1. Introduction
The low-level jet (LLJ) is a strong and narrow airstream typically observed within the planetary boundary layer (PBL) (Stull 1988). The LLJ wind speed profile usually takes the shape of a nose, namely, a maximum with a fast decay of the wind speed with height both below and above it. Deviations from this “canonical” shape have been observed in the literature, consisting of wind speed profiles with two noses simultaneously observed along the vertical. These “atypical” LLJs will be referred to as double-nosed LLJs in the current study, although the term multiple LLJ is sometimes used when more noses are present (e.g., Banta et al. 2002; Tuononen et al. 2017).
Despite no field campaigns have been conducted to specifically study double-nosed LLJs, several investigations have reported on their occurrences, mostly in flat and rarely in complex terrain. Typically observed over the Great Plains of the United States, nocturnal double-nosed LLJs developing in flat terrain mostly occurred within the first 400–600 m AGL (Banta et al. 2002; Pichugina et al. 2007; Banta 2008), with sporadic cases within 1300–1600 m AGL (Klein et al. 2015). Their frequency has been rarely reported, with Pichugina et al. (2007) recording double-nosed LLJs in up to the 15% of the total number of measured profiles within a single night. Double-nosed LLJs have been also observed over southern China (Du et al. 2012; Du and Chen 2018, 2019a,b; Zhang and Meng 2019), where an LLJ below 1 km AGL coexisted with a synoptically driven LLJ within 1–3 km AGL, and in diurnal profiles over flat plains (Hoecker 1963) and coastal areas (Tuononen et al. 2017). In complex terrain, several studies have focused on the western United States finding double-nosed LLJs within the first 700–1000 m AGL (Banta et al. 2004; Savage et al. 2008), with the first nose close to the surface, at 50 m AGL according to Savage et al. (2008).
Existing literature has delved into the mechanisms that drive the LLJ formation within the PBL (e.g., Stull 1988; Stensrud 1996), with inertial oscillations being the most frequently observed under stable stratification, both in flat and complex terrains. Other mechanisms can be fronts and baroclinic weather patterns in flat terrain, orographic (wind splitting, ducting, or confluence around mountain barriers) and thermal effects (mountain and valley winds) in complex terrain. Few studies also delved into the mechanisms leading to a modification of the LLJ profile, but the formation of multiple noses was neither considered nor observed. For example, Sun et al. (2002) concluded that momentum transport associated with an intermittent-turbulence event altered the LLJ nose height. Similarly, Viana et al. (2009) observed a destructive interaction between the LLJ and a mesoscale buoyancy wave which was horizontally ducted and propagated in the opposite direction of the LLJ.
Conversely to LLJs, it appears that existing literature on double-nosed LLJs is limited and largely observational. No numerical or analytical studies of double-nosed LLJs have been reported so far, at least to our knowledge. Moreover, observational studies have not delved into the dynamics and formation mechanisms of double-nosed LLJs, but a few hypotheses have been suggested. Concerning the flat terrain, Banta et al. (2002) suggested (without validation) that each nose might be driven by inertial oscillations associated with horizontal pressure gradients at different spatial scales. Du et al. (2012) proposed that a synoptic forcing drove the upper nose formation at 1–3 km AGL above an LLJ driven by inertial oscillations below 1 km AGL. In the context of complex terrain, Banta et al. (2004) suggested that the intrusion of local drainage flows may force a secondary nose formation, although this hypothesis has not been further investigated.
The present study explores some of the open questions from the literature, and conducts a data analysis of recent double-nosed LLJ observations in complex terrain with the aim of improving our understanding of the double-nosed LLJ phenomenon and underlying driving mechanisms. The paper is structured as follows. Section 2 describes the double-nosed LLJ driving mechanisms proposed in the current study. Section 3 describes the measurement site, the equipment, and the data processing. It also defines the criteria to identify the double-nosed LLJs. Section 4 is devoted to the discussion of the results. Finally, section 5 draws the conclusions.
2. Double-nosed low-level jet driving mechanisms
As discussed in the following sections, nocturnal LLJs were frequently observed during quiescent nights (weak synoptic forcing) of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) experiment (Fernando et al. 2015). These LLJs were typically confined in the lowest 100–150 m and persisting through the entire night. A peculiar feature of these LLJs was the formation of an additional nose which temporarily altered the canonical shape of the LLJ profile. Hereafter, we will refer to this transient nose as a secondary nose to differentiate it from the one which instead persisted throughout the night (called primary nose). On the basis of these observations, we propose two different mechanisms induced by boundary layer processes, that can drive the formation of a transient double-nosed LLJ in nocturnal (from sunset to sunrise) stably stratified flows in complex terrain. Necessary conditions to observe such double-nosed LLJs, and for the mechanisms to explain their formation are (i) the presence of a weak-synoptic forcing, (ii) the existence of an already-established LLJ, (iii) the occurrence of a local flow perturbation, and (iv) the development of a local wave capable of vertically transporting the flow momentum (required for one mechanism only).
Given this preamble, the proposed mechanisms are named 1) wind driven and 2) wave driven. The wind-driven mechanism describes noses associated with different air masses simultaneously flowing one above the other from different directions. The double-nosed shape of the wind speed profile coincides with a sharp and pronounced variation of the wind direction between the primary and the secondary nose. Double-nosed LLJs traceable to this mechanism have already been observed in the literature (e.g., Savage et al. 2008). Conversely, the wave-driven mechanism has never been considered before. It assumes that the secondary-nose formation is driven by the vertical momentum transport carried by a wave, the latter being caused by an external flow perturbation. The occurrence of the secondary nose is always concomitant with wave activity and the wind direction describes a small and nearly constant rotation with the height within the PBL.
In this paper, we classify the observed double-nosed LLJs based on the change in wind direction with height into wave-driven and wind-driven cases, but we will only analyze the wave-driven cases in more detail. While the formation mechanism for the other cases is not discussed further, the wind-driven mechanism seems the most plausible hypothesis. In absence of synoptic forcing, no other mechanisms suggest a sharp wind turning rather than the intrusion of a local flow (i.e., our wind-driven mechanism). For example, the presence of waves in the PBL can induce a secondary nose but do not explain a sharp wind turning. Nevertheless, a robust verification of such a hypothesis would have required additional measurements and analyses beyond the scope of this investigation.
Conceptual figure illustrating the wave-driven mechanism. (a) canonical LLJ, (b) wave generation, (c) secondary-nose formation, (d) secondary-nose dissipation. The green lines represent hypothetical streamlines of wind speed, the red lines the wind speed profile, the blue arrow the phase velocity of the wave, the gray triangles a generic sloping terrain, and the gray shading in the background the surrounding topography.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
3. Data collection and processing
a. Measurement site and equipment
To investigate the driving mechanisms of double-nosed LLJs in complex terrain, a suitable dataset has been extracted from the MATERHORN data repository (https://data.eol.ucar.edu/master_lists/generated/materhorn-x/). This dataset includes measurements collected over Dugway Valley (40.121 360°, −113.129 070°), Utah (marked with the black box in Fig. 2), where nocturnal LLJs were frequently observed during both fall (September–October 2012) and spring (May 2013) campaigns. Dugway Valley is approximately 30-km wide (from southwest to northeast) and 40-km long (from southeast to northwest). The valley floor is a gentle-sloping terrain (0.06°) at 1300 m above the mean sea level, characterized by arid soil and desert shrub. Dugway Valley is bounded by Granite Peak to the west (840 m AGL) and Dugway Range to the south (a mountain chain with a maximum altitude of 770 m AGL), the two of them separated by a 5-km gap.
Measurement site of the current study. The black rectangle indicates Dugway Valley, the red dot the Sagebrush site, the blue dot the radar, the pink dots the MINISAMS, the blue arrow the downvalley-flow direction (i.e., the direction of the main circulation within the valley), and the red arrows the direction of secondary flows that can perturb the downvalley flow. The circumferences show the distances from Sagebrush (10 km: yellow; 20 km: orange) the gray lines the cardinal points.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
In the present study, we analyze data collected during quiescent intensive observing periods (IOPs; listed in Table 1), characterized by wind speed at 700 hPa smaller than 5 m s−1 (Fernando et al. 2015). These quiescent IOPs are negligibly affected by synoptic forcing, aiding the development of a nocturnal stable boundary layer and the formation of a downvalley flow from southeast to northwest (blue arrow, Fig. 2) as the main circulation within Dugway Valley. This thermal circulation drives the early-evening development of the LLJ while being progressively superimposed by the inertial oscillations that regulate the subsequent nocturnal evolution of the LLJ up to the sunrise. This is argued in the complementary work by Barbano et al. (2021) where, after the initial growth of the nocturnal boundary layer, the observed LLJ evolution is indeed well reproduced by the analytical model of Van De Wiel et al. (2010), which calculates the horizontal wind field as inertial oscillations around the nocturnal equilibrium profile (represented by the Ekman spiral). However, complex terrain (unlike flat terrain) is intrinsically characterized by multiple flows interactions that perturb the main valley circulation. In our specific case, downslope flows from the surrounding mountains (red arrows, Fig. 2) may intrude the Dugway Valley and alter the LLJ circulation, by triggering the generation of waves that are able to redistribute momentum and energy, and modify the local atmospheric conditions (Sun et al. 2015b).
Quiescent IOPs analyzed and respective operational period. The local time is MDT = UTC − 6 h.
To analyze the LLJ vertical structure and detect the presence of multiple wind maxima, this study uses data collected from a DigiCORA tethered-balloon system (TTS111, Vaisala, Helsinki, Finland; Fernando 2017) deployed at the Sagebrush site (i.e., at the center of the valley, Fig. 2) during each IOP. This balloon allowed the retrieval of multiple vertical profiles of wind speed and direction, temperature, relative humidity, mixing ratio, and pressure (from which the height is derived using the hydrostatic equation) through an intensive sequence of ascents and descents during each IOP. Ascents of approximately 20 min were collecting data up to 400 m AGL with a vertical resolution of 1 m. Measurements collected during balloon descents (lasting approximately 10 min) are discarded as affected by large uncertainties caused by the restoring mechanism of the balloon, bringing the instrumentation to the ground through an uneven and yanking path.
Each tethered-balloon ascent is preliminary checked to remove nonphysical values. Except for the wind direction, we use nonaveraged profiles to avoid losing information on the narrower noses oftentimes observed at the surface. This choice is motivated by the coherency of each tethered-balloon measurement with the height. In fact, each quantity has a smaller vertical variation than the respective instrumental uncertainty, which ensures that using nonaveraged data will not affect the double-nosed LLJ identification method defined in section 3b. Vertical averages are only used to deal with multiple measurements collected at the same height, caused by small vertical fluctuations (of order 1 m) of the balloon oftentimes observed at the takeoff from the ground. For the wind direction only, the 5-m mode was applied by counting the number of the wind-direction data belonging to each 10° bin the wind rose is divided into. Finally, no time average is applied (following Baas et al. 2009) as the tethered balloon already captures a sequence of snapshots of the mean state of the boundary layer.
Balloon measurements were complemented with ground-based observations to better characterize the near-surface valley circulation. These included data from the Surface Atmospheric Measurement Stations Mini Network (MINISAMS; Pace 2016), a permanent array of fifty-one 10-m towers evenly distributed within the valley area (pink dots, Fig. 2). Each tower is equipped with vane anemometers (05103, R. M. Young, Traverse City, Michigan) and temperature and relative-humidity probes (HMP45C-L, Campbell Scientific, Logan, Utah) at two vertical levels (2 and 10 m). Data from MINISAMS were stored at 1-min average, and further averaged over 5-min intervals to align with the flux-tower measurements. Nearby the balloon launching site (red dot, Fig. 2), a 20-m flux tower (Pace et al. 2017) was fully instrumented with three-axis sonic anemometers (CSAT3, Campbell Scientific) coupled with temperature and relative-humidity probes (HMP45C-L, Campbell Scientific) placed at five levels (0.5, 2, 5, 10, and 20 m) for continuous measurement of turbulent and mean meteorological variables. Temperature and relative-humidity data were sampled at 1 Hz, while sonic-anemometer data at 20 Hz. The flux-tower dataset is preliminary checked to detect and remove nonphysical values or instrumental failures. Following the method by Hojstrup (1993), a despiking procedure is applied over a 5-min data interval (Vickers and Mahrt 1997) to replace outliers with the nearest finite values in the time series. The despiked wind components are rotated to align the wind vector to the mean streamline direction (McMillen 1988) and then averaged on 5-min intervals. Selected subsets are averaged on smaller intervals to investigate specific processes.
To observe the possible occurrence of mesoscale-flow perturbations, data collected using a frequency-modulated continuous-wave (FMCW) radar (Pace 2017) were also analyzed. This radar was located 9 km to the north-northwest of Sagebrush site (blue dot, Fig. 2), and equipped with a bistatic parabolic antenna which allows collecting data up to 6 km AGL using a bandwidth centered at 2.9 GHz. Data preprocessing involved the use of a phased-locked-loop digital frequency synthesizer to reduce the noise at 200 MHz.
b. Identification criteria for double-nosed low-level jets
Regarding maxima, we adopt the absolute threshold proposed by Banta et al. (2002) for Umax < 7.5 m s−1, and the relative threshold by Baas et al. (2009) for Umax ≥ 7.5 m s−1. Regarding minima, we adopt a three-condition method. The threshold of 1.0 m s−1 proposed by Baas et al. (2009) is adopted for Umin > 4 m s−1, while it is reduced to 0.5 m s−1 (according to our instrumental uncertainty) for Umin < 2 m s−1. The relative threshold adopted in the range 2 ≤ Umin ≤ 4 m s−1 is obtained imposing a linear fit between the two previous conditions to guarantee that the minima identification in Eq. (9) follows a continuous piecewise function covering the whole range of wind speed.1
Sketch of a double-nosed LLJ profile that reports the main variables used to characterize a nose: Umax is the wind speed maximum, Umin the wind speed minimum below (superscript b) and above (superscript a) it, and nw the nose width defined as the difference between an upper height hu and a lower one hl, with 1 indicating the primary nose and 2 the secondary nose.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
c. Wave-motion characterization
As key components of the wave-driven mechanism, physical characteristics of the inertial–gravity waves must be delineated with precision, to investigate their role in the secondary-nose formation. The necessary accuracy is ensured by the high-frequency flux-tower measurements, enabling the observation of the wave generation driven by surface-flow perturbations and the wave propagation up to 20 m AGL.
4. Results and discussion
a. Double-nosed low-level jet classification
Among the 94 nocturnal profiles measured with the tethered balloon, 78 (the 83%) fulfill the criterion in Eqs. (8) and (9): 58 LLJs are single nosed (the 62% of the total number of nocturnal profiles) while 20 are double-nosed LLJs (the 21%). These percentages are in line with Pichugina et al. (2007), where canonical and double-nosed LLJs were observed in the 70% and 15% of the total number of profiles, respectively. The 20 double-nosed LLJs are unevenly distributed among the IOPs (see Table 2). In each double-nosed LLJ, the secondary nose is above the primary one. The primary-nose maximum is between 10 and 150 m AGL, while the secondary-nose one is between 90 and 340 m AGL. Overall, the wind speed maximum of the primary nose follows a Gaussian distribution within the range 2.9–7.0 m s−1; in 15 cases, the wind direction is in the range 90° ≤ Φ ≤ 180°. The nose width oscillates in the range 6 ≤ nw ≤ 77 m. The secondary-nose variability is typically larger than the primary one. The wind speed maxima are in the range 3.1–9.3 m s−1, following a Gaussian distribution skewed toward the smallest edge. For this reason, the secondary-nose maxima can be either larger (observed 9 times) or smaller (11 times) than the primary one. The wind direction also shows a larger variety, being 9 times in the range 180° ≤ Φ ≤ 270°, 7 within 90° ≤ Φ ≤ 180°. Finally, the nose width oscillates in the range 21 ≤ nw ≤ 177 m.
Double-nosed LLJ profiles identified in MATERHORN data during quiescent nights. The detection time and period corresponds to the sounding takeoff time if the double-nosed LLJ is observed in a single profile. The detection time and period corresponds to the takeoff times of the first and last soundings enclosing the observation period if the double-nosed LLJs are observed in consecutive profiles. No. is the total number of consecutive ascents that shows the same double-nosed LLJ event; Φ1 and Φ2 are the mode and associated standard deviations of wind direction within the primary and secondary noses, respectively, as delimited by hl,i and hu,i (i = 1, 2) defined in Eqs. (10) and (11); |ΔΦ| = |Φ2 − Φ1| is the wind direction difference between the noses and associated error. Double-nosed LLJs associated with cases A, B, and C are classified as wave driven.
Among the observed double-nosed LLJs, 15 profiles are classified as wind driven and 5 as wave driven, considering the wind-direction difference between primary and secondary noses as a sole discriminant. As defined in section 2, the wave-driven double-nosed LLJs are characterized by a small and nearly constant rotation of the wind direction with the height. The result is a secondary nose directed approximately as the primary one, as highlighted in Table 2 where the values of |ΔΦ| are always smaller than the associated errors during the wave-driven events. Conversely, the wind-driven double-nosed LLJs show values of |ΔΦ| of at least 47°, but typically above 80°. An example of double-nosed LLJ identified as wind driven is given in Fig. 4. The sharp and sudden wind-direction variation (88°) creates a discontinuity at the base of the secondary nose, providing a modification in the LLJ dynamics. Compared to the primary nose, the secondary one shows similar wind speed maximum and potential-temperature gradient evolving into isothermal conditions in the atmosphere above the nose. These observations support the hypothesis of an upper-airflow intrusion while excluding wave/turbulence activity and expected rotations with the height (e.g., the Ekman spiral) as driving mechanisms. Nevertheless, additional upper-air measurements and analyses would be required to investigate this secondary flow in the surrounding of the tethered-balloon site, leaving the wind driven as a plausible suggestion.
Example of (a) wind speed, (b) wind direction, and (c) potential-temperature profiles during the first wind-driven double-nosed LLJ event observed within the IOP0 (0913–1016 UTC).
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
Concerning the wave-driven profiles, three events have been observed: case A during IOP4, cases B and C within IOP8. Both cases A and B show a single double-nosed LLJ profile, named A-2 and B-2, respectively. During case C, three double-nosed LLJ profiles are consecutively measured, namely, C-2a, C-2b, and C-3. We adopt a unique nomenclature for the first two double-nosed LLJs of case C because they share similar flow characteristics. Figures 5 and 6 show the evolution of wind speed, wind-direction, and potential-temperature profiles for each wave-driven double-nosed LLJ from immediately before its occurrence (A-1, B-1, and C-1) to its dissipation (A-3, B-3, and C-4) in which the unperturbed LLJ is restored. Table 3 lists the main characteristics of the wave-driven double-nosed LLJs. During cases A and B, the dissipation is observed after approximately 30 min. During case C, primary and secondary noses move above 100 and 300 m AGL, respectively, passing from condition C-2b to C-3. Then the secondary-nose dissipation is observed in C-4, after approximately 90 min. The different duration of these events could be related to the different perturbations that trigger the double-nosed LLJs formation. While cases A and B will be associated with a surface perturbation (section 4c), C may be linked with a mesoscale flow and a downward wave-momentum transport from it as supported by the secondary-nose characteristics in both C-2a and C-2b. Differently from A-2 and B-2, the secondary-nose maximum is approximately 35% larger than the primary one. However, given the limited vertical extension of the tethered-balloon profiles (400 m), the hypothesis of a mesoscale perturbation as triggering mechanism will only be qualitatively investigated in section 4c.
Evolution of the wind speed, wind-direction, and potential-temperature profiles before and after (a)–(c) case A and (d)–(f) case B.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
Evolution of the (a) wind speed, (b) wind direction, and (c) potential-temperature profiles before and after case C.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
Wave-driven double-nosed LLJ characterization by the estimation of wind speed maximum Umax, height of wind speed maximum hmax, wind direction Φ, and nose width nw for both noses. The last row reports the wind direction difference Δ Φ between the noses. The reported time is the sampling period in which each profile is detected by tethered balloon in Sagebrush.
b. Wave-driven mechanism verification
The following analysis is focused on the corroboration of the wave-driven mechanism suggested in section 2. Specifically, cases A and B are analyzed to prove that an inertial–gravity wave can drive the secondary-nose formation by means of momentum transported from the primary nose.
In both cases, the appearance of the double-nosed LLJ is preceded by a reduction-shift perturbation composed of a wind speed reduction and a wind-direction shift. The first occurs at all the tower levels (Figs. 7a,c), and the second decreases with the height almost disappearing at 20 m AGL (Figs. 7b,d), suggesting that the flow perturbation is confined at the surface and cannot generate or directly contribute to the formation of the secondary nose. At the same time, this reduction-shift perturbation induces a wavelike motion (Fig. 7) that is associated with an inertial–gravity wave. Note that no significant turbulent activity is detected at this stage; thus the double-nosed LLJs formation is not driven by intermittent-turbulence events at the surface. The wave physical parameters are computed according to section 3c and listed in Table 4. The inertial–gravity waves observed during cases A and B have similar vertical phase velocity cpz, period T, and intrinsic frequency ω, in line with typical values at midlatitude (Holton 2004). The hypothesis of predominant vertical momentum transport is ensured for all three cases by the propagation angle ϕ. Despite ϕ is close to 90°, ω is buoyant-dominated, as the buoyancy term N2 cos2ϕ and inertial term
The 5-min-average time series obtained from flux-tower measurements of the (left) wind speed and (right) wind direction before and after (a),(b) case A and (c),(d) cases B and C. The red line indicates the time in which the tethered balloon starts to measure A-2 in (a) and (b), B-2 in (c) and (d). The blue lines in (c) and (d) indicate the time interval in which the tethered balloon measures C-2.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
Inertial–gravity wave characterization by flux-tower data at 5, 10, and 20 m. N is the Brunt–Väisälä frequency during the wave generation, T the period, ω the intrinsic frequency, cpz the vertical phase velocity, and ϕ the propagation angle.
The delay time dt between the concomitant observation of the perturbation–wave generation and the double-nosed LLJ observation ensures the wave has enough time to cover the distance between the ground and the secondary-nose layer. Note that this delay time is calculated considering the instant when the tethered balloon reaches the secondary-nose layer, thus adding few minutes (the exact amounts depend on the case) to the sounding time (i.e., the takeoff time). During case A, we start observing the effects of the perturbation-wave generation at Sagebrush site at 0911 UTC (Figs. 7a,b). The observation of the secondary nose in A-2 (occurring at 0938 UTC) is therefore delayed by dt = 27 min. During case B, the reduction-shift perturbation lasts 30 min from 0432 to 0502 UTC (possibly caused by the larger values of N with respect to case A), when the wave is generated (Figs. 7c,d). In this second case, the observation of the secondary nose in B-2 occurs at 0522 UTC, with a delay dt = 20 min from the wave generation. Considering the vertical phase velocity, the waves need dtpz = 10 and 12 min, respectively, in cases A and B to move from the surface to the secondary-noses height. Since the delay times dt are larger than the wave propagation times dtpz, the waves have enough time to subtract momentum from the primary noses and carry it to the atmospheric layer of the secondary ones.
To verify that the momentum carried by the wave is sufficient to justify the formation of a secondary nose, the methods proposed in section 2 are applied to cases A and B, comparing the wave momentum transported by the inertial–gravity wave with the observed bulk-momentum variation at the secondary nose occurrence. As determined in Eq. (4), the computation of the wave momentum relies on the evaluation of the wave kinematic momentum fluxes
Bulk Mb (blue) and wave Mw (red) momentum profiles for (a) case A and (b) case B, as computed using Eqs. (6) and (4), respectively.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
A wave motion is also detected during case C using sonic-anemometer data (Fig. 7c). As reported in Table 4, this wave is characterized by a larger period than cases A and B, and by a negative vertical phase velocity, accounting for downward momentum propagation. As hypothesized in section 4a, this downward propagation may be a symptom of the wave-driven mechanism triggered by a mesoscale perturbation at higher elevations. However, there is no evidence that the surface wave and the mesoscale perturbation are correlated, leaving again case C as a suggestion.
c. Flow perturbation
As hypothesized in section 2 the generation of the inertial–gravity wave is caused by a flow perturbation. In this section, we explore different perturbations as possible causes of wave generation. As measured from the MINISAMS network, the wind-velocity field at 2 m shows that the flow perturbation occurs at the surface as a shallow downslope flow intruding the valley from Dugway Range (case A) and Granite Peak (case B), altering the previously unperturbed valley circulation (Figs. 9a and 10a). At the onset of the perturbation of case A, a bending of the wind direction starts at 0855 UTC, turning the wind vectors from southeast to southwest in the proximity of Dugway Range (Fig. 9b). The downslope flow propagates northward to the valley center, involving Sagebrush at 0910 UTC (Fig. 9c). After 20 min from the perturbation onset, the downvalley direction is restored in Sagebrush at 0915 UTC (Fig. 9d). A small temperature decrease (Δθ ≈ 0.5 K) is observed during the intrusion (Fig. 9c), suggesting the downslope flow has similar temperatures to the surface unperturbed circulation in the valley.
Evolution of the temperature and wind speed fields at 2 m in Dugway Valley during case A. The fields are retrieved from the MINISAMS data averaged over 5 min. The white arrows start from MINISAMS position. The red dot indicates the position of the Sagebrush site.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
Evolution of the temperature and wind speed fields at 2 m in Dugway Valley during case B. The fields are retrieved from the MINISAMS data averaged over 5 min. The white arrows start from MINISAMS position. The red dot indicates the position of the Sagebrush site.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
Case B shows a similar evolution. The wind vectors start bending northeastward in the proximity of the southern end of Granite Peak at 0430 UTC (Fig. 10b). The perturbation propagates toward northeast (Fig. 10c) involving Sagebrush at 0445 UTC and ceases at 0455 UTC (Fig. 10d), after 25 min from its onset. During case B, the air-temperature decrease (Δθ ≈ 1.5 K) involves the entire Dugway Valley.
Since the MINISAMS did not retrieve a surface-flow perturbation before case C, the origin of this perturbation is probably not local nor surface driven. In this case, the formation of the double-nosed LLJ may be induced by a mesoscale-flow perturbation, as suggested by the backscatter signal measured by the radar, showing an oscillatory motion around 1400 m from 0810 UTC (Fig. 11). This perturbation starts propagating downward at 0850 UTC, possibly inducing the flow oscillation reported at 600 m between 0915 and 0930 UTC. This flow oscillation fits with the first observation of the double-nosed LLJ in C-2a, providing a suggestion for the wave generation. Moreover, it would also explain the large intensity of the secondary noses as they would directly scale with the mesoscale flow. However, the evidence provided by radar data is not sufficient to evaluate the characteristics of the wave. Supplementary data would have been necessary to detail the mesoscale flow to verify the downward momentum transport as the driven mechanism of the double-nosed LLJ during case C.
Continuous time–height display of the returned-signal-relative amplitude measured by the FMCW radar before and during case C. The red line highlights the mesoscale flow perturbation, and the yellow line the flow oscillation during C-2.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
5. Conclusions
This paper presented an in-depth observational analysis of double-nosed LLJs developing within the PBL, where a transient wind speed nose (secondary nose) appears above the LLJ maximum (primary nose). The analysis is carried out using the comprehensive dataset collected during the first two MATERHORN field experimental campaigns carried out in Dugway Valley, Utah, in autumn 2012 and late spring 2013. Two different mechanisms have been presented as responsible for the formation of nocturnal double-nosed LLJs, namely, wind-driven and wave-driven mechanisms. In the wind driven, the secondary nose is systematically associated with a sharp, large variation in wind direction, driven by a flow intrusion above the primary nose. In the wave driven, a nearly constant wind direction is maintained between the two noses, with the secondary one driven by the vertical momentum transport associated with a wave, the latter generated by a flow perturbation. This paper has attempted to the formalization and verification of the wave-driven mechanism.
A total of 20 nocturnal double-nosed LLJ profiles have been identified during quiescent periods and upon a careful redefinition of the LLJ identification criteria. Five of them have been classified as wave driven. During two of these events, the flow perturbation occurred at the surface as a downslope flow intruding the main circulation within the valley from the surrounding slopes. From these flow collisions, an inertial–gravity wave is generated, breaking the already-established LLJ dynamics and transporting momentum from the primary nose to a higher elevation where the secondary nose is observed. The canonical (single-nosed) shape of the LLJ is then restored after the dissipation of the secondary nose.
Further field studies could be encouraged to improve our knowledge of the double-nosed LLJs driving mechanisms. Additional observations are required to provide a robust verification and generalization of the two mechanisms proposed in the current study, especially of the wind-driven mechanism which is only hypothesized. Reinterpretation of past studies in this context may also be of use. From the pure process perspective, further studies may reveal different causes of the wave generation in the wave-driven mechanism (starting from the suggestion on the submesoscale perturbation given in the current investigation), and provide an exhaustive interpretation for the dissipation process of the secondary nose.
Acknowledgments
The research was partially funded by the Office of Naval Research Award N00014-11-1-0709, Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program.
Data availability statement
The dataset analyzed during the current study is available in the EOL data archive (https://data.eol.ucar.edu). This dataset was derived from the following public domain resource: MATERHORN data, https://data.eol.ucar.edu/master_lists/generated/materhorn-x.
APPENDIX
Estimation Method for the Vertical Velocity Component
The 30-min-averaged (a) streamwise and (b) cross-stream wind velocity components comparison between sonic anemometer data on the abscissa and tethered-balloon data (previously interpolated to obtain a regular time series) on the ordinate. Circles encompass the IOP4 night and stars the IOP8 night. Colors identify the observation levels: 2 m in red, 5 m in yellow, 10 m in purple, and 20 m in green.
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
(a) The 30-min-averaged ψS as a function of
Citation: Journal of the Atmospheric Sciences 78, 12; 10.1175/JAS-D-20-0274.1
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From an operational perspective, if Eq. (9) fails, we identify the minima in a (single-nosed) LLJ profile by visual inspection following simple rules: 1) below the nose, we impose Umin = 0 m s−1, and 2) above the nose, Umin is assigned as the smallest wind speed measured above it. In the contingency of a double-nosed LLJ profile, the previous rules are integrated with the following: 3) between two noses, Umin is assigned as the smallest wind speed between them, and 4) above the second nose, Umin is assigned as the smallest wind speed measured above it.
