scholarly journals The role of nocturnal Low-Level-Jet in nocturnal convection and rainfalls in the west Mediterranean coast: the episode of 14 December 2010 in northeast of Iberian Peninsula

2012 ◽  
Vol 8 (1) ◽  
pp. 27-31 ◽  
Author(s):  
J. Mazón ◽  
D. Pino
Keyword(s):  
Iberian Peninsula ◽  
Mesoscale Model ◽  
Return Flow ◽  
Inertial Oscillation ◽  
Air Mass ◽  
Cold Air ◽  
Low Level ◽  
Radar Images ◽  
Synoptic Wind ◽  
Low Level Jet

Abstract. The night of 14 December 2010 radar images of the Spanish Weather Agency recorded a large rain band that moved offshore at the Northeast coast of the Iberian Peninsula. MM5 mesoscale model is used to study the atmospheric dynamics during that day. A Nocturnal Low Level Jet (NLLJ) generated by an inertial oscillation that brings cold air to the coast from inland has been simulated in the area. This cold air interacts with a warmer air mass some kilometers offshore. According to the MM5 mesoscale model simulation, the cold air enhances upward movements of the warm air producing condensation. Additionally, there is a return flow to the coastline at 600–900 m high. This warm air mass interacts again with the cold air moving downslope, also producing condensation inland. The simulation for the night before this episode shows large drainage winds with a NLLJ profile, but no condensation areas. The night after the 14th the simulation also shows drainage winds but without a NLLJ profile. However, an offshore convergence area was produced with a returned flow, but no condensation inland occurred. This fact is in agreement with radar observations which reported no precipitation for these two days. Consequently, NLLJ in combination with a synoptic wind over the sea could enhance condensation and eventually precipitation rates in the Mediterranean Iberian coast.

On the Role of Sloping Terrain in the Forcing of the Great Plains Low-Level Jet

2010 ◽  
Vol 67 (8) ◽  
pp. 2690-2699 ◽  
Author(s):  
Thomas R. Parish ◽  
Larry D. Oolman
Keyword(s):  
Great Plains ◽  
Mesoscale Model ◽  
Horizontal Resolution ◽  
Inertial Oscillation ◽  
Western Boundary ◽  
Boundary Current ◽  
Wind Maximum ◽  
Low Level ◽  
Low Level Jet ◽  
Sloping Terrain

Abstract The summertime Great Plains low-level jet (LLJ) has been the subject of numerous investigations during the past several decades. Characteristics of the LLJ include nighttime development of a pronounced wind maximum of typically 15–20 m s−1 at levels 300–800 m above the surface and a clockwise rotation of the wind maximum during the course of the night. Maximum frequency of occurrence of the LLJ is found in the southern Great Plains. Theories proposed to explain the diurnal wind maximum of the Great Plains LLJ include inertial oscillation of the ageostrophic wind, the diurnal oscillation of the horizontal pressure field associated with heating and cooling of the sloping terrain, and the western boundary current interpretations. A simple equation system and output from the 12-km horizontal resolution Weather Research and Forecasting Nonhydrostatic Mesoscale Model (NAM) for July 2008 are used to provide evidence as to the importance of the Great Plains topography in driving the LLJ. Summertime heating of the sloping terrain is critical in establishing the climatological position for the Great Plains LLJ. Heating enhances the background geostrophic flow associated with the Bermuda high, resulting in a maximum low-level mean summertime flow over the Great Plains region. Maximum geostrophic winds in the NAM are found during late afternoon, providing a large background wind on which frictional decoupling can act. The nighttime LLJ maximum is the result of an inertial oscillation of the unbalanced components that arise fundamentally from frictional decoupling. Diurnal heating of the sloping terrain forces a cycle in the geostrophic wind that is out of phase with the wind maximum.

Spatial and temporal variability of the Iberian Peninsula coastal low-level jet

2017 ◽  
Vol 38 (4) ◽  
pp. 1605-1622 ◽  
Author(s):  
Nadia Rijo ◽  
Alvaro Semedo ◽  
Pedro M. A. Miranda ◽  
Daniela Lima ◽  
Rita M. Cardoso ◽  
...  
Keyword(s):  
Iberian Peninsula ◽  
Temporal Variability ◽  
Spatial And Temporal Variability ◽  
Low Level ◽  
Low Level Jet

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

2019 ◽  
Vol 147 (9) ◽  
pp. 3481-3493 ◽  
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.

The interaction between the nocturnal Amazonian low-level jet and convection in CESM

2021 ◽  
pp. 1-37
Author(s):  
Hedanqiu Bai ◽  
Courtney Schumacher
Keyword(s):  
Air Temperature ◽  
Climate Models ◽  
Coupled Model ◽  
Reanalysis Data ◽  
Equatorial Atlantic ◽  
Cold Air ◽  
Low Level ◽  
Temperature Bias ◽  
Early Afternoon ◽  
Low Level Jet

AbstractA nocturnal Amazonian low-level jet (ALLJ) was recently diagnosed using reanalysis data. This work assesses the ability of CESM1.2.2 to reproduce the jet and explores the mechanisms by which the ALLJ influences convection in the Amazon. The coupled CESM simulates the nocturnal ALLJ realistically, while CAM5 does not. A low-level cold air temperature bias in the eastern Amazon exists in CAM5, thus the ALLJ is weaker than observed. However, a cold SST bias over the equatorial North Atlantic in the coupled model offsets the cold air temperature bias, producing a realistic ALLJ. Climate models significantly underestimate March-April-May (MAM) precipitation over the eastern Amazon. We ran two sensitivity experiments using the coupled CESM by adding bottom-heavy diabatic heating at noon and midnight for 2.5 hours along the coastal Amazon during MAM to mimic the occurrence of shallow precipitating convection. When heating is added during the early afternoon, coastal convection deepens and the ALLJ transports moisture inland from the ocean, preconditioning the environment for deep convective development during the ensuing hours. The increased convection over the eastern Amazon also moderately alleviates the equatorial Atlantic westerly wind bias, leading to deepening of the east Atlantic thermocline in the following months and partially improving the simulated June-July-August (JJA) Atlantic cold tongue in the coupled model. When heating is added at night, coastal convection does not strengthen as much and the ALLJ transports less moisture. Improvements in the simulated Atlantic winds and SST are negligible. Therefore, diurnal circulations matter to the organization of convection and rain across the Amazon, with impacts over the equatorial Atlantic.

Numerical Simulation of the Effects of Warm Temperature Perturbation on Mesoscale Vortex and Rainfall in Col Field

2020 ◽  
Author(s):  
Yongqiang Jiang ◽  
Chaohui Chen ◽  
Hongrang He ◽  
Yudi Liu ◽  
Hong Huang ◽  
...  
Keyword(s):  
Mesoscale Model ◽  
Vertical Wind Shear ◽  
Temperature Perturbation ◽  
Strong Wind ◽  
Mesoscale Vortex ◽  
Pressure Drops ◽  
Low Level ◽  
Long Time ◽  
Low Level Jet ◽  
Upper Level

<p>The col field (a region between two lows and two highs in the isobaric surface) is a common pattern leading to the generation of mesoscale vortex and heavy rainfall in China. The mesoscale vortex usually forms near the col point and the dilatation axis of the col field in the low-level troposphere.</p><p>The Mesoscale model WRF was used to numerically simulate a rainfall process in col field. A temperature perturbation column (TPC) was introduced into the low-level col field near the col point, and the effects of TPC on mesoscale vortex and rainfall was analyzed.</p><p>It was shown that in the region of strong wind background, the TPC moves downstream and has little effect on the environment, while near the col point, the wind speed and the vertical wind shear are small, the TPC can stay in the col field for a long time, which can have a greater impact on the environment. The strong TPC near the col point can trigger the vortex. As the temperature of the air column increases, the pressure drops, leading to the low-level convergence and the upper-level divergence, and the low-level cyclonic vorticity form under the effect of ageostrophic winds, which is favor of the formation of mesoscale vortex in the weak wind field. The formation of vortex promotes the intensification of precipitation. The release of the latent heat of the condensation induced by the TPC makes a positive feedback for the mesoscale vortex. The southwestly low-level jet enhances through the thermodynamic action, resulting in convergence of the leeward low-level jet and increase of precipitation, and divergence of the upwind low-level jet and decrease of precipitation, respectively. The col field is a favorable circumstance for the formation of mesoscale vortex.</p><p>Acknowledgements. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41975128 and 41275099).</p>

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

2016 ◽  
Vol 144 (8) ◽  
pp. 2963-2979 ◽  
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.

Turbulence Measurements in a Young Cyclone Over the Ocean1

1957 ◽  
Vol 38 (1.1) ◽  
pp. 13-16 ◽  
Author(s):  
Andrew F. Bunker
Keyword(s):  
Heat Flow ◽  
Transition Zone ◽  
Thermal Structure ◽  
Air Mass ◽  
Cold Air ◽  
Turbulence Measurements ◽  
Heat Flows ◽  
Low Level ◽  
Strong Turbulence ◽  
Warm Sector

A flight was made through a young coastal storm with a PBY-6A aircraft equipped to measure both mean temperatures and rapid variations of the temperature and turbulent gust velocities. Low level observations were obtained which show the thermal structure of the cyclone and the magnitude of the turbulence, the shearing stresses and the heat flows. Particularly strong turbulence was noted in the transition zone between the cool air of the anticyclone and the faster moving air of the warm sector. Stable air and downward heat flow was observed in the cold air mass.

scholarly journals Regional Model Simulations of the Bodélé Low-Level Jet of Northern Chad during the Bodélé Dust Experiment (BoDEx 2005)

2008 ◽  
Vol 21 (5) ◽  
pp. 995-1012 ◽  
Author(s):  
Martin C. Todd ◽  
Richard Washington ◽  
Srivatsan Raghavan ◽  
Gil Lizcano ◽  
Peter Knippertz
Keyword(s):  
Diurnal Cycle ◽  
Climate Models ◽  
Mesoscale Model ◽  
Regional Climate ◽  
Dust Emission ◽  
Horizontal Resolution ◽  
Emission Model ◽  
Low Level ◽  
Low Level Jet ◽  
Key Features

Abstract The low-level jet (LLJ) over the Bodélé depression in northern Chad is a newly identified feature. Strong LLJ events are responsible for the emission of large quantities of mineral dust from the depression, the world’s largest single dust source, and its subsequent transport to West Africa, the tropical Atlantic, and beyond. Accurate simulation of this key dust-generating atmospheric feature is, therefore, an important requirement for dust models. The objectives of the present study are (i) to evaluate the ability of regional climate models (RCMs) and global analyses/reanalyses to represent this feature, and (ii) to determine the driving mechanisms of the LLJ and its strong diurnal cycle. Observational data obtained during the Bodélé Dust Experiment (BoDEx 2005) are utilized for comparison. When suitably configured, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) RCM can represent very accurately many of the key features of the jet including the structure, diurnal cycle, and day-to-day variability. Surface winds are also well reproduced, including the peak winds, which activate dust emission. Model fidelity is, however, strongly dependent on the boundary layer parameterization scheme, surface roughness, and vertical resolution in the lowest layers. A model horizontal resolution of a few tens of kilometers is sufficient to resolve most of the key features of the LLJ, while in global analyses/reanalyses many features of the LLJ are not adequately represented. Idealized RCM simulations indicate that under strong synoptic forcing the surrounding orography of the Tibesti and Ennedi Mountains acts to focus the LLJ onto the Bodélé and to accelerate the jet by ∼40%. From the RCM experiments it is diagnosed that the pronounced diurnal cycle of the Bodélé LLJ is largely a result of varying eddy viscosity, with elevated heating/cooling over the Tibesti Mountains to the north as a second-order contribution.

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