Characteristics Of Low-level Jet Research Articles

Study on the Characteristics of Low-level Jets in Short-term Heavy Rainfall under the Influence of Northeast Cold Vortex in Shenyang Based on Wind Profile Radar

To enhance the understanding of the mechanisms behind heavy rainstorms influenced by the northeast cold vortex in Shenyang, we analyzed the intensity, height, and index of low-level jets during short-term heavy rainfall events from May to September between 2017 and 2021. The findings are as follows: (1) The maximum wind speed below 2 km generally follows a normal distribution, with low-level jets already present prior to approximately 30% of the heavy rainfall events. The maximum wind speed at corresponding percentiles shows a slight increase as the rainstorm approaches. However, the frequency of low-level jets decreases, and wind speeds significantly drop during the rainstorm. (2) The height distribution of low-level jets increases with altitude, reaching its lowest point 2 to 3 hours before the onset of heavy rainfall. At this time, low-level jets exhibit their strongest and most vigorous development. Combined with the warm, moist air from the southwest providing moisture and thermal energy, these conditions can trigger the onset of heavy rainstorms. (3) The hourly distribution of the low-level jet index (I) remains relatively consistent leading up to a rainstorm. As the rainstorm nears, wind speeds decrease, and the minimum height of low-level jets rises, resulting in a decrease in the low-level jet index (I).

Offshore low-level jet observations and model representation using lidar buoy data off the California coast

Abstract. Low-level jets (LLJs) occur under a variety of atmospheric conditions and influence the available wind resource for wind energy projects. In 2020, lidar-mounted buoys owned by the US Department of Energy (DOE) were deployed off the California coast in two wind energy lease areas administered by the Bureau of Ocean Energy Management: Humboldt and Morro Bay. The wind profile observations from the lidars and collocated near-surface meteorological stations (4–240 m) provide valuable year-long analyses of offshore LLJ characteristics at heights relevant to wind turbines. At Humboldt, LLJs were associated with flow reversals and north-northeasterly winds, directions that are more aligned with terrain influences than the predominant northerly flow. At Morro Bay, coastal LLJs were observed primarily during northerly flow as opposed to the predominant north-northwesterly flow. LLJs were observed more frequently in colder seasons within the lowest 250 m a.s.l. (above sea level), in contrast with the summertime occurrence of the higher-altitude California coastal jet influenced by the North Pacific High, which typically occurs at heights of 300–400 m. The lidar buoy observations also validate LLJ representation in atmospheric models that estimate potential energy yield of offshore wind farms. The European Centre for Medium-Range Weather Forecasts Reanalysis version 5 (ERA5) was unsuccessful at identifying all observed LLJs at both buoy locations within the lowest 200 m. An extension of the National Renewable Energy Laboratory (NREL) 20-year wind resource dataset for the Outer Continental Shelf off the coast of California (CA20-Ext) yielded marginally greater captures of observed LLJs using the Mellor–Yamada–Nakanishi–Niino (MYNN) planetary boundary layer (PBL) scheme than the 2023 National Offshore Wind dataset (NOW-23), which uses the Yonsei University (YSU) scheme. However, CA20-Ext also produced the most LLJ false alarms, which are instances when a model identified an LLJ but no LLJ was observed. CA20-Ext and NOW-23 exhibited a tendency to overestimate the duration of LLJ events and underestimate LLJ core heights.

Characteristics of the low‐level jets observed over Dunkerque (North Sea French coast) using 4 years of wind lidar data

AbstractThis study investigates the occurrence and characteristics of low‐level jets (LLJs) up to 200 m above sea level in the North Sea area, specifically in the southernmost part of the basin, at the entrance of the English Channel. Using a short‐range Doppler lidar installed in Dunkerque Port, on the northern coast of France, wind profiles were recorded for 4 years and analyzed statistically. LLJs were detected on more than 11,000 of the 10‐min average wind profiles (5% of time), with similar jet core height and core speed distributions as in other sites in the southern North Sea area, and a similar annual cycle. However, there were differences in the core directions and the daily cycle, with afternoon northeastly jets being the most frequent in Dunkerque, whereas southwesterly nocturnal jets were dominant in the other North Sea sites. This suggests that wind channeling in the Dover Strait is likely a major factor for LLJ formation in Dunkerque region. The study also examined the conditions of LLJ occurrence using European Centre for Medium‐Range Weather Forecast (ECMWF) reanalysis v5 and ultrasonic anemometer data, but the different types of LLJs sharing the same core direction and occurrence conditions made it difficult to quantify specific jet formation mechanisms. This is the first experimental study on the North Sea shore, and at the entrance of the English Channel, providing valuable insights into the LLJ behavior in the region.

Occurrence of Low-Level Jets over the Eastern U.S. Coastal Zone at Heights Relevant to Wind Energy

Two years of high-resolution simulations conducted with the Weather Research and Forecasting (WRF) model are used to characterize the frequency, intensity and height of low-level jets (LLJ) over the U.S. Atlantic coastal zone. Meteorological conditions and the occurrence and characteristics of LLJs are described for (i) the centroids of thirteen of the sixteen active offshore wind energy lease areas off the U.S. east coast and (ii) along two transects extending east from the U.S. coastline across the northern lease areas (LA). Flow close to the nominal hub-height of wind turbines is predominantly northwesterly and southwesterly and exhibits pronounced seasonality, with highest wind speeds in November, and lowest wind speeds in June. LLJs diagnosed using vertical profiles of modeled wind speeds from approximately 20 to 530 m above sea level exhibit highest frequency in LA south of Massachusetts, where LLJs are identified in up to 12% of hours in June. LLJs are considerably less frequent further south along the U.S. east coast and outside of the summer season. LLJs frequently occur at heights that intersect the wind turbine rotor plane, and at wind speeds within typical wind turbine operating ranges. LLJs are most frequent, intense and have lowest core heights under strong horizontal temperature gradients and lower planetary boundary layer heights.

Interactions Between the Nocturnal Low-Level Jets and the Urban Boundary Layer: A Case Study over London

Understanding the physical processes that affect the turbulent structure of the nocturnal urban boundary layer (UBL) is essential for improving forecasts of air quality and the air temperature in urban areas. Low-level jets (LLJs) have been shown to affect turbulence in the nocturnal UBL. We investigate the interaction of a mesoscale LLJ with the UBL during a 60-h case study. We use observations from two Doppler lidars and results from two high-resolution numerical-weather-prediction models (Weather Research and Forecasting model, and the Met Office Unified Model for limited-area forecasts for the U.K.) to study differences in the occurrence frequency, height, wind speed, and fall-off of LLJs between an urban (London, U.K.) and a rural (Chilbolton, U.K.) site. The LLJs are elevated (approx 70 m) over London, due to the deeper UBL, while the wind speed and fall-off are slightly reduced with respect to the rural LLJ. Utilizing two idealized experiments in the WRF model, we find that topography strongly affects LLJ characteristics, but there is still a substantial urban influence. Finally, we find that the increase in wind shear under the LLJ enhances the shear production of turbulent kinetic energy and helps to maintain the vertical mixing in the nocturnal UBL.

Characteristics of Low-level jets during 2015–2016 and the effect on fog in Tianjin

Characteristics of wind direction, wind speed, wind shear, and height of low-level jets (LLJs) are obtained and studied using the wind profile data collected by a wind-profile radar located at Xiqing station in Tianjin from 2015 to 2016. Results on the general characteristics of LLJs show that the dominate wind direction of LLJs ranges from 180° to 270° (47%), which is also the prevailing wind direction of strong LLJs (wind speed larger than 14 m s–1). The wind speeds of LLJs are mostly lower than 14 m s−1, and the occurrence proportion of strong LLJs is only 22.8%. The occurrence height distributions of LLJs are relatively uniform throughout the year except the peak at the lower heights (500–600 m). Moreover, the seasonal and diurnal variations of LLJs are investigated, respectively, and the results show that the occurrence frequency of LLJs during warm season is larger than that during cold season, which may be due to the increasing amplitude of inertial oscillations in the boundary layer. The diurnal variation of all categories of LLJs shows a pronounced a unimodal pattern, with the peak of occurrence number appearing at around midnight. In addition, the relationship between LLJs and fog events in Tianjin is investigated based on the data obtained from the surface automatic weather station (AWS) and a 255-m atmospheric boundary layer meteorological tower, since LLJs frequently occur in foggy days (almost 50%) in Tianjin. Eleven fog events accompanied by LLJs are analyzed and statistical results show LLJ plays a critical role in fog life cycle. The effect of northerly LLJs on fog is to enhance turbulent mixing in the fog layer, which is induced by wind shear associated with northerly LLJs and propagates downward to fog layer. There is a threshold relationship between the northerly LLJs and fog development, strong LLJs lead to inversion layer collapse and fog dissipation, while weak LLJs weaken the strength of inversion and atmospheric stability, which are favorable for fog development. Therefore, for fog events accompanied by northerly LLJs, there are negative correlations between fog durations and the strength of LLJs. However, the main contribution of northerly LJJs to fog is to result in fog dissipation in Tianjin, since the duration of fog with northerly LLJs is shorter than that without LLJs. The main contribution of the southwesterly LLJs are to transport water vapor to fog area and increase the humidity in fog layer. Therefore, the southwesterly LLJs are favorable for the fog formation, development and prolong the duration of fog.

Characteristics of the atmospheric vertical structure with different sea ice covers over the Pacific sector of the Arctic Ocean in summer

During the Chinese National Arctic Research Expedition in 2018 (CHINARE 2018), 84 radiosondes were released over the Pacific sector of the Arctic Ocean. Using these data, the characteristics of the atmospheric vertical structure with different sea ice covers were studied. The results showed that (1) typical temperature inversions were found in the low-level atmosphere with various characteristics over different regions. The inversions over the open water were the weakest and elevated. Oppositely, the inversions over the very pack ice zone were the strongest and located at the lowest altitude. Warm air advection, radiative cooling and surface melt together provided favorable conditions for the occurrence of an inversion over the very close pack ice zone. As a transition region, the marginal ice zone presented relatively moderate inversions that were mainly controlled by warm air advection and surface melt. (2) The upper-level jet over the very close pack ice zone was the weakest, and large differences were observed in the characteristics of low-level jets (LLJs) over different regions. With increasing sea ice, the frequency and strength of the LLJ increased and tended to enhance, respectively. Most LLJs were located below the tops of the inversions over the very close pack ice zone. Additional observation data are required to confirm that sea ice has significant impacts on the atmospheric vertical structure.

Characterizing mesoscale variability in low-level jet simulations for CBLAST-LOW 2001 campaign

A low-level jet (LLJ) event observed during a frontal passage in the 2001 Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds campaign was simulated using the Weather Research and Forecasting model (WRF). The sensitivity of the modeled LLJ characteristics, such as formation time, height and the strength of the LLJ core, to the choice of initial and boundary conditions, planetary boundary layer (PBL) schemes and vertical resolution was evaluated with a suite of diagnostic tools. The model simulations were compared against available soundings from the campaign observations as well as with surface observations from the Automated Surface Observing Systems. The simulation initialized with ERA-interim reanalysis and using the Mellor–Yamada–Nakanishi–Niino PBL scheme gave the best mix of diagnostic scores for surface temperature and wind speed predictions. The choice of boundary conditions introduced a stronger variability in the LLJ characteristics than the changes in PBL schemes or vertical resolution. The variability emerged primarily due to the timing of the frontal passage in the boundary condition datasets.

The influence of climate change on low‐level jet characteristics over the South‐Central Plains as simulated by CMIP5 models

AbstractPrevious studies have projected the Central Plains low‐level jet (LLJ) to form more frequently over the South‐Central Plains region in the future, with limited projected changes in its speed and height. This work's objective was to assess future LLJ characteristics with wind speed outputs obtained from the Phase 5 of the Coupled Model Intercomparison Project (CMIP5) suite of models. Improvements in model resolution and simulation of climate dynamics relevant to LLJs were made in CMIP5, and a multi‐model study that focuses specifically on the LLJ's future state using these models is yet to be done. Occurrences of LLJs were identified by applying an LLJ definition based on wind shear to six‐hourly vertical wind profiles derived from the model outputs. Changes in future LLJ speed, frequency, and height versus a historical control were subsequently calculated on seasonal and diurnal temporal scales. As seen in previous work, the largest expected changes in LLJ characteristics are those of its frequency, especially in summer, autumn, and at night. Evidence was also found for large increases in frequency in the northern half of the study region, consistent with previous expectations. CMIP5 models with finer horizontal and vertical resolutions tended to project greater increases in LLJ frequency than those with coarse resolutions. Projected changes in LLJ speed and height were by contrast largely insignificant, also consistent with previous findings. However, these characteristics, along with frequency, exhibited notable diurnal variability in historical and future simulations, especially in winter. An increase in cold‐season LLJ variability could have implications for the consistency of synoptically driven LLJs. This finding, along with the projected warm‐season poleward shift in the LLJ's frequency maximum over the study region, elucidates the LLJ's future state as projected by a selection of CMIP5 models, and the consequences for processes that depend upon the LLJ's consistency.

Assessment of INSAT-3D/INSAT-3DR Derived Atmospheric Motion Vectors Using Wind Profiler Observations Located at Gadanki, India

The accurate estimation of atmospheric motion vectors (AMVs) derived using sequences of images from geostationary satellites is very important because of their extensive use in improving initial conditions of the numerical weather prediction (NWP) models. To assess the quality of retrieved AMVs, validation of these winds with ground-based observation is very important. In the present study, the evaluation of AMVs retrieved using the data from geostationary satellites, viz. INSAT-3D/INSAT-3DR, has been carried out with respect to wind profiler data and radiosonde data. The wind profiler used in the present study is an L-band radar lower atmosphere wind profiler and is installed at the National Atmospheric Research Laboratory, Gadanki, located at 13.27° N, 79.10° E. This instrument measures continuous high-resolution wind measurements in the lower atmosphere. The radiosonde data available near Gadanki are used for validation. In this study, normal infrared (IR) AMV derived using 10.5–11.5 μm channel of INSAT-3D and INSAT-3DR satellites and staggering-mode IR AMV retrieved using INSAT-3D/INSAT-3DR combination are used. AMV accuracy assessment was performed for the years 2017 and 2018 by calculating statistical scores, viz. root mean square vector difference, wind speed bias and a number of matching points. In addition, the INSAT-3D/INSAT-3DR AMV and profiler wind data are also used to study the characteristics of low-level jet during active and break period of the Indian summer monsoon of 2017 and 2018. The accuracy assessment through this study shows the usefulness of AMVs for improving the initial conditions in NWP models in future.

Oklahoma's future wind energy resources and their relationship with the Central Plains low-level jet

Previous work has projected increases in wind speed/wind power density (WPD) over the United States' South-Central Plains, which could be linked to future increases in frequency of the Central Plains Low-Level Jet (LLJ). This work serves to both update projections of this region's wind energy resources, and to assess the LLJ's importance for influencing these resources. Projected changes in several wind energy metrics (including ramp event frequency and cut-off frequency) were assessed specifically over the Oklahoma Panhandle. These projections were computed by using a climatological delta method to statistically downscale changes in near-surface wind speed from multiple climate models onto observations from the Oklahoma Mesonet. The results suggest a pattern of overall reductions in wind speed and WPD, slight reductions in ramp event frequency, and little change in cut-off frequency, with projected changes often being larger and of statistical significance at night and during summer and autumn. Highly distinguishable diurnal and seasonal variability in ramp event and cut-off frequency was found in this work, with these frequencies most notably maximizing during the day. This climatological variability is consistent with that seen in observations of these wind energy metrics. Regression of LLJ characteristics against wind energy metrics identified consistent and conceptually understandable links between increased LLJ frequency and decreased ramp event frequency in future decades. Such a result attests to a more stable and easier to manage future wind energy resource over the Oklahoma Panhandle.

An Overview of Low‐Level Jet Winds and Corresponding Mixed Layer Depths During PECAN

AbstractThirty southerly low‐level jet (LLJ) events were observed during the Plains Elevated Convection at Night (PECAN) field campaign in the Great Plains region of the United States during summer 2015. Here we present Doppler lidar wind data from three PECAN instrumentation sites to explore characteristics of LLJs and the boundary layer as well as some of the heterogeneities possible within the wind field of a LLJ. Southerly LLJs were observed on 66% of nights at the southwestern site (Greensburg, KS) but only 56% and 53% of nights at the eastern and northern sites, respectively (Hesston and Ellis, KS). The northernmost site had a relative abundance of weaker jets or nonjet conditions due to fronts or convective systems that only affected part of the observation domain. Plotting mean wind fields of each LLJ type reveals that the strongest LLJs tend to develop under very similar conditions but begin to show variability in wind profile evolution after several hours. A robust mixed layer height retrieval algorithm is used to investigate the interplay between the jets and the turbulent convective boundary layer, showing that stronger LLJs are preceded by deeper afternoon mixed layers and often have a later decoupling of mixing between the upper convective mixed layer and the near‐surface layer. Only the strongest LLJs generated a shallow mixing layer overnight. Comparing jet strength and direction to pristine nocturnal convection initiation shows that the strongest southerly LLJs yielded the most pristine nocturnal convection initiation events per night, and the pristine nocturnal convection initiation occurred farther north.

Low-level jets over the North Sea based on ERA5 and observations: together they do better

Abstract. Ten years of ERA5 reanalysis data are combined with met-mast and lidar observations from 10 offshore platforms to investigate low-level jet characteristics over the Dutch North Sea. The objective of this study is to combine the best of two worlds: (1) ERA5 data with a large spatiotemporal extent but inherent accuracy limitations due to a relatively coarse grid and an incomplete representation of physical processes and (2) observations that provide more reliable estimates of the measured quantity but are limited in both space and time. We demonstrate the effect of time and range limitations on the reconstructed wind climate, with special attention paid to the impact on low-level jets. For both measurement and model data, the representation of wind speed is biased. The limited temporal extent of observations leads to a wind speed bias on the order of ±1 m s−1 as compared to the long-term mean. In part due to data-assimilation strategies that cause abrupt discontinuities in the diurnal cycle, ERA5 also exhibits a wind speed bias of approximately 0.5 m s−1. The representation of low-level jets in ERA5 is poor in terms of a one-to-one correspondence, and the jets appear vertically displaced (“smeared out”). However, climatological characteristics such as the shape of the seasonal cycle and the affinity with certain circulation patterns are represented quite well, albeit with different magnitudes. We therefore experiment with various methods to adjust the modelled low-level jet rate to the observations or, vice versa, to correct for the erratic nature of the short observation periods using long-term ERA5 information. While quantitative uncertainty is still quite large, the presented results provide valuable insight into North Sea low-level jet characteristics. These jets occur predominantly for circulation types with an easterly component, with a clear peak in spring, and are concentrated along the coasts at heights between 50 and 200 m. Further, it is demonstrated that these characteristics can be used as predictors to infer the observed low-level jet rate from ERA5 data with reasonable accuracy.

The Climatology of Low‐Level Jet in Beijing and Guangzhou, China

AbstractThe important roles of low‐level jet (LLJ) in transport of heat, moisture, and pollutants have long been recognized. However, partly due to the lack of long‐term observations, the characteristics of LLJs in Beijing and Guangzhou have not been well understood. Using long‐term wind profiler observations and numerical simulations, the basic climatological characteristics of LLJ in Beijing and Guangzhou were documented, and their relationships with large‐ and local‐scale forcings were investigated. The occurrence frequencies of LLJ in both megacities approximately exhibit a bimodal distribution during an annual cycle. In terms of the seasonality, the LLJs in Beijing occur more often in spring and winter, and those in Guangzhou appear more frequently from October to December and from February to April. Pronounced diurnal variations of LLJ are found in Beijing, where most LLJs occurred during the nighttime, and the diurnal variations of LLJ in Guangzhou are less prominent. Favorable synoptic conditions for LLJ formations in Beijing are those having a southwest‐northeast or southeast‐northwest pressure gradient across Beijing. Dominant synoptic patterns associated with LLJs in Guangzhou are those having a low‐pressure or high‐pressure system to the northwest. Overall, the LLJs are primarily induced by synoptic forcings and modulated by the diurnal evolution of planetary boundary layer. The key findings from this study provide us a better understanding of the LLJs in these highly populated regions.

Investigation of low-level jet characteristics based on wind profiler observations

Low-level jets (LLJs) have received increasing concerns over the decades due to their close connection with various branches of meteorological and engineering applications. This study aims to provide a comprehensive investigation of LLJ characteristics based on two-year wind profiler observations in Hong Kong, in which a wide variety of LLJ characteristics, including jet speed maximum, jet height and distribution of jet direction, frequency of occurrence, statistics of wind shear coefficients associated with LLJs are explored and discussed in detail. The effect of wind speed criteria for LLJ identification on the LLJ characteristics is identified. Furthermore, the connection between LLJ characteristics and seasonality, directionality, as well as jet intensity is investigated individually. For the study of directionality, two typical jet directions derived in this study (i.e. N and E-ESE) are selected for further analysis, while for the study of jet intensity, the LLJ observations are categorized into weak, moderate and strong cases depending on the jet classification method proposed by Bonner (1968). The analyzed results suggest that the variability of the LLJ characteristics has a strong dependence on the change in seasonality, directionality and jet intensity. The discussions and conclusions from this study are expected to further enhance the understanding of the LLJ characteristics, and provide valuable implications for practical applications related to wind engineering.

A Case Study of Low‐Level Jets in Yerevan Simulated by the WRF Model

AbstractCapabilities of high‐resolution (3 km) Weather Research and Forecasting (WRF) simulations to reproduce topographically induced mountain‐valley winds and low‐level jets (LLJs) in Yerevan have been evaluated using high‐frequency observational and modeled data. High sensitivities of simulations of near‐surface winds and LLJ characteristics observed on 4 July 2015 to both boundary layer and initial and lateral boundary conditions setup have been demonstrated. Among the nine tested planetary boundary layer (PBL) parameterization schemes the MYJ, QNSE, and TEMF PBL schemes showed greater skill in simulation of near‐surface valley winds over Yerevan, while the other PBL schemes tend to significantly underestimate the strength of valley winds, with the BouLac PBL scheme being the worst performer. Most of PBL schemes simulate well‐defined LLJs in Yerevan associated with evening valley winds. The simulated jet cores are mostly located between 150 and 250 m above ground with magnitudes varying from 12 to 21 m s−1. However, the intensity of the observed nocturnal LLJ in Yerevan (located at 110 m above ground) is strongly underestimated by most of the WRF runs while the Shin and Hong and YSU PBL schemes simulate nocturnal LLJs higher than the observed LLJ. The WRF runs initiated with newly released European Centre for Medium‐Range Weather Forecasts ERA‐5 data set showed improved simulation of near‐surface winds and nighttime potential temperatures in Yerevan relative to those forced by the Global Forecast System fields.

Low-Level Jets over Utö, Finland, Based on Doppler Lidar Observations

AbstractOver two years of meteorological observations from Utö, a small island in the Finnish outer archipelago in the Baltic Sea, were used to investigate the occurrence and characteristics of low-level jets (LLJs) and the diurnal and seasonal variations in these properties. An objective LLJ identification algorithm that is suitable for high-temporal-and-vertical-resolution Doppler lidar data was created and applied to wind profiles obtained from a combination of Doppler lidar data and two-dimensional sonic anemometer observations. This algorithm was designed to identify coherent LLJ structures and requires that they persist for at least 1 h. The long-term mean LLJ frequency of occurrence at Utö was 12%, the mean LLJ wind speed was 11.6 m s−1, and the vast majority of identified LLJs occurred below 150 m above ground level. The LLJ frequency of occurrence was much higher during summer (21%) and spring (18%) than in autumn (8%) and winter (3%). During winter and spring, the LLJ frequency of occurrence is evenly distributed throughout the day. In contrast, the LLJ frequency of occurrence peaks at night (1900–0100 UTC) during summer and during the evening hours (1700–1900 UTC) in autumn. The highest and strongest LLJs come from the southwest, which is also the predominant LLJ direction in all seasons. LLJs below 100 m are common in spring and summer, are weaker, and do not show a strong directional dependence.

Potential of collocated radiometer and wind profiler observations for monsoon studies

Collocated observations from microwave radiometer and wind profiler are used in a pilot study during the monsoon period to derive information on the thermodynamics and winds and association with rainfall characteristics. These instruments were operated throughout the monsoon season of 2015. Continuous vertical profiles of winds, temperature and humidity show significant promise for understanding the low-level jet, its periodicity and its association with moisture transport, clouds and precipitation embedded within the monsoon large-scale convection. Observations showed mutually beneficial in explaining variability that are part of the low frequency oscillations and the diurnal variability during monsoon. These observations highlight the importance of locally driven convective systems, in the presence of weak moisture transport over the area. The episodic moisture convergence showed a periodicity of 9 days which matches with the subsequent convection and precipitation and thermodynamic regimes. Inferences from the diurnal cycle of moisture transport and the convective activity, relationship with the low-level jet characteristics and thermodynamics are also illustrated.

Influence of heterogeneous refractivity on optical wave propagation in coastal environments

Spatial variations of refractivity significantly dictate the characteristics of optical wave propagation through the atmosphere. Consequently, the ability to simulate such propagation is highly dependent upon the accurate characterization of refractivity along the propagation path. Unfortunately, the scarcity of high spatiotemporal resolution observational data has forced many past studies of optical wave propagation to assume horizontally homogeneous (HH) atmospheric conditions. However, the (adverse) impact of such an assumption has not been quantified in the literature. In this paper, we attempt to fill this void by utilizing a mesoscale modeling-based approach to explicitly simulate atmospheric refraction. We then compare the differences of the HH refractivity fields to the mesoscale model-derived refractivity fields by means of a realistic atmospheric event and through ray tracing simulations. In this study, we model a coastal low-level jet, a common coastal atmospheric phenomenon which is associated with heterogeneous thermal and refractivity fields. Observational data from a radiosonde and a radar wind profiler near the northeastern region of the United States are used for model validation. The observed characteristics of low-level jet (e.g., evolution, intensity, location) and associated temperature inversion are found to be reasonably well captured by the mesoscale model. The simulated nighttime refractivity gradient field manifests significant spatial heterogeneity; over land, the refractivity gradient is much stronger and amplified near the ground, whereas it becomes much weaker over the ocean. We quantify the effect of this heterogeneity on optical ray trajectories by simulating a suite of rays and documenting the variability of their altitudes at certain propagation ranges. It is found that the altitude of optical rays may vary tens of meters during a diurnal cycle, and at nighttime the rays may bend downward by more than 150 m at a range of 100 km. We run additional ray tracing simulations using refractivity profiles from a single location and assuming HH refractivity along the propagation path. It is observed that the HH approach yields instantaneous ray bending magnitudes up to 30 % less than the ray bending based on the refractivity simulated by the mesoscale model. At the same time, it is found that the mesoscale model-based refractivity fields may have uncertainty introduced by different factors associated with the model configuration. Of these factors, turbulence parameterization is explored in-depth and found to be responsible for more uncertainty than spatial grid resolution. To be more specific, different turbulence parameterizations are found to produce significantly varying temperature inversion parameters (e.g., height, magnitude), which are critical factors influencing ray trajectories. Collectively, these results highlight the potential advantages and disadvantages of utilizing a mesoscale model to simulate refractivity in coastal areas as opposed to assuming HH refractivity.

Observational Characteristics of Low-Level Jets in Central Western Mexico

During field experiments using surface weather stations and a Sodar-RASS (radio acoustic sounding system) combination, low-level jets were observed in the Colima Valley of central western Mexico. These low-level jet events have not been previously observed in this part of the tropics. The present study focuses on events from one specific field period from December 2011 to April 2013 performed at a specific location. From this dataset, it was found that these phenomena typically develop at night at around 0300 local time and they reach their maximum speed 20 min after sunrise and end at 1100 local time. For the considered period, low-level jet events occurred during 88 % of nights, and most of these events were fully characterized; thus, 73 % of the nights presented with categorized low-level jets following the classical literature on this mesoscale phenomenon in other parts of the world. The low-level wind maxima occurred with events ranging from category sub-zero to category 3. The strongest category 3 events are around 25 m s $$^{-1}$$ and the longest event was 57.5 h, but they are more typical of category 1. Their average speed is 11.7 m s $$^{-1}$$ and their height distribution is bimodal, with a group of events occurring at lower levels, with an average height of 190 m; higher levels have a mean height of 480 m. The higher cases are more typical of the rainy season. During all seasons, their direction is predominantly from the north-north-west to north-north-east with some dispersion occurring during the rainy season. The region of study is typically considered to have low wind speeds in the atmospheric boundary layer, making these events super-geostrophic.