Pristine Nocturnal Convective Initiation: A Climatology and Preliminary Examination of Predictability

Abstract The prediction of convective initiation remains a challenge to forecasters in the Great Plains, especially for elevated events at night. This study examines a subset of 287 likely elevated nocturnal convective initiation events that occurred with little or no direct influence from surface boundaries or preexisting convection over a 4-month period of May–August during the summer of 2015. Events were first classified into one of four types based on apparent formation mechanisms and location relative to any low-level jet. A climatology of each of the four types was performed focusing on general spatial tendencies over a large Great Plains domain and initiation timing trends. Simulations from five convection-allowing models available during the Plains Elevated Convection At Night (PECAN) field campaign, along with four versions of a 4-km Weather Research and Forecasting (WRF) Model, were used to examine the predictability of these types of convective initiation. A dual-peak pattern for initiation timing was revealed, with one peak near 0400 UTC and another around 0700 UTC. The times and prominence of each peak shifted depending on the region analyzed. Positive thermal advection by the geostrophic wind was present in the majority of events for three types but not for the type occurring without a low-level jet. Models were more deficient with location than timing for the five PECAN models, with the four 4-km WRF Models showing similar location errors and problems with initiating convection at a lower altitude than observed.

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Mesoscale Moisture Transport by the Low-Level Jet during the IHOP Field Experiment

Monthly Weather Review ◽

10.1175/2008mwr2421.1 ◽

2008 ◽

Vol 136 (10) ◽

pp. 3781-3795 ◽

Cited By ~ 11

Author(s):

Edward I. Tollerud ◽

Fernando Caracena ◽

Steven E. Koch ◽

Brian D. Jamison ◽

R. Michael Hardesty ◽

...

Keyword(s):

Great Plains ◽

Cross Sections ◽

Moisture Transport ◽

Large Scale ◽

Wrf Model ◽

The United States ◽

Small Scale ◽

Low Level ◽

Convective Scale ◽

Low Level Jet

Previous studies of the low-level jet (LLJ) over the central Great Plains of the United States have been unable to determine the role that mesoscale and smaller circulations play in the transport of moisture. To address this issue, two aircraft missions during the International H2O Project (IHOP_2002) were designed to observe closely a well-developed LLJ over the Great Plains (primarily Oklahoma and Kansas) with multiple observation platforms. In addition to standard operational platforms (most important, radiosondes and profilers) to provide the large-scale setting, dropsondes released from the aircraft at 55-km intervals and a pair of onboard lidar instruments—High Resolution Doppler Lidar (HRDL) for wind and differential absorption lidar (DIAL) for moisture—observed the moisture transport in the LLJ at greater resolution. Using these observations, the authors describe the multiscalar structure of the LLJ and then focus attention on the bulk properties and effects of scales of motion by computing moisture fluxes through cross sections that bracket the LLJ. From these computations, the Reynolds averages within the cross sections can be computed. This allow an estimate to be made of the bulk effect of integrated estimates of the contribution of small-scale (mesoscale to convective scale) circulations to the overall transport. The performance of the Weather Research and Forecasting (WRF) Model in forecasting the intensity and evolution of the LLJ for this case is briefly examined.

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A Unified Theory for the Great Plains Nocturnal Low-Level Jet

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0307.1 ◽

2016 ◽

Vol 73 (8) ◽

pp. 3037-3057 ◽

Cited By ~ 72

Author(s):

Alan Shapiro ◽

Evgeni Fedorovich ◽

Stefan Rahimi

Keyword(s):

Great Plains ◽

Slope Angle ◽

Geostrophic Wind ◽

Linear Functions ◽

Free Atmosphere ◽

Wind Maximum ◽

Maximum Surface ◽

Low Level ◽

Low Level Jet ◽

Surface Buoyancy

Abstract A theory is presented for the Great Plains low-level jet in which the jet emerges in the sloping atmospheric boundary layer as the nocturnal phase of an oscillation arising from diurnal variations in turbulent diffusivity (Blackadar mechanism) and surface buoyancy (Holton mechanism). The governing equations are the equations of motion, mass conservation, and thermal energy for a stably stratified fluid in the Boussinesq approximation. Attention is restricted to remote (far above slope) geostrophic winds that blow along the terrain isoheights (southerly for the Great Plains). Diurnally periodic solutions are obtained analytically with diffusivities that vary as piecewise constant functions of time and slope buoyancies that vary as piecewise linear functions of time. The solution is controlled by 11 parameters: slope angle, Coriolis parameter, free-atmosphere Brunt–Väisälä frequency, free-atmosphere geostrophic wind, radiative damping parameter, day and night diffusivities, maximum and minimum surface buoyancies, and times of maximum surface buoyancy and sunset. The Holton mechanism, by itself, results in relatively weak wind maxima but produces strong jets when paired with the Blackadar mechanism. Jets with both Blackadar and Holton mechanisms operating are shown to be broadly consistent with observations and climatological analyses. Jets strengthen with increasing geostrophic wind, maximum surface buoyancy, and day-to-night ratio of the diffusivities and weaken with increasing Brunt–Väisälä frequency and magnitude of minimum slope buoyancy (greater nighttime cooling). Peak winds are maximized for slope angles characteristic of the Great Plains.

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Clarifying the Baroclinic Contribution to the Great Plains Low-Level Jet Frequency Maximum

Monthly Weather Review ◽

10.1175/mwr-d-19-0024.1 ◽

2019 ◽

Vol 147 (9) ◽

pp. 3481-3493 ◽

Cited By ~ 2

Author(s):

Joshua G. Gebauer ◽

Alan Shapiro

Keyword(s):

Great Plains ◽

Geostrophic Wind ◽

Inertial Oscillation ◽

Low Level ◽

Differential Heating ◽

Oklahoma Mesonet ◽

Nocturnal Jet ◽

Low Level Jet ◽

Relative Prevalence ◽

East West

Abstract The frequency and intensity of the Great Plains nocturnal low-level jet (LLJ) are enhanced by baroclinicity over the sloped terrain of the region. A classical description of baroclinic-induced diurnal wind oscillations over the Great Plains considers differential heating of the slope with respect to air at the same elevation far removed from the slope, but with buoyancy constant along the slope (Holton mechanism). Baroclinicity can also occur due to differential heating of the slope itself, which creates a gradient in buoyancy along the slope. The relative prevalence of the two types of baroclinicity in this region has received scant attention in the literature. The present study uses 19 years of data from the Oklahoma Mesonet to evaluate the characteristics of along-slope buoyancy gradients over the region. A mean negative afternoon along-slope buoyancy gradient (east–west gradient) is found over Oklahoma. The sign of this afternoon buoyancy gradient is favorable for LLJ formation, as it results in the strongest southerly geostrophic wind near the ground around sunset, which is conducive to nocturnal jet formation via the inertial oscillation mechanism. The negative afternoon buoyancy gradient is at least partially created by an east–west gradient in diurnal heating and is stronger and more consistent in the summer months, which is when LLJs are most frequent. The contribution of the along-slope buoyancy gradient to the low-level geostrophic wind was found to be as important as the contribution of the Holton mechanism. Overall, these results indicate that along-slope buoyancy gradients should be accounted for in studies of LLJ dynamics over the Great Plains.

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WRF Model Study of the Great Plains Low-Level Jet: Effects of Grid Spacing and Boundary Layer Parameterization

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-17-0361.1 ◽

2018 ◽

Vol 57 (10) ◽

pp. 2375-2397 ◽

Cited By ~ 6

Author(s):

Elizabeth N. Smith ◽

Jeremy A. Gibbs ◽

Evgeni Fedorovich ◽

Petra M. Klein

Keyword(s):

Boundary Layer ◽

Great Plains ◽

Wrf Model ◽

High Temporal Resolution ◽

Model Stability ◽

Low Level ◽

Model Study ◽

Vertical Grid ◽

Low Level Jet ◽

Normal Scale

AbstractPrevious studies have shown that the Weather Research and Forecasting (WRF) Model often underpredicts the strength of the Great Plains nocturnal low-level jet (NLLJ), which has implications for weather, climate, aviation, air quality, and wind energy in the region. During the Lower Atmospheric Boundary Layer Experiment (LABLE) conducted in 2012, NLLJs were frequently observed at high temporal resolution, allowing for detailed documentation of their development and evolution throughout the night. Ten LABLE cases with observed NLLJs were chosen to systematically evaluate the WRF Model’s ability to reproduce the observed NLLJs. Model runs were performed with 4-, 2-, and 1-km horizontal spacing and with the default stretched vertical grid and a nonstretched 40-m vertically spaced grid to investigate which grid configurations are optimal for NLLJ modeling. These tests were conducted using three common boundary layer parameterization schemes: Mellor–Yamada Nakanishi Niino, Yonsei University, and Quasi-Normal Scale Elimination. It was found that refining horizontal spacing does not necessarily improve the modeled NLLJ wind. Increasing the number of vertical levels on a non-stretched grid provides more information about the structure of the NLLJ with some schemes, but the benefit is limited by computational expense and model stability. Simulations of the NLLJ were found to be less sensitive to boundary layer parameterization than to grid configuration. The Quasi-Normal Scale Elimination scheme was chosen for future NLLJ simulation studies.

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A Comparative Study of the 3 June 2015 Great Plains Low-Level Jet

Monthly Weather Review ◽

10.1175/mwr-d-16-0071.1 ◽

2016 ◽

Vol 144 (8) ◽

pp. 2963-2979 ◽

Cited By ~ 9

Author(s):

Thomas R. Parish

Keyword(s):

Great Plains ◽

Cross Sections ◽

Geostrophic Wind ◽

Gradient Force ◽

Inertial Oscillation ◽

Pressure Gradient Force ◽

Wind Maximum ◽

Airborne Measurements ◽

Low Level ◽

Low Level Jet

Abstract Detailed ground-based and airborne measurements were conducted of the summertime Great Plains low-level jet (LLJ) in central Kansas during the Plains Elevated Convection at Night (PECAN) campaign. Airborne measurements using the University of Wyoming King Air were made to document the vertical wind profile and the forcing of the jet during the nighttime hours on 3 June 2015. Two flights were conducted that document the evolution of the LLJ from sunset to dawn. Each flight included a series of vertical sawtooth and isobaric legs along a fixed track at 38.7°N between longitudes 98.9° and 100°W. Comparison of the 3 June 2015 LLJ was made with a composite LLJ case obtained from gridded output from the North American Mesoscale Forecast System for June and July of 2008 and 2009. Forcing of the LLJ was detected using cross sections of D values that allow measurement of the vertical profile of the horizontal pressure gradient force and the thermal wind. Combined with observations of the actual wind, ageostrophic components normal to the flight track can be detected. Observations show that the 3 June 2015 LLJ displayed classic features of the LLJ, including an inertial oscillation of the ageostrophic wind. Oscillations in the geostrophic wind as a result of diurnal heating and cooling of the sloping terrain are not responsible for the nocturnal wind maximum. Net daytime heating of the sloping Great Plains, however, is responsible for the development of a strong background geostrophic wind that is critical to formation of the LLJ.

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A 1D Theoretical Analysis of Northerly Low-Level Jets over the Great Plains

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0333.1 ◽

2017 ◽

Vol 74 (10) ◽

pp. 3419-3431 ◽

Cited By ~ 4

Author(s):

Joshua G. Gebauer ◽

Evgeni Fedorovich ◽

Alan Shapiro

Keyword(s):

Great Plains ◽

Equations Of Motion ◽

Geostrophic Wind ◽

Inertial Oscillation ◽

Diurnal Changes ◽

Convective Boundary ◽

Low Level ◽

Low Level Jets ◽

Low Level Jet ◽

Negative Buoyancy

Abstract The forcing of northerly low-level jets over the eastward-sloped terrain of the U.S. Great Plains was studied using a one-dimensional (1D) nonstationary analytical model based on the Boussinesq-approximated equations of motion and thermal energy. For northerly low-level jets, the forcing from diurnal changes in surface heating of the sloped terrain (Holton mechanism) is out of phase with the nocturnal inertial oscillation resulting from the cessation of turbulent mixing at sunset (Blackadar mechanism), which results in weaker northerly nocturnal low-level jets when compared to southerly nocturnal low-levels jets with the same-magnitude background pressure gradient forcing. Because of the Blackadar and Holton mechanisms acting out of phase, nocturnal northerly low-level jets cannot solely explain the northerly low-level jet maximum over the Great Plains found in climatological studies. It is shown that negative buoyancy values over the eastward-sloped terrain enhance the low-level northerly geostrophic wind, which can cause low-level jetlike wind profiles that do not necessarily depend on the diurnal cycle. However, nocturnal northerly low-level jets primarily caused by an inertial oscillation still occur when daytime mixing is strong and buoyancy is small at sunset. These conditions are possible when strong capping inversions are present in the daytime convective boundary layer. The occurrence of both types of northerly low-level jets, those caused by negative buoyancy values over the sloped terrain and those driven by an inertial oscillation, better explains the findings of previous low-level jet climatologies.

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Understanding the influence of ENSO on the Great Plains low-level jet in CMIP5 models

Climate Dynamics ◽

10.1007/s00382-017-3970-9 ◽

2017 ◽

Vol 51 (4) ◽

pp. 1537-1558 ◽

Cited By ~ 3

Author(s):

James F. Danco ◽

Elinor R. Martin

Keyword(s):

Great Plains ◽

Cmip5 Models ◽

Low Level ◽

Low Level Jet

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Assimilation of Satellite-Derived Soil Moisture for Improved Forecasts of the Great Plains Low-Level Jet

Monthly Weather Review ◽

10.1175/mwr-d-20-0185.1 ◽

2020 ◽

Vol 148 (11) ◽

pp. 4607-4627

Author(s):

Craig R. Ferguson ◽

Shubhi Agrawal ◽

Mark C. Beauharnois ◽

Geng Xia ◽

D. Alex Burrows ◽

...

Keyword(s):

Soil Moisture ◽

Wind Speed ◽

Data Assimilation ◽

Great Plains ◽

Wrf Model ◽

Added Value ◽

Low Level ◽

Convective Systems ◽

South Central ◽

Mesoscale Convective

AbstractIn the context of forecasting societally impactful Great Plains low-level jets (GPLLJs), the potential added value of satellite soil moisture (SM) data assimilation (DA) is high. GPLLJs are both sensitive to regional soil moisture gradients and frequent drivers of severe weather, including mesoscale convective systems. An untested hypothesis is that SM DA is more effective in forecasts of weakly synoptically forced, or uncoupled GPLLJs, than in forecasts of cyclone-induced coupled GPLLJs. Using the NASA Unified Weather Research and Forecasting (NU-WRF) Model, 75 GPLLJs are simulated at 9-km resolution both with and without NASA Soil Moisture Active Passive SM DA. Differences in modeled SM, surface sensible (SH) and latent heat (LH) fluxes, 2-m temperature (T2), 2-m humidity (Q2), PBL height (PBLH), and 850-hPa wind speed (W850) are quantified for individual jets and jet-type event subsets over the south-central Great Plains, as well as separately for each GPLLJ sector (entrance, core, and exit). At the GPLLJ core, DA-related changes of up to 5.4 kg m−2 in SM can result in T2, Q2, LH, SH, PBLH, and W850 differences of 0.68°C, 0.71 g kg−2, 59.9 W m−2, 52.4 W m−2, 240 m, and 4 m s−1, respectively. W850 differences focus along the jet axis and tend to increase from south to north. Jet-type differences are most evident at the GPLLJ exit where DA increases and decreases W850 in uncoupled and coupled GPLLJs, respectively. Data assimilation marginally reduces negative wind speed bias for all jets, but the correction is greater for uncoupled GPLLJs, as hypothesized.

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