPa), air temperature (°C), relative humidity (RH; %), parcel acceleration (103 m s−2) along the Lagrangian air parcel back trajectory arriving at 500 hPa on 31 Dec 2005 1500 UTC over REV for (left) CTRL and (right) NLH simulations.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Figure 16 shows the moisture flux patterns in the lower troposphere at 1500 UTC 31 December 2005 for both the CTRL and NLH simulations. Most evident are the larger magnitudes of fluxes for the CTRL simulation, particularly early in the period at 1200 UTC (not shown). The downstream moisture flux maxima at 800 hPa in the CTRL experiment at 1500 UTC are larger in the region of importance for the Truckee River watershed between TRK and REV. In the NLH, the downstream maxima features are shifted westward over TVL and the immediate leeside of the mountains. Thus, the CTRL showed a more realistic and effective transport of water vapor over and downstream from the mountains, enhancing the moisture availability for spillover. The surge of momentum between 800 and 500 hPa most evident in Fig. 12 and inferred from the vertical shear in Fig. 13 is analogous to the midlevel diabatically forced jetlet as described in Kaplan et al. (1998, 2009). The larger θe magnitudes in the CTRL simulation versus the NLH in Fig. 11 support this increase in midtropospheric cross-mountain moisture fluxes and leeside lifting. A summary of the analyzed mesoscale structures seen in CTRL and NLH simulations is listed in Table 1.

Fig. 16.
Fig. 16.

WRF-diagnosed absolute magnitudes of moisture fluxes at 1-km resolution (|ρVq|, where ρ is the density air, V is the horizontal velocity vector, and q is the water vapor mixing ratio) at 800 hPa (shaded; 102 kg m−2 s−1; local maxima are shown in boxes) and vectorial representation at 500 hPa (normalized vector lengths), valid at 1500 UTC 31 Dec 2005 from the (a) CTRL and (b) NLH simulations. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Table 1.

Summary of the multiscale interactions from CTRL and NLH experiments.

Table 1.

d. Similarities and differences in the 1997 and 2005 case studies

Both cases contained a strong jet streak that affected the Sierra Nevada with an exit region indirect ageostrophic circulation, a strong midlevel cross-mountain jet, unseasonably warm temperatures, midlevel moisture plume, low-level mountain-parallel jet, and upstream condensation with generally reduced static stability. While there are many similarities in the 2005 and the 1997 case studies to the effects of upstream/windward-side latent heat release, there are some subtle differences in the synoptic background. The events differed in the period of strong west–southwesterly midlevel flow; that is, the period in the 1997 case is longer than in the 2005 case (see also Fig. 5). The jet in the 2005 case has more curvature, and the midtropospheric static stability is lower than the 1997 case. The 1997 case exhibits stronger cross-mountain flow and larger moisture amounts in the midtroposphere. The simulated fields indicate that these environmental differences result in a leeside upward vertical motion plume in the 1997 case more upstream of the 2005 case, which is consistent with lower static stability in the 2005 case. This is also reflected in the deeper trough aloft in the 2005 case with a stronger leeside pressure fall signal than in the 1997 case.

Furthermore, the cross-mountain moisture flux is greater for the 1997 case, as the airflow is stronger, and the midtroposphere is warmer with higher dewpoint temperatures than in the 2005 case. These stability and transport differences account for the key overarching signals in the observed and simulated precipitation differences between the 2005 and 1997 cases with 1) more upslope precipitation and immediate leeside spillover precipitation in the 1997 case, 2) somewhat farther northward displacement in the spillover precipitation in the 1997 case, and 3) farther downstream displacement of spillover precipitation in the 2005 case (Figs. 8a and 8b). The lower static stability tends to displace the precipitation downstream in the 2005 case more substantially than in the 1997 case over the western high plateau of Nevada. Thus, the increased moisture flux across the Sierra Nevada in the 1997 case reflects the stronger midlevel jet and moisture plume above 750 hPa. The downstream displaced lifting and increased relative humidity due to adiabatic cooling is more significant in the 2005 case. Both case studies respond to the hypothesis in a similar way concerning the effect of windward-side latent heat release on leeside spillover precipitation (Figs. 8, 13, 14, and 16).

e. Summary of key processes—Forecast paradigm

Based on the results from CTRL and NLH simulations for both case studies, it is possible to define a sequence of events that constitute a forecast paradigm for heavy spillover precipitation. Early in the process, there is the arrival of an upper-level polar jet over the Sierra Nevada with its exit region associated with a rightward-directed cross-stream ageostrophic flow over the entire Sierra Nevada (Figs. 2 and 5). These features create a favorable environment for velocity divergence aloft—primarily but not exclusively on the left side of the jet exit region. This divergence pattern is consistent with a large-scale transverse thermally indirect secondary circulation. The development of upstream precipitation and associated diabatic heating acts to change the downstream (leeside) circulation. The polar jet becomes more anticyclonically curved aloft in response to upstream latent heating and coherently builds a ridge on the windward side in response to the upstream heating (Fig. 10). A secondary jet develops at midlevels to the southeast of the upper-level anticyclonic outflow. This moisture-rich flow propagates over the mountain crest and leads to substantial horizontal moisture fluxes over to the leeside precipitation zone. This is a diabatically modified or split jet field (absent in NLH simulations) (Figs. 13 and 16). A train of transient internal gravity waves develop in the NLH simulation that are normal to the polar jet (Fig. 10) with little signal of the leeside midlevel jet found in the CTRL simulation.

Furthermore, upstream diabatic forcing acts to keep a relatively persistent leeside mesolow near the region of heavy precipitation. This mesolow or trough contains a stationary mesoscale plume of ascent within the deep leeside trough that represents a thermally direct secondary circulation associated with the vertical stretching of the moist isentropic surfaces. The destabilization and midlevel moisture fluxes are strongly linked to the slow-moving plume of ascent. This facilitates a river of midlevel moisture advected over to the leeside of the mountains. Significant spillover precipitation is generated above the mesolow and within this thermally direct secondary circulation on the leeside. This is enhanced by a near-surface mountain-parallel low-level jet that increases the boundary layer convergence into the mesolow. The near-surface jet represents a low-level isallobaric ageostrophic wind response into the leeside mesolow.

One can also explain the analogous free surface/shallow water feature that flows over the mountain for the CTRL versus NLH simulation, as described in Lin (2007). This is schematically shown in Fig. 3. As can be seen in the CTRL (NLH), the free surface tends to indicate troughing (ridging) structure over the leeside. The Froude number (; where g = gravitational acceleration, U = ambient wind speed, and H = height of the free surface) regimes are remarkably different with a transition from supercriticality (subcriticality; Fr < 1) to subcriticality (supercriticality; Fr > 1) in the CTRL (NLH). These different regimes are a direct response to upstream Froude numbers in which lower (higher) upstream static stability and weaker divergent (stronger convergent) flow prevail in the CTRL (NLH). These differences in the two simulations are consistent with differences in energy conversions. In the CTRL (NLH), potential (kinetic) energy is being converted to kinetic (potential) energy over the mountain crest. The differences between the NLH and CTRL are consistent with upstream latent heating and deflected mid–upper tropospheric flow in the CTRL and weaker midtropospheric and stronger upper-tropospheric cross-mountain flow in the NLH. Thus moist, divergent, and ascending flow dominate the leeside in the CTRL producing copious spillover precipitation and dry high momentum/downslope-descending flow dominates the NLH simulation on the leeside of the mountain range largely devoid of precipitation. The results support a different mountain-wave structure in the CTRL and NLH because of the effects of upstream latent heating. Thus, the windward-side convective heating and outflow jet have a profound impact on the upstream Froude numbers and downstream mountain-wave structures.

5. Summary and conclusions

This study complements the large-scale studies of two leeside heavy precipitation events (1–3 January 1997 and 31 December 2005–1 January 2006) over the Sierra Nevada range (Underwood et al. 2009; Kaplan et al. 2009). Special emphasis is placed on the mesoscale manifestations in the mid- and upper-tropospheric circulations to test and validate the following hypothesis: multiscale jet streak adjustments in response to diabatic heating over the windward side of the Sierra Nevada lead to a set of mesoscale jets and associated pressure systems that create a favorable environment for local condensation and heavy precipitation on the leeside of the mountain range. To diagnose the role of latent heating, blocking on the windward side, and consequent leeside mesoscale dynamical and thermodynamical adjustment processes, a set of numerical experiments using the Weather Research and Forecasting Model (WRF) were conducted: 1) CTRL (full microphysics) and 2) simulation in the absence of condensation, evaporation, melting, freezing, heating, and cooling—that is, equivalent to no latent heating (NLH) simulations.

Most notably, the CTRL simulations exhibited strong latent heating on the windward side of the Sierra Nevada. This heating significantly modulates the leeside circulation, which is consistent with the theory of blocking. In effect, our results refine the details not typically available in observations by virtue of defining the finer patterns of multiscale jets, mesolows, static stability, and moisture fluxes. When diabatic heating is absent in the simulations, the transient jet-perpendicular gravity wave signals tend to dominate leeside dynamics. The response to upstream diabatic forcing is significantly different from dry adjustments as evidenced by an expanding spatial and temporal scale of the vertical motions downstream. Also the structure of the mountain wave differs between the CTRL and NLH simulations. The CTRL is wetter and the mountain-wave structure is shifted such that it creates a strong midlevel cross-mountain flow and leeside ascent. In the NLH, strong downslope flow and high surface momentum develop as the wave shifts downstream. It is also much drier in the absence of a midlevel outflow maximum. The static stability and vertical wind shear are radically different between the CTRL and NLH.

The differences in the CTRL and NLH clearly indicate the role of upstream latent heating in organizing a convectively driven circulation, downstream mass fluxes, and leeside destabilization/ascent, thereby favoring downstream spillover precipitation. The simulated soundings also confirm the moist neutrality above 750 hPa collocated with this ascending leeside feature in both case studies. The moist isentropic depression is clearly collocated with upper-level divergence, low-level inflow, reduced static stability, and strong ascent consistent with a thermally direct leeside mesoscale circulation (i.e., an ascending node within the warm air). This supports higher relative humidity and maxima of hydrometeors, not seen in the NLH simulations, and thus confirming the local leeside forcing of condensational processes.

The most notable differences between CTRL and NLH are the radically different simulated vertical motions, static stability, and vertical wind shear. The vertical motions in Fig. 11b for the NLH are indicative of shorter wavelength features with near-quadrature relationships while the moist isentropic surfaces are indicative of transient gravity waves. Additionally, the vertical separation between the moist isentropes is greater in the CTRL than in the NLH. The vertical shear from southerly near-surface flow to west–southwesterly midtropospheric flow is evident in the CTRL relative to the NLH simulations (Figs. 12 and 13). The larger θe magnitudes in the CTRL simulation versus the NLH (Fig. 11) support this increase in midtropospheric cross-mountain moisture fluxes and leeside lifting. Although the adjustment processes exhibit subtle differences in the simulations performed on both flooding case studies, the major theme is the same. This theme is one of downstream mass fluxes facilitated by the upstream latent heat release through a sequence of jets and adjustment processes that lead to leeside precipitation, which is the central outcome of this study.

The leeside spillover precipitation process is synthesized in this study and follows a sequence of mesoscale events: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet’s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow. This sequence completes the low-level and upper-level branches of a thermally direct mesoscale circulation with rising motion in the leeside and unstable warm air resulting in the spillover precipitation.

The dynamic simulations of the two extreme precipitation events on the leeside of the Sierra Nevada Range indicate that spillover precipitation cannot be simply viewed as falling hydrometeors advected from the windward side of the mountain but to be seen in the context of radically different mountain-wave structures. Rather, analyses of a sequence of dynamical interactions on both synoptic and subsynoptic scales are essential for a more complete understanding of the phenomenon.

Acknowledgments

This work supported by the National Science Foundation Grant 0447416 and UCAR/COMET Grant S06-58387, DoD/Army Grant N61339-04-C-0072, and a DRI/IPA Grant. We would also like to thank Jim Wallmann and Rhett Milne of the Reno Forecast Office of the National Weather Service for their collaboration in this work. We thank the anonymous reviewers for their valuable comments and suggestions, which significantly improved the initial manuscript. The author Ramesh Vellore thanks R. Krishnan of the Center for Climate Change Research, Indian Institute of Tropical Meteorology, for his enthusiastic suggestions.

APPENDIX

Simulation Sensitivity to WRF Microphysical Schemes

We briefly highlight the sensitivity of modeled precipitation amounts on the leeside of the Sierra Nevada to different WRF microphysical schemes in an effort to demonstrate 1) the importance of upstream moist physical processes to the evolution of leeside (spillover) precipitation and 2) the fidelity of the precipitation simulations. The sensitivity experiments are conducted only for the 2005 case study. Among the different bulk microphysical schemes used in the sensitivity experimentation (see section 3), the Thompson (Thompson et al. 2004, 2008) and Morrison schemes (Morrison et al. 2005) use double-moment microphysics that predict both mass mixing ratios and number concentration of water substances—namely, water vapor, cloud water, rainwater, cloud ice, snow, and graupel.

The spatial distribution of 36-h precipitation accumulations ending at 0000 UTC 1 January 2006 using different choices of WRF microphysical schemes (see section 3 for details) is shown in Figure A1. All microphysical schemes invariably showed the spillover precipitation regime on the leeside except for the Purdue Lin scheme (Figures A1 and A2). Also, the simulated 36-h precipitation amounts from the Thompson (230 mm) and the Morrison (220 mm) schemes showed good correspondence with the observed estimate (214 mm) on the windward slope at BLU. Purdue Lin and WSM6 schemes showed copious amounts and widespread precipitation on the windward side, while GSFC showed more precipitation over the crest of the Sierra Nevada. In particular, among the different WRF microphysical parameterizations the Thompson scheme simulated the timing and the amount of the spillover precipitation in good agreement with the observations (see also Fig. 6). The observed (CTRL) 36-h precipitation accumulations on the leeside at TRK, REV, CSN, and TVL are 88, 44, 142, and 125 mm (133, 38, 120, and 168 mm), respectively (see also Fig. 8a).

Fig. A1.
Fig. A1.

WRF-simulated spatial distribution of 36-h precipitation accumulations at 1-km resolution (mm; 1 in. = 25.4 mm; also shown in boxes) ending at 0000 UTC 1 Jan 2006 using (a) Purdue Lin, (b) Morrison, (c) WSM6, and (d) GSFC microphysical schemes. The state line divides California and Nevada at 120°W (see also Fig. 1b). Stations TRK, BLU, and TVL (REV and CSN) are in California (Nevada).

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

Fig. A2.
Fig. A2.

Observed and diagnosed from 5-km WRF grid ending at 1600 UTC 24-h precipitation accumulations (mm) for (a) 30 Dec 2005 and (b) 2 Jan 1997 for different river basins (see also Fig. 1b) in Nevada and California.

Citation: Journal of Hydrometeorology 13, 4; 10.1175/JHM-D-11-06.1

The Morrison scheme also showed the signatures of double maxima of hydrometeor mixing ratios over the Sierra upslope and west of the Carson Range similar to the Thompson scheme (see Fig. 9). The observed and CTRL-simulated precipitation accumulations for different river basins (Fig. 1b) in the area of study are shown in Figure A2. The simulated precipitation accumulations showed an overall correlation of 0.78 for the windward side and 0.94 for the spillover zone with the observed amounts for the river basins in the area of study.

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