1. Introduction
Measurement of the spatial distribution and temporal evolution of supercooled liquid water (SLW) over complex terrain is critical for understanding natural ice particle nucleation, growth, and fallout within orographic cloud systems, as well as evaluating the potential for cloud seeding to increase winter precipitation (Rauber et al. 2019). A period of intensive field experimentation over coastal and interior ranges between the 1970s and 1980s focused on measuring the magnitude, distribution, and controls of SLW over complex orography, and employed a variety of instrumentation that included aircraft probes, microwave radiometers, balloonborne instruments, and surface measurements (Hobbs 1975; Reinking 1979; Hill and Woffinden 1980; Cooper and Vali 1981; Heggli et al. 1983; Politovich and Vali 1983; Rauber et al. 1986; Rauber and Grant 1986; Sassen et al. 1986, 1990; Rauber and Grant 1987; Rogers and Vali 1987; Heggli and Rauber 1988; Demoz et al. 1993). International studies were subsequently conducted over Australia’s Great Dividing Range and Snowy Mountains (Long and Carter 1996) and the central mountains of Japan (Kusunoki et al. 2004, 2005). More recent work has been reported by Ikeda et al. (2007) over the Cascades, by Dorsi et al. (2015) over the Park Range, and by Pokharel et al. (2015) and Rasmussen et al. (2018) over the Sierra Madre and Medicine Bow Mountains of Wyoming.
These studies demonstrated that the presence of SLW in orographic clouds is most frequently associated with updrafts strong enough to ensure that the condensate supply rate is greater than the diffusional growth rate of ice, allowing for the formation of SLW (e.g., Rauber and Tokay 1991; Ikeda et al. 2007). Vertical air circulations over the terrain can be described as both fixed, their magnitudes mechanically driven by wind near and over the terrain, the terrain steepness, and stability profile (i.e., stationary waves), and transient, temporally varying vertical circulations related to vertical wind shear, conditional instability, and thermodynamic effects such as evaporation within passing synoptic-scale cyclones. Under strong unblocked flow, a mountain ridge produces a significant stationary updraft, typically less than 70 cm s−1, that depends on the mountain slope, wind speed, and stability, whereas winter storms crossing mountain ranges typically contain transient updrafts (such as waves, turbulence, or convection) dependent on shear and potential instability on the order of 1–10 m s−1 (Zaremba et al. 2022b).
Fixed orographically induced gravity waves, locked to the terrain, are generated by winds blowing across a mountain range causing forced ascent on the windward side and descent on the leeward side. These mountain waves have been extensively studied in relation to their response to ambient wind and stability profiles, mainly using idealized soundings or simulations with idealized terrain (see review by Smith 2019). The distribution of fixed forced ascent and descent across a mountain range depends on the geometry of the terrain, such as the height, width, and length of the range (e.g., Sinclair et al. 1997; Chater and Sturman 1998; Colle 2004; Colle et al. 2008; Colle and Zeng 2004; Jiang 2006; see chapter 5 in Lin 2007 for a review). The magnitude of the updrafts and downdrafts vary with the strength of the mean low-level winds, stability over the terrain, and prevailing wind direction (e.g., Held and Ting 1990; Colle 2004). In situations where there are strong low-level winds, and weak (moist) stratification larger ridges will produce a deep wave response whereas smaller ridges will produce shallower vertical motion dipoles. The interaction of hydrometeors with consecutive fixed waves in moist flow over complex terrain is not well understood. However, SLW has commonly been observed within fixed mountain-induced gravity waves (e.g., Reinking et al. 2000; Bruintjes et al. 1994; Garvert et al. 2007; Geerts et al. 2023) suggesting that SLW peaks on the upwind side of terrain features (Rauber and Grant 1986).
Orographically forced updrafts can be modified by a variety of transient processes over the terrain, mainly convection and turbulence, in cold-season storms. Although orographic clouds are primarily stratiform, they often contain shallow or deep convection, which results from the release of potential instability associated with orographic lifting (Shafer et al. 2006; Ikeda et al. 2007; Jing and Geerts 2015; Geerts et al. 2015; Kirshbaum et al. 2018). Smaller-scale transient updrafts can also be associated with shallow convective cloud-top generating cells (e.g., Rauber and Tokay 1991; Kumjian et al. 2014; Keeler et al. 2016a,b, 2017), shear-induced Kelvin–Helmholtz billows (Houze and Medina 2005; Geerts and Miao 2010; Barnes et al. 2018; Medina and Houze 2015; Grasmick and Geerts 2020; Cann et al. 2022; Grasmick et al. 2021), or boundary layer turbulence (Lee 1988; Geerts et al. 2011; Chu et al. 2018; Majewski and French 2020).
There are few observational and modeling studies that link fixed and transient updrafts to cloud supercooled liquid water content (SLWC) and cloud droplet number concentrations over complex terrain in wintertime orographic storms. Garvert et al. (2007) presented a case study from the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) and found that fixed waves over the Oregon Cascades exhibited a mixture of evanescent and vertically propagating characteristics that modulated modeled cloud water content and snow mixing ratios. Medina et al. (2005, 2007) and Colle et al. (2008) also examined an enhanced reflectivity signature and precipitation enhancement coinciding with a fixed wave observed using Doppler radar and dual-Doppler observations during IMPROVE-2. Kingsmill et al. (2016) presented a case study from the Colorado Airborne Multi-Phase Study (CAMPS; Dorsi et al. 2015) over the Park Range and showed that cloud liquid water content and precipitation distributions were not consistent along the 13 spatially and temporally varying flight legs across the ridgeline. They suggested that small-scale, short-lived transient updrafts propagating over the domain modified expected fixed updraft driven SLWC and precipitation distributions. Dorsi et al. (2015) used observations from 10 frontal and postfrontal storms during CAMPS and composited mixed-phase percentage, SLWC, and ice particle concentrations over the Park Range ridgeline in Colorado. These cases all had westerly cross-barrier winds and encompassed periods when stable air was passing over the ridge resulting in fixed waves driven by the terrain. The cases also sampled elevated convective cells triggered by orographic ascent, turbulence caused by the elevated terrain, and shallow convection triggered by frontal forcing. A peak in mixed-phase frequency in the composites was observed 5 km west of the windward ridge axis, with mixed-phase frequency sharply decreasing east of the ridgeline where fixed downdrafts were commonly present. None of these studies examined cloud droplet number concentrations or the potential controls that fixed and transient updrafts have on SLWC over the terrain. Quantifying the magnitudes of fixed and transient updraft structures in relation to SLWC and cloud droplet number concentrations within orographic clouds is an important step toward understanding where and when supercooled water may be present and where wintertime orographic cloud seeding opportunities may exist.
The Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE; Tessendorf et al. 2019) provided an opportunity to address questions related to vertical motions in orographic clouds and their control on SLWC and cloud droplet number concentrations over complex terrain. SNOWIE occurred between 7 January and 16 March 2017 over the Payette River basin of western Idaho with 23 research flights (RFs) lasting ∼4 h, each one corresponding to an intensive operation period (IOP). As part of this project, Zaremba et al. (2022a, Part I) developed methodology to retrieve vertical air velocity (w) and flight-leg-averaged particle terminal velocity profiles (
2. Orography of the Payette River basin
The Payette River and its tributaries flow from the south and west sides of the Salmon River Mountains of Idaho into the Snake and Columbia River basin. The region is made up of a series of north–south ridges with elevations ranging from ∼2.5 to 2.8 km and valleys with elevations ranging from ∼0.9 km (South Fork Valley) to ∼1.4 km (North Fork Valley). The Payette River drains into the Snake River, which subsequently drains through Hells Canyon to the west of the Payette Basin (Fig. 1a). Passing cyclones often transport warm, moist air across the Snake River basin, commonly from the west-southwest. Near-surface air commonly flows from the south in the valleys of the Payette River basin under stratified conditions during these events (Tessendorf et al. 2019).
(a) SNOWIE domain within Idaho outlined in black. Elevation in meters MSL is color shaded (0.125° × 0.125° grid). (b) An expansion of the box in a with 900-m-grid terrain elevation. The Payette River basin is outlined in gray. Plotted in blue are the three flight tracks flown during SNOWIE. Rawinsonde launch locations are noted by black dots. The location of the radiometer at Smith’s Ferry is denoted by the black star (see section 4). Ranges and valleys are labeled (adapted from Zaremba et al. 2022b).
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
During SNOWIE, the University of Wyoming King Air (UWKA) flew repeated flight legs with reverse headings along one of three tracks over the Payette River basin parallel to mean midlevel (∼700 hPa) flow in a west–east (flight track A) or southwest–northeast direction (flight tracks B and C; Fig. 1b). Along track A the UWKA crossed the Western Range, North Fork Range, and the Western and Eastern Salmon River Ranges, as well as the Payette River tributaries between these ranges. On flight track B, the UWKA flew over the same ranges and tributaries but at an ∼35° angle relative to the ridge lines. Most flights were either along track A or B, and data presented here are from flights along those tracks.
3. Data sources
a. Wyoming Cloud Radar
The University of Wyoming Cloud Radar (WCR; Wang et al. 2012) is a 3-mm wavelength, pulsed Doppler cloud radar operated onboard the University of Wyoming King Air (UWKA) during SNOWIE. Data used herein are from the WCR fixed antennas nominally pointed at zenith and nadir during straight, level flight. In this configuration, the WCR measured the equivalent reflectivity factor (Ze) and radial velocity (Vr). Data were sampled at 30-m range resolution along the radar beams and 4.5–7.5 m along the flight track depending on the ground-relative aircraft speed resulting in a nominal pulse volume of 10 × 10 × 30 m3 at 1-km range. Zaremba et al. (2022a) present an algorithm to retrieve vertical air velocity from the WCR measured Vr, aircraft attitude parameters, and rawinsonde wind measurements. Using the retrieved vertical motions, Zaremba et al. (2022b) provided a quantitative analysis of the magnitude of updrafts and downdrafts over the Payette Basin associated with both mechanical forcing fixed to the topography and transient circulations associated with conditional instability and shear within passing weather systems. Retrieved vertical motions using the Zaremba et al. (2022a) algorithm are used herein to examine the impact of these fixed and transient vertical circulations on SLW production and cloud droplet concentrations over the terrain. Cloud-top heights reported herein are based on the WCR echo tops, which were determined by applying a noise threshold to the WCR Ze and Vr measurements.
b. Rawinsondes
The Idaho Power Company (IPC) launched Lockheed Martin LMS6 rawinsondes from sites in Crouch and Lowman, Idaho (Figs. 1a,b). These rawinsondes were used to remove the horizontal wind contribution to Vr using the retrieval methodology in Zaremba et al. (2022b), estimate mean wind speeds over the terrain along fixed flight tracks, and estimate cloud-top temperature. The manufacturer-stated accuracy of IPC rawinsondes was ±0.2°C for temperature and ±0.2 m s−1 for wind.
Rawinsonde data and WCR measurements were used to estimate cloud-top height (CTH) and cloud-top temperature (CTT) directly overhead of SLW measurements at flight level. The altitude of the top of the highest range gate directly above the position of the aircraft associated with cloud was matched to rawinsonde temperature data that was interpolated to the nearest 0.1 m and closest in time and location to the aircraft. This allowed for 1-Hz estimates of CTH and CTT along a given flight track.
c. Gust probe data
During SNOWIE, a gust probe located on the UWKA’s nose boom–measured vertical air velocity wgp at flight level at a rate of 1 Hz. To obtain wgp, aircraft motion was first removed from the gust probe’s raw winds using data from the aircraft’s inertial navigation system and aircraft attitude parameters. After removing flight-leg averages in this analysis, wgp from the gust probe is accurate to at least 0.1 m s−1 (Lenschow 1972; Geerts and Miao 2005).
d. Microwave radiometer
Data from a dual-channel (23.8 and 31.4 GHz) radiometer located at Smith’s Ferry were used in real-time during SNOWIE to assess the potential for orographic cloud seeding. The radiometer provided continuous (∼4-min resolution) measurements of both liquid water and vapor integrated through the depth of the atmosphere. The radiometer at Smith’s Ferry was located in the Middle Fork Payette River Valley near the midpoint of the research flight tracks (44.18°N, 116.04°W; Fig. 1b) at an elevation of 1316 m (lower than the surrounding terrain peaks). A blower system blew air across the microwave window to prevent the formation of dew and accumulation of snowflakes and light drizzle on the window. The blower was ineffective during rain. IOPs 2, 3, 20, 22, 23, and 24 had rain in the valleys based on WCR Ze and Vr cross sections that showed distinct melting layers within various valleys, including the valley in which the radiometer was located. Data where rain was observed within valleys were excluded from analyses that follow. The radiometer was also not operational during IOPs 14–17. From the remaining 14 IOPs, 88 total flight legs had WCR measurements while the radiometer was operational. Sources of error in radiometric measurements of liquid and vapor are associated with estimating the mean radiating temperature, brightness temperature, and attenuation coefficients of vapor and liquid (Friedrich et al. 2012; Bianco et al. 2017). The radiometer had calibrated temperature accuracy of ±0.5°C at 25°C.
e. Particle probes and SLWC measurements
UWKA acquired in situ measurements of SLWC and droplet size distributions from liquid hydrometeors up to roughly 50 μm in diameter and hydrometeor size, shape, and concentration of larger hydrometeors along its flight track.
1) 2DS and 2DP probes
In situ measurements of hydrometeor size, shape, and concentration for particles with diameters larger than about 50 μm were provided by two optical array probes (OAPs) on board the UWKA, a Stratton Park Engineering Company (SPEC), Inc., 2D stereo probe (2DS; Lawson et al. 2006) and a Particle Measuring System (PMS) 2D precipitation probe (2DP). Data from these OAPs were processed using the University of Illinois–Oklahoma Optical Array Probe Processing Software (UIOOPS; McFarquhar et al. 2018). Only “center in” particles were accepted in this analysis and diameters were determined using the minimum enclosing circle. The size ranges used for the 2DS and 2DP probes were determined as follows: 1) A lower threshold (100 μm) for the 2DS was chosen in accordance with previous studies to minimize uncertainty in the depth of field (e.g., Lawson et al. 2006) and to remove potentially shattered artifacts (e.g., Korolev et al. 2011; Jackson et al. 2014). 2) An upper threshold (1200 μm, or 1.2 mm) was selected as the 2DS–2DP cutoff point. The 1.2-mm threshold was selected because the concentrations from the 2DS and 2DP most consistently overlapped at this size, even though this size is larger than the approximately 1-mm threshold that has been used in several prior studies (e.g., Hu et al. 2021). Combined particle concentrations from the two probes are reported in this study as ice concentrations or Nt,Ice.
Supercooled drizzle droplets with diameters larger than 100 μm were observed on some flights by the 2DS, including IOP1, during SNOWIE, as reported by Majewski and French (2020). In such cases, Nt,Ice is not a reliable indicator of observed ice concentrations. While such situations were not common during SNOWIE, reported ice concentrations may rarely be influenced by supercooled drizzle droplets.
2) Cloud droplet probe
A cloud droplet probe (CDP; Lance 2012; Lance et al. 2010) provided measurements of SLWC (all measurements described herein were acquired at flight levels where temperatures were <0°C) through the integration of cloud droplet size distributions for particles with diameters 2 < D < 50 μm. The CDP also provided droplet size distributions with 2-μm resolution (Lance et al. 2010; Faber et al. 2018). The CDP is designed to measure cloud droplets but can respond to ice particles. Typically, CDP measurements of ice are often one or more orders of magnitude less than the actual count of liquid droplets particularly in smaller bins. It has also been demonstrated that the CDP is minimally affected by ice-shattering artifacts (Lance et al. 2010; Khanal 2013). A CDP concentration of 5 cm−3 was chosen as the threshold for the presence of SLW rather than the 10 cm−3 threshold typically used in past studies (e.g., Cober et al. 2001; Um et al. 2018; Finlon et al. 2019). The choice was made based on analysis of the observed CDP concentrations while sampling heavily precipitating ice clouds. Figures 2a and 2c show two examples of Ze from flight legs during RF04 where deep, heavily precipitating clouds were observed over the Payette River basin with no SLW detected in any of the other probes. Particle concentrations from the 2DS (Nt,2DS) and 2DP (Nt,2DP) combined (Nt,Ice) ranged from 20 to 80 L−1 along both flight legs with concentrations in isolated pockets approaching 100 L−1 (Figs. 2b,d). During these flight legs, in the presence of high background ice concentrations, CDP concentrations were always less than 1 cm−3 and typically less than 0.15 cm−3 (Figs. 2b,d). For all cases during SNOWIE, when Nt,2DS was >15 L−1 background concentrations detected by the CDP were typically <0.15 cm−3 and always less than 1 cm−3. For SNOWIE, a threshold of 5 cm−3 for detecting SLW was considered conservative and is used in all subsequent analyses.
(a) Ze and (b) Nt,CDP (black line; left axis) and Nt,Ice (red line; right axis) during 2214:40–2230:30 UTC 18 Jan 2017. (c),(d) As in (a) and (b), but during 2233:30–2243:35 UTC 18 Jan 2017.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
3) Particle volume monitor (Gerber) probe
The particle volume monitor (Gerber) probe measures SLWC by measuring the amount of laser light scattered by droplets within a sample volume of 1.25 cm−3 (Gerber et al. 1994). The Gerber probe provides measurements for droplets with diameters between 3 and 50 μm but the sampling efficiency of the instrument begins to decrease when the mean volume diameter of the sampled droplets increases above 20 μm (Baumgardner et al. 2017). The probe operates across a temperature range of −70° to 40°C and the full range of relative humidities encountered during SNOWIE.
4) Nevzorov probe
A Nevzorov liquid water and total water content probe (Korolev et al. 1998) also provided measurements of SLWC. Power supplied to the Nevzorov probe is related to an element temperature as particles are vaporized upon impacting the element surface area. Convective losses due to moist airflow over the sensor transfer energy from the collector elements and are quite large at aircraft flight speeds. This is accounted for to some degree by a rear-facing reference element that does not collect condensate and is used to estimate the convective heat losses as a function of flight conditions. However, there remains a baseline drift of the Nevzorov-derived SLWC measurements (Abel et al. 2014; Faber et al. 2018). Nevzorov data were manually corrected during periods when the CDP SLWC was near 0 g m−3. The baseline was adjusted only when a baseline shift was apparent. Also, to account for impact of ice particles on the liquid water sensing element, a subtraction of 5% of the difference between the total water content and SLWC was also applied to SLWC measurements in order to account for ice build-up on the probe. The Nevzorov collection efficiency for the liquid sensing element also decreases with increasing droplet diameter (>30 μm) (Schwarzenboeck et al. 2009).
5) Comparison of SLWC probes
Faber et al. (2018) provide laboratory and in-flight evaluation of measurement uncertainties of the UWKA CDP and a comparison with the UWKA Nevzorov probe-derived LWC. Their in-flight evaluation included both SNOWIE data and data from the Precipitation and Cloud Measurements for Instrument Characterization and Evaluation (PACMICE) campaign over eastern Wyoming and western Nebraska. The latter campaign focused on collecting measurements in precipitating stratiform and convective systems, primarily in the fall and spring seasons. They found that the CDP LWC exceeded the Nevzorov LWC by ∼20 %, with expected sizing errors from laboratory measurements resulting in less than 10% error in CDP LWC.
Herein, we evaluate the consistency of SLWC measurements between three different probes (CDP, Gerber, Nevzorov) only during SNOWIE. Data points with Nt,CDP > 5 cm−3 and measured SLWC > 0 g m−3 from all three probes are compared in Fig. 3. A total of 42 911 1 Hz data points met these criteria. Overall, the data from the probes compared well. The CDP had a median SLWC of 0.08 g m−3 and an interquartile range of 0.04–0.16 g m−3, the Gerber probe, a median of 0.09 g m−3 and interquartile range of 0.05–0.15 g m−3, the Nevzorov a median of 0.06 g m−3 and an interquartile range of 0.03–0.12 g m−3 (Fig. 3).
SLWC comparison between the CDP, Gerber, and Nevzorov probes for all research flights during the SNOWIE field campaign for periods when SLWC for all probes was >0 g m−3 and Nt,CDP > 5 cm−3. Boxes represent the 25th and 75th percentiles. The red line represents the median value, while whiskers represent the 1st and 99th percentiles.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
Figures 4a–f show a 1:1 comparison of SLWC measurements between each probe, with Pearson’s correlation coefficients ranging from 0.94 to 0.96. The mean absolute error between these probes was 0.03 g m−3, with CDP values slightly higher than those from the other probes, especially at SLWC > 0.25 g m−3 (Figs. 4a,b). Because the Nevzorov probe data were manually adjusted for drift, and the Gerber probe had increased uncertainty when larger cloud droplets were sampled, the CDP will be used for the remainder of the analysis, as it provided consistent measurements of SLWC that compared well with other probes while also providing droplet concentration and size spectra measurements.
SLWC comparison plots (g m−3) when SLWC for all probes was >0 g m−3 and Nt,CDP > 5 cm−3 (a) CDP and Nevzorov probes, (b) CDP and Gerber probes, and (c) Gerber and Nevzorov probes. The black line is the 1:1 line. Each dot is a 1-Hz sample.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
4. Controls on SLWC and cloud droplet number concentrations
This section explores controls on SLWC and cloud droplet number concentrations over the Payette River basin and Salmon River Mountains during SNOWIE and their relation to fixed and transient vertical motions observed over the terrain.
During SNOWIE, 27.4% of 1 Hz in situ CDP samples (48 429 out of 176 930) were in an environment containing SLW (where Nt,CDP, > 5 cm−3). Figures 5a and 5b show a probability density function (PDF) of Nt,CDP from all research flights during the campaign. The mean Nt,CDP was 30.8 cm−3, the median 22.8 cm−3, and the interquartile range 13.3–37.2 cm−3 (Figs. 5a,b). Cloud droplet concentrations encountered during SNOWIE were low, with 95th-percentile values <80 cm−3. Possible reasons for these low values will be explored in section 4b. Excluding periods when no SLW was observed, the mean and median values of SLWC were 0.13 and 0.08 g m−3, respectively, while the interquartile range was 0.02–0.18 g m−3 (Figs. 5c,d). On some flights, in isolated regions of convection and strong shear-induced turbulence, SLWC approached 0.40–1.75 g m−3.
(a) Nt,CDP (cm−3) for all 1-Hz samples during SNOWIE where Nt,CDP > 5 cm−3; (b) as in (a), but the y axis is on a log scale to show the more extreme values; (c) CDP derived SLWC (g m−3) for all 1-Hz samples during SNOWIE where Nt,CDP > 5 cm−3; (d) as in (c), but for SLWC.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
a. Factors limiting SLWC
Past studies using radiometers in orographic clouds over the Colorado’s Park Range and the central Sierra Nevada have shown that deeper clouds with lower cloud-top temperatures typically have lower integrated liquid water content values compared to shallow clouds, likely due to the increased ice particle concentrations within them (Rauber et al. 1986; Heggli and Rauber 1988). We therefore expected the occurrence of SLWC in clouds over Idaho to be related to cloud-top temperature. To examine this quantitatively, the retrieved CTH from the WCR was matched to a nearby rawinsonde to estimate CTT. Cumulative distribution functions (CDFs) of SLWC with respect to CTT, including periods where no SLWC was observed, were created from all SNOWIE flight legs (Fig. 6). CTTs were segregated in 5°C increments. The percentage of 1-Hz measurements with SLW present decreased as CTT decreased. When CTTs were less than −40°C, SLWC measured at flight level was typically lower, and the CDF shows that >80% of the SLWC values equaled 0 g m−3. For all flight legs, 19.4%, 7.4%, and 6.7% of samples with CTTs between −20° to −25°C, −35° to −40°C, and −50° to −55°C, respectively had SLWC exceeding 0.1 g m−3 (Fig. 6).
CDF of CDP SLWC for all 1-Hz data points for different cloud-top temperature ranges noted by changes in color for all SNOWIE data. Several curves overlap.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
Figures 7a–c show the distribution of SLWC as a function of CTT, flight-level temperature, and ice particle concentration (Nt,Ice) for times when SLWC was present. The median value of SLWC decreased from 0.15 to 0.02 g m−3 as rawinsonde and WCR estimated CTT decreased from −15° to −40°C with the greatest spread in SLWC occurring when CTT were >−20°C (Fig. 7a). When CTT < −35°C, the 75th- and 95th-percentile values of SLWC were 0.15 and 0.35 g m−3, respectively (Fig. 7a). During SNOWIE, SLWC measurements were made at flight-level temperatures ranging from −3° to −33°C, with most measurements concentrated between −5° and −19°C because the UWKA primarily flew between 3 and 5 km MSL to target potential seeding signatures (Friedrich et al. 2021). Values of SLWC had the greatest spread at temperatures between −5° and −20°C, where pockets of SLWC approached 0.40–1.75 g m−3 (Fig. 7b). During the campaign, Nt,Ice was typically <20 L−1 although values as large as 150 L−1 were observed (Figs. 7c,f). The highest SLWC values (0.40–0.80+ g m−3) occurred when background Nt,Ice < 5 L−1 (Fig. 7c).
(a) CDP SLWC (g m−3) vs CTT (°C) binned every 0.025 g m−3 and 0.5°C; (b) CDP SLWC (g m−3) vs flight-level temperature (°C) binned every 0.025 g m−3 and 0.5°C; (c) CDP SLWC (g m−3) vs Nt,Ice (L−1) binned every 0.025 g m−3 and 1 L−1; (d) Nt,CDP (cm−3) vs CTT (°C) binned every 1 cm−3 and 0.5°C; (e) Nt,CDP (cm−3) vs flight-level temperature (°C) binned every 1 cm−3 and 0.5°C; (f) Nt,CDP (cm−3) vs Nt,Ice (L−1) binned every 1 cm−3 and 1 L−1. The data exclude periods when no SLW was present.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
These results were consistent with radiometric measurements of vertically integrated SLW. CTT and CTH data 10 km due east and west of the radiometer were averaged along each flight leg and compared to average radiometric measurements of integrated liquid during those times. Figures 8a and 8b show a comparison of integrated liquid, CTH and CTT. A least squares polynomial fit was used to assess the trend with increasing CTH (decreasing CTT). On average, as CTH increased and CTT decreased, clouds contained less integrated liquid. The correlation coefficient for CTH was 0.66 and 0.59 for CTT. In any particular cloud system, the highest integrated water was more often associated with shallower clouds with CTTs > −20°C.
Comparison of integrated liquid measured by a radiometer at Smith’s Ferry (see Fig. 1a) compared with (a) mean cloud-top temperature (°C) and (b) cloud-top height (km) along each flight leg (yellow dots). The black lines are the least squares polynomial fits. Each dot represents an average during the time of a UWKA flight leg.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
Overall, the data in Figs. 6–8 suggest that cloud-top temperature and background ice concentrations provide controls on SLWC magnitudes, with the highest values occurring in clouds with higher cloud-top temperatures and lower background ice particle concentrations. The data provide evidence that the amount of SLWC within clouds over the Payette River basin is somewhat related to CTT and ice particle concentrations, similar to past findings over other mountain ranges (e.g., Rauber et al. 1986; Rauber and Grant 1986; Heggli and Rauber 1988), although the scatter suggests that other processes may be involved.
b. Airmass source region relationship to cloud droplet number concentrations
Figures 7d–f show the distribution of Nt,CDP at the flight level as a function of CTT, flight-level temperature, and ice particle concentration (Nt,Ice) for times when SLWC was present. The median value of Nt,CDP decreased from 16 to 4 cm−3 as rawinsonde estimated CTT decreased from −15° to −40°C (Fig. 7d). When CTT < −35°C, the 75th- and 95th-percentile values of Nt,CDP were 34.8 and 119 cm−3. Nt,CDP measurements at flight level exhibited the highest values between −3° and −8°C with isolated regions having Nt,CDP between 250 and 320 cm−3. Regions with Nt,CDP > 150 cm−3 were only observed at temperatures > −14.2°C (Fig. 7e). The highest Nt,CDP values (50–320 cm−3) occurred when background Nt,Ice < 5 L−1 (Fig. 7f). Supercooled cloud droplet concentrations in clouds over the Payette River basin were consistent with those reported in one storm over the Sierra Nevada (Rauber 1992) and several events over Colorado’s Park Range (Rauber and Grant 1986). Rauber and Grant (1986) also reported cases where cloud drop concentrations reached ∼300 cm−3, similar to one storm over the Payette River basin. Aside from these few measurements of high droplet concentrations, the droplet concentrations over the Payette River basin were lower than those in many marine clouds. One possible reason is that the airmasses in which these clouds originate have been decoupled from the boundary layer and may have passed through clouds over upstream mountains where cloud condensation nuclei have previously been scavenged.
To investigate the relationship between airmass source regions and cloud droplet number concentrations over the Payette River basin, 72-h Global Data Assimilation System (GDAS) 0.5° HYSPLIT (Stein et al. 2015) back trajectories were run starting at flight level along each flight leg between 3 and 6 km at three longitudes (115.6°, 116.0°, and 116.4°W) along flight track A (Fig. 9). Statistics on cloud droplet number concentrations were calculated for all periods when SLW was present (Nt,CDP > 5 cm−3) along flight legs during each research flight. Trajectories typically underwent 2–4 km of ascent prior to arrival over the Payette River basin as they passed over the Cascade Range, North Coastal Range, or the Sierra Nevada. Past work also found that the Burney Gap, north of the Sierra Nevada’s in California, was an important pathway for Pacific moisture to reach southwestern Idaho with near zonal flow, with ascent over the Cascade Mountains of Oregon being the second most common moisture pathway (Cann and Friedrich 2020). Trajectories that approached the basin from the west-southwest underwent greater ascent and had lower median Nt,CDP with values ranging from 8.2 to 21.0 cm−3 (RF03, RF09, RF21, and RF23). Interquartile ranges during these cases ranged from 5.9–13.2 to 12.4–28.5 cm−3. Trajectories that passed over the terrain from the west underwent less ascent and had median Nt,CDP between 19.6 and 44.6 L−1 (RF01, RF11, RF12, RF22). Interquartile ranges in these cases ranged from 11.6–32.8 to 23.8–63.1 cm−3. Analysis of the trajectories during RF03, RF21, and RF23 revealed that the airmass that affected the Payette River basin may have interacted with the oceanic boundary layer because they passed through altitudes below 1 km between 8 and 41 h before the research flight. For these IOPs, the median Nt,CDP values were 21, 13.4, and 8.2 cm−3, respectively. Notably, there were no significant differences in cloud droplet concentrations associated with the trajectories originating at lower and higher altitudes over the Pacific Ocean. Trajectories during RF09 originated between 3- and 4-km altitude off the southern coast of California before turning clockwise northward and then over the Payette River basin. These trajectories underwent gradual ascent and had a lower median cloud droplet concentration of 13.3 cm−3.
(a) Boxplots of Nt,CDP for 1-Hz periods when Nt,CDP > 5 cm−3 for each flight leg flown along flight track A. Boxes represent the 25th and 75th percentiles and the whiskers represent the 5th and 95th percentiles. (b) Map of trajectories from each research flight flown along flight track A. 72-h GDAS back trajectories were run for each flight leg starting at 42.4°N, 116.4°, 116.0°, and 115.6°W starting at the closest hour and at the mean height of the aircraft during a given leg. (c)–(j) Height of back trajectories with time. The x axis is hours before and the y axis of the plot shows the trajectory start height.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
This analysis showed that trajectories approaching the basin from the west-southwest (RF03, RF21, and RF23) had lower median cloud droplet concentrations than those passing over the coastal terrain directly from the west (RF01, RF11, RF12, and RF22). The trajectories of southwest origin underwent more ascent and may have had lower cloud droplet concentrations due to scavenging of aerosol associated with precipitation over upwind ranges. Furthermore, trajectories typically passed over the continental and marine boundary layer at altitudes > 2–3 km, preventing mixing with the boundary layer.
c. Relationship of SLWC and Nt,CDP to vertical motions fixed to orography
To assess the relationship between updrafts and downdrafts associated with orography and the distribution of SLWC and Nt,CDP over the terrain, 1 Hz SLWC and Nt,CDP samples were binned every 0.01° longitude (∼800 m) along flight tracks A and B for all flight legs. Composites of vertical motion (w) from all flight legs along tracks A and B were then constructed using the approach presented in Zaremba et al. (2022b). As noted by Zaremba et al. transient vertical motions vary in time and space, so averaging vertical motions over the cross sections from a large number of flight legs effectively removes the contribution of transient updrafts while retaining vertical circulations tied to the terrain. Figures 10a and 11a show the distribution of fixed orographic updrafts over the terrain along flight tracks A and B, respectively. Localized peaks were associated with fixed orographic wave couplets noted by vertical black lines. Median updraft and downdraft magnitudes were on the order of 0.3 to 0.5 m s−1. The strongest vertical motions, ∼0.7 m s−1, were located at lower levels (<4 km) over the steepest terrain along the Western Salmon River Range.
(a) WCR composites of w along flight track A from 8 research flights (IOP01, IOP03, IOP09, IOP11, IOP12, IOP21, IOP22, and IOP23) consisting of 82 flight legs during SNOWIE. Data are binned every 30 m in altitude and 0.005° longitude. Bins with less than 10 flight legs were excluded. (b) Number of 1-Hz counts in each 0.01° longitude bin (800 m) corresponding to (c) and (d) (black) and to (e) and (f) (red). (c) Boxplots of SLWC (g m−3) in each 0.01° (800 m) longitude bin. Boxes represent the 25th and 75th percentiles; whiskers represent the 5th and 95th percentiles. The solid black boxplot lines represent the median SLWC (g m−3). This panel includes all SLWC (g m−3) measurements, including zero values, during SNOWIE. (d) As in (c), but boxplots of Nt,CDP (cm−3). (e) As in (c), but data only include samples where supercooled water was present. (f) As in (d), but data only include samples where supercooled water was present. Bins with less than 75 one-Hz measurements were excluded in (c)–(f).
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
As in Fig. 10, but showing data along flight track B from 11 research flights (IOP02, IOP04, IOP08, IOP10, IOP13, IOP15, IOP16, IOP17, IOP19, IOP20, and IOP24). The data are from 126 flight legs.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
Flight track A included 82 flight legs during 8 research flights. Along A, there were 62 122 1 Hz CDP samples. Of those, 72.5% had no SLW present, while 27.5% had SLW present (where Nt,CDP > 5 cm−3). Along flight track A, SLWC and Nt,CDP magnitudes and distributions closely mirrored the strongest vertical motions over the terrain, with the largest SLWC and Nt,CDP values associated with the strongest vertical motions (0.3–0.5 m s−1) over the Western Salmon River Range, but somewhat downwind of the Eastern Salmon River Range (Figs. 10b,c). SLWC and Nt,CDP magnitudes were distributed normally over terrain peaks associated with fixed waves, with peak values over the highest terrain, except over the Eastern Salmon Range, where the SLWC and Nt,CDP maxima were offset eastward of the terrain peak. The eastward displacement in this region can be explained by consideration of the three-dimensional topography in that region (Fig. 1). During IOPs along flight track A, low-level flow in the valleys was southerly veering to westerly with height at the flight level. Terrain within the valley east of the Eastern Salmon River Range slopes upward significantly northward normal to the flight track so that southerly flow in the valley has a strong upward orographic component near the location of the observed maximum SLWC. The 75th-percentile value of SLWC increased from 0.02 g m−3 over the valleys to 0.14 g m−3 near the terrain peak (Fig. 10c). The 95th-percentile SLWC maxima associated with the fixed wave over the Western Salmon River Range was 0.3 g m−3. The 75th-percentile value of Nt,CDP increased from 11 cm−3 over the valleys to 22 cm−3 over the Western Salmon River Range Ridge (Fig. 10c). The 95th-percentile Nt,CDP maxima, 43 cm−3, was also associated with the fixed wave over the Western Salmon River Range. Over peaks such as the North Fork Range and Squaw Butte Ridge, the 95th percentile of Nt,CDP peaked between 25 and 40 cm−3.
Flight track B included 126 flight legs on 11 research flights. Along B, there were 81 757 one-Hz CDP samples. Of those, 72.3% had no SLW present, while 27.7% had SLW present. The fixed updraft and downdraft structure again aligned with terrain features with maximum vertical motion magnitudes of 0.3–0.5 m s−1 near the surface and decreasing with altitude (Fig. 11a). As with track A, SLWC and Nt,CDP distributions and magnitudes mirrored the strongest vertical motions over the highest terrain with the largest magnitudes over the Western Salmon River Range. Even smaller ridgelines showed bell-shaped SLWC and Nt,CDP distributions with Nt,CDP and SLWC peaking along terrain ridges albeit with smaller magnitudes. The 75th-percentile SLWC approached 0.05 g m−3, and 95th-percentile SLWC approached 0.30 g m−3 over the Western Salmon River Range ridge (Fig. 11b). The 75th-percentile Nt,CDP increased from 6 cm−3 over the valleys to ∼22 cm−3 over smaller terrain peaks and 30 cm−3 over the Western Salmon River Range ridgeline (Fig. 11c). These analyses show that SLWC and Nt,CDP magnitudes are strongly related to orographic forcing over the higher terrain.
Figures 12a and 12b compare Nt,CDP and Figs. 12c,d compare SLWC to UWKA gust probe vertical motions measured at 1 Hz and WCR data (averaged at to 1 Hz over a ±300-m distance above and below the aircraft flight level). Nt,CDP and SLWC were normally distributed about 0 m s−1 over the complex terrain of the Payette River basin. This was not surprising given that SLW occurred more frequently over ridgelines associated with fixed orographic waves.
(a) 2D histogram of w measured by the gust probe vs CDP SLWC binned every 0.01 g m−3 and 0.1 m s−1. (b) As in (a), but for mean radar derived w in a 600-m layer around the aircraft centered at flight level. (c) As in (a), but for Nt,CDP (cm−3). (d) As in (b), but for Nt,CDP (cm−3).
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
d. Relationship of drop size distributions to fixed updrafts
Mean size distributions were created along flight track A in order to examine how these varied within the fixed updrafts and downdrafts present over the Western Salmon River Range where terrain-forced flow generated the strongest fixed vertical motions (Fig. 13a). In this case, 6540 one-Hz periods with Nt,CDP > 5 cm−3 were available within the fixed updraft (west of −115.78°) and 5679 1 Hz samples within the fixed downdraft (east of −115.78°). Figure 13b shows composite cloud droplet size distributions upwind and downwind of the terrain peak. Both distributions were approximately the same in terms of concentration and shape with bimodal distributions. These resulted from compositing narrower size distributions centered around 10 μm during some events, and wider size distributions centered at 20 μm sampled during other events. Relatively few larger cloud droplets were sampled within the fixed wave over the Western Salmon River Range. Boxplots in Fig. 13c show that the typical spread in concentration in both the fixed updrafts and downdrafts closely mirrored one another in magnitude across all size bins. The upwind–downwind symmetry in both the SLWC and cloud droplet size distributions in Figs. 12 and 13 suggest that the supercooled water condensed on the upwind side largely evaporated on the downwind side, rather than being converted to ice through nucleation or riming.
(a) Composited WCR w associated with the fixed updrafts and downdrafts over the Western Salmon River Range crest (−115.758°); (b) mean size distribution in the updraft (red line; between −115.85° and −115.758°) and downdraft (blue line; between −115.758° and −115.685°); (c) range of size distributions represented by boxplots where red boxplots represent distributions upwind of the mountain crest and blue boxplots represent distributions downwind of the mountain crest.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
e. Potential for supercooled water above and below aircraft altitude in fixed updrafts
Measurements of SLW were only available at aircraft altitudes during the flights. To estimate the likelihood of supercooled water production at other altitudes, air parcel trajectories associated with fixed updrafts and downdrafts were calculated every 100 m in altitude between 2.5 and 8 km (Figs. 14a,e) to determine the vertical displacement of air associated with fixed vertical motions. The trajectories were calculated using the mean wind component along cross sections A and B averaged from all rawinsondes during the IOPs composited in Figs. 10 and 11, respectively. Mean wind speeds increased from ∼5 m s−1 near the surface to 45 m s−1 at 10 km along flight track A and from ∼5 m s−1 near the surface to 40 m s−1 near cloud top along flight track B (Figs. 14c,g).
(a) WCR composites of w along flight track A with airmass trajectories every 100 m in altitude superimposed. The trajectories were calculated using the composite w field and mean wind component parallel to the cross-section direction calculated using all rawinsondes launched at Crouch and Lowman during flights along track A. (b) Maximum vertical displacement of each trajectory shown in (a). (c) Mean mixing ratio (red) and mean rawinsonde-measured wind component parallel to cross section (black) during flights along track A. (d) Vertical displacement of selected air parcels along track A. (e)–(h) As in (a)–(d), but for flight track B.
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
The total vertical displacement of air parcels along the trajectories are shown for parcels starting at 3, 4, 5 and 8 km on the west side of the cross section in Figs. 14d and 14h. The successive ridges were close enough that the air parcel trajectories experienced multiple ascents and descents as they traversed through the mountain waves over the terrain. Along both flight tracks A and B, air parcels lower in the clouds (<5 km MSL) experienced larger vertical displacements because they experienced the strongest updrafts and downdrafts and because they resided in the vertical circulations for longer periods of times due to weaker wind speeds closer to the terrain. The vertical displacement associated with the waves over the terrain decreased from a maximum of 360 m for parcels starting at 3-km altitude to ∼100 m at 8-km altitude. The largest vertical displacement occurred over the Western Salmon Range at low levels where fixed updraft and downdraft magnitudes were greatest (Figs. 14d,h). Figures 14c and 14g show vertical profiles of the average mixing ratio during the flight legs. The higher mixing ratios, together with the largest vertical air parcel displacements at lower altitudes, suggest that SLW production was most likely to be produced at low altitudes close to the terrain slope, consistent with earlier findings using slantwise radiometric measurements over northern Colorado’s Park Range (Rauber et al. 1986). Conversely, except in cloud-top generating cells (Rauber and Grant 1986; Plummer et al. 2014; Tessendorf et al. 2023, manuscript submitted to J. Atmos. Sci.), the presence of supercooled water at altitudes above the aircraft was less likely, since the vertical parcel displacements were smaller, the mixing ratios lower, and the temperatures colder.
f. Relationship of Nt,CDP and SLWC to transient updrafts
SLWC and Nt,CDP magnitudes and distributions varied along individual flight tracks as a result of vertical motions associated with local variations in instability and shear within passing weather systems (Zaremba et al. 2022b). Figures 10e, 10f, 11e and 11f show that when only nonzero SLWC samples are averaged, SLWC and Nt,CDP appears more uniformly distributed over the flight track with little discernable terrain dependence. SLWC and Nt,CDP peaks no longer directly correspond to mountain peaks and their associated fixed orographic waves. Median SLWC values (excluding zeros) along flight tracks A and B typically ranged from 0.05 to 0.1 g m−3, with 75th-percentile SLWC values approaching 0.1–0.35 g m−3, and 95th-percentile values reaching a maximum of 0.45 g m−3 (Figs. 10c and 11c). Median Nt,CDP along flight tracks A and B typically ranged from 15 to 25 cm−3 with 75th-percentile Nt,CDP values approaching 25–55 cm−3 and 95th-percentile values reaching a maximum of 100 cm−3 (along flight track B). In this section we examine case studies where transient updrafts were dominant features over the terrain and directly affected the distribution and magnitude of SLWC over the terrain.
g. IOP12: Elevated convection
Ten research flight legs were flown along flight track A during IOP12 between 2013 and 2303 UTC 7 February 2017. The UWKA flew at altitudes between 3.4 and 4.5 km, sampling elevated convection in a strongly sheared, potentially unstable layer over the terrain. Based on the soundings, the elevated convection originated within a potentially unstable layer between 4.5 and 6 km where equivalent potential temperature decreased with height. The equilibrium level from the sounding conformed closely to the top of the convective cells on radar (see Zaremba et al. 2022b, Fig. 12). During this flight, Ze together with rawinsonde measured temperatures showed that CTTs ranged between −3.4° and −39.4°C. The flight-level temperatures ranged between −5.5° and −15.2°C. Elevated convective cells were typically 3–4 km deep, often protruding above the surrounding low-level cloud (Fig. 15a), and had vertical motion magnitudes on the order of −4 to 5 m s−1 (Fig. 15b). The updraft structures were transient, varying substantially from flight leg to flight leg in terms of depth, spatial coverage, and magnitude as convective cells formed and were transported across the mountain ranges. Maximum values of SLW sampled by the UWKA within cells ranged from 0.2 to 0.4 g m−3 with maximum Nt,CDP values ranging from 10 to 30 cm−3 (Figs. 15c,d).
Elevated convection observed between 2050:00 and 2059:30 UTC 7 Feb 2017. (a) Ze. (b) WCR retrieved w. (c) w measured by the gust probe at flight level (black line) and CDP SLWC observed at flight level (red line; g m−3). (d) Nt,CDP (cm−3).
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
h. IOP22: Cloud-top generating cells
During IOP22, the UWKA sampled a split cloud layer. The lower cloud layer exhibited weak instability near cloud top that manifested in a series of cloud-top generating cells. During this IOP, eight research flight legs were flown between 1422 and 1646 UTC 9 March 2017 along flight track A. Tessendorf et al. (2023, manuscript submitted to J. Atmos. Sci.) provide a thorough analysis of the generating cells in this and other cases during SNOWIE. Here, we limit our discussion to the supercooled water measurements. The UWKA porpoised up and down through the top 2 km of cloud top in order to sample variations in SLWC, Nt,CDP, and ice particles within the cloud-top generating cells and their subsequent reflectivity plumes. The measured temperature at flight level varied between −8.7° and −16.1°C. Within the generating cells, WCR updrafts had vertical motion magnitudes on the order of −2 to 2 m s−1 typical of generating cells in other environments (e.g., Plummer et al. 2014, 2015; Rosenow et al. 2014). Figure 15 shows an example of cloud-top generating cells observed between 15:35:40 and 1546:30 UTC 9 March 2017. The UWKA flew downward from ∼200 to ∼400 m below cloud top between 1535:40 and 1539:00 UTC before rising out of cloud. During this time, SLWC was widespread with SLWC > 0.2 g m−3, approaching 0.4 g m−3 in isolated pockets, and Nt,CDP ranged from 7 to 56 cm−3. The UWKA reentered cloud top at 1540:15 UTC and descended to ∼1 km below cloud top at 1542:30 UTC. SLWC peaked at 0.36 g m−3 before decreasing to 0.01 g m−3 beneath the generating cells, while Nt,CDP peaked at 55 cm−3. Between 1542:00 and 1544:00 UTC a fixed wave was sampled over the terrain associated with the Western Salmon River Range but appeared to have only a minor effect on the SLWC magnitudes, which appeared to be forced by the transient updrafts associated with the generating cells. Nt,CDP peaked at 80 cm−3 within the fixed wave beneath cloud top but SLWC in this region was less than 0.05 g m−3. As the UWKA increased in altitude and flew up and out of cloud SLWC and Nt,CDP again increased closer to cloud top with SLWC magnitudes reaching 0.38 g m−3 and Nt,CDP magnitudes near ∼50 cm−3.
i. IOP20: Gravity waves
Gravity waves not fixed to the underlying topography were observed during IOP20 (1200–1630 UTC 5 March 2017) ahead of an advancing cold front along flight track B (Fig. 16). The gravity waves appeared as a wave packet with uniform wavelength along part of the sampled cross section. Maximum updrafts and downdrafts within the gravity wave layer, between 4 and 6 km, were on the order of 2–3 m s−1 but increased to 6 m s−1 later in the flight as the wavelength of the wave decreased. The gravity waves also appeared to induce vertical motions above the gravity wave layer although these vertical motions (ranging up to 3–4 m s−1) appeared more chaotic and turbulent in nature. During this flight, CTTs were between −22.2° and −56.1°C and the measured flight-level temperatures ranged between −13.4° and −21.5°C. The UWKA sampled the wave on one flight leg between 1212:40 and 1222:35 UTC. On subsequent flight legs, the UWKA was flying beneath the waves. Within the waves, the gust probe measured vertical motions oscillating between −2 and 2 m s−1 (Fig. 17c). SLWC and Nt,CDP magnitudes followed a similar wavelike pattern peaking at the downwind edge of the updrafts at 0.5 g m−3 and 57 cm−3, respectively, and then decreasing to near zero values at the downwind edge of the downdrafts. Outside of the waves, SLWC ranged from 0 to 0.2 g m−3, with Nt,CDP ranging from 0 to 25 cm−3. Higher values associated with weak background turbulence were also sampled by the UWKA (Figs. 17c,d). Between 1218:00 and 1220:00 UTC there was also a broader region with higher Nt,CDP (∼35–45 cm−3) yet low SLWC (<0.2 g m−3) compared to the waves (Figs. 17c,d). The SLW showed no evidence of being tied to fixed terrain-induced updrafts along the flight leg.
Cloud-top generating cells observed between 1535:40 and 1546:30 UTC 9 Mar 2017. (a) Ze. (b) WCR retrieved w. (c) CDP SLWC observed at flight level (g m−3); gust probe vertical motions were unavailable during this leg. (d) Nt,CDP (cm−3).
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
A gravity wave observed between 2050:00 and 2059:30 UTC 7 Feb 2017. (a) Ze. (b) WCR retrieved w. (c) w measured by the gust probe at flight level (black line) and CDP SLWC observed at flight level (red line; g m−3). (d) Nt,CDP (cm−3).
Citation: Journal of Applied Meteorology and Climatology 62, 10; 10.1175/JAMC-D-23-0080.1
Case studies of transient updrafts revealed that SLWC and Nt,CDP were associated with individual transient updraft features like elevated convection, cloud-top generating cells, and gravity waves. In some cases, SLW was advected out of the updraft into an adjacent downdraft where evaporation occurred. Transient updrafts on the order of 1–5 m s−1 were commonly observed compared to the fixed updrafts on the order of 0.1–0.5 m s−1. When analyzing composited non-zero SLWC samples, the distribution of SLWC and Nt,CDP appeared more uniform over the flight track, indicating that terrain dependence was minimal due to the time and spatially varying nature of transient updrafts associated with passing weather systems over the terrain.
5. Conclusions
This paper examined the controls on supercooled liquid water content (SLWC; measurements that were acquired at flight levels where temperatures were less than 0°C and hence all cloud water was assumed to be supercooled) and cloud drop number concentrations (Nt,CDP; cloud droplets with diameters between 2 and 50 μm) over the Payette River basin and Salmon River Mountains of Idaho observed during the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE). A comparison of three SLWC probes (Gerber, Nevzorov, and CDP) on 23 research flights found that the probes had similar SLWC magnitudes. The cloud droplet probe was chosen for additional analysis of microphysical properties because it provided information on droplet number concentrations and droplet size spectra (between 2 and 50 μm). During SNOWIE, 27.4% of 1 Hz in situ CDP samples (48 429 out of 176 930) were in an environment containing SLW (where Nt,CDP, > 5 cm−3). The interquartile range of SLWC (excluding zeros) was found to be 0.02–0.18 g m−3 and 13.3–37.2 cm−3 for droplet concentrations, with the most extreme values reaching 0.40–1.75 g m−3 and 150–320 cm−3 in isolated regions of convection and strong shear-induced turbulence on some flights. SLWC and Nt,CDP distributions were shown to be directly related to cloud-top temperature (CTT) and ice particle concentrations, consistent with past research over other mountain ranges. For all flight legs, 19.4%, 7.4%, and 6.7% of samples with CTTs between −20° to −25°C, −35° to −40°C, and −50° to −55°C, respectively, had SLWC exceeding 0.1 g m−3 (see Fig. 6).
Two classes of vertical motions were analyzed as potential controls on SLWC and Nt,CDP, the first forced by the orography and fixed in space relative to the topography (stationary waves), and the second transient, triggered by vertical shear and instability within passing synoptic-scale cyclones. Composites of vertical motion from all flight legs along fixed tracks were constructed using the approach outlined in Zaremba et al. (2022b). Transient vertical motions varied in time and space, so averaging vertical motions over cross sections from a large number of flight legs effectively removed the contribution of transient updrafts while retaining vertical circulations tied to the terrain. Composited fixed updraft fields were then compared to distributions of SLWC and Nt,CDP along two flight tracks. SLWC occurrence and magnitudes, and cloud droplet concentrations associated with fixed updrafts were found to be normally distributed about ridgelines when SLW was present. Over ridgelines, the 75th- and 95th-percentile values of SLWC over the highest terrain, including zeros when no SLW was present, was 0.04 and 0.32 g m−3, while the 75th and 95th percentiles of Nt,CDP were 20 and 40 cm−3. Airmass trajectories over the Payette River basin were also run through fixed vertical motion fields. The largest vertical air parcel displacements near the terrain, together with the highest mixing ratios at lower altitudes, implied that SLW production was most likely at low altitudes close to the terrain slope associated with fixed waves.
Case studies showing elevated convection, cloud-top generating cells, and nonorographic gravity waves were also explored. These showed regions of SLWC and Nt,CDP associated with individual transient updraft features. In some cases, SLW was advected out of the updraft into an adjacent downdraft where evaporation occurred. When considering only composited non-zero SLWC samples, SLWC and Nt,CDP appear more uniformly distributed over the flight track with little discernable terrain dependence as a result of time and spatially varying transient updrafts associated with passing weather systems. SLWC (excluding zeros) across the terrain had 75th-percentile values of 0.10–0.35 g m−3 and a 95th percentile of 0.45 g m−3. The 75th-percentile Nt,CDP (excluding zeros) across the terrain ranged from 25 to 55 cm−3 with 95th percentile Nt,CDP approaching 100 cm−3. To investigate the relationship between airmass source regions and drop number concentrations over the Payette River basin, 72-h HYSPLIT back trajectories were run to determine if air parcels interacted with the boundary layer prior to arrival over the basin. This analysis showed that trajectories approaching the basin from the west-southwest had lower median droplet concentrations than those passing over the coastal terrain directly to the west of the Payette River basin. These trajectories underwent more appreciable ascent and may have had lower droplet concentrations due to scavenging of aerosol associated with precipitation over upwind ranges.
Field campaigns like SNOWIE are crucial to improving our conceptual understanding of SLW distributions over complex terrain because of its importance to successful cloud seeding. The results indicate that aircraft seeding is more likely to be successful in shallower clouds with warmer cloud-top temperatures. These clouds are likely to have SLW in generating cells near their tops, and/or also may have SLW associated with transient features.
The SLW associated with fixed waves over the orography is more difficult to target with aircraft because of the proximity of the SLW to the terrain. For ground-based seeding, placing generators on the upwind ridges would likely target the low-level SLW occurring within the mountain waves downstream. The likelihood of SLW associated within these waves decreases with cloud depth and increasing natural ice particle concentrations, so again shallower clouds would be more favorable targets. Although transient updrafts have minimal impact on particle trajectories generated by seeding (Heimes et al. 2022), it would be difficult to target them during airborne cloud seeding because of their time and spatially varying nature. They could, however, be targeted with continuous ground-based seeding provided that seeding material would loft into clouds. More physical process and modeling studies are needed to better understand SLWC and Nt,CDP magnitudes and distributions relative to fixed and transient vertical motions observed over the terrain, especially over other mountain ranges throughout the intermountain west as this would improve targeting for orographic cloud seeding operations and increase the likelihood of seeding success.
Acknowledgments.
We thank the crew from the University of Wyoming King Air (UWKA) as well as all students from the Universities of Colorado, Wyoming, and Illinois for their help operating and deploying instruments during the campaign. Funding for the UWKA and WCR during SNOWIE was provided through the National Science Foundation (NSF) Award AGS-1441831. This research was supported by NSF Grants AGS-1547101, AGS-1546963, AGS-1546939, AGS-2016106, AGS-2015829, and AGS-2016077. We also thank Dr. Tony Lyza and two anonymous reviewers for comments that helped substantially improve the quality of the paper.
Data availability statement.
All data presented here are publicly available through the SNOWIE data archive website (https://data.eol.ucar.edu/master_lists/generated/snowie/) maintained by the Earth Observing Laboratory (EOL) at the National Center for Atmospheric Research (NCAR). Individual instrument DOIs are as follows: Wyoming Cloud Radar Data: https://doi.org/10.15786/M2CD4J. Nevzorov Corrected Data: https://doi.org/10.26023/2QRK-XSBA-RS0P. University of Wyoming King Air Flight Level Data: https://doi.org/10.15786/M2MW9F. University of Wyoming King Size Spectra: https://doi.org/10.5065/D6GT5KXK. University of Colorado-Boulder Microwave Radiometer Data at Smith’s Ferry: https://doi.org/10.5065/D6W957WJ
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