Numerical Simulation of Baroclinic Waves with a Parameterized Boundary Layer

2007 ◽  
Vol 64 (12) ◽  
pp. 4383-4399 ◽  
Author(s):  
R. S. Plant ◽  
S. E. Belcher
Keyword(s):  
Boundary Layer ◽  
Lower Troposphere ◽  
Conveyor Belt ◽  
Static Stability ◽  
Turbulent Fluxes ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Ekman Pumping ◽  
Basic States

Abstract A dry three-dimensional baroclinic life cycle model is used to investigate the role of turbulent fluxes of heat and momentum within the boundary layer on midlatitude cyclones. Simulations are performed of life cycles for two basic states: with and without turbulent fluxes. The different basic states produce cyclones with contrasting frontal and mesoscale flow structures. The analysis focuses on the generation of potential vorticity (PV) in the boundary layer and its subsequent transport into the free troposphere. The dynamic mechanism through which friction mitigates a barotropic vortex is that of Ekman pumping. This has often been assumed to also be the dominant mechanism for baroclinic developments. The PV framework highlights an additional, baroclinic mechanism. Positive PV is generated baroclinically due to friction to the northeast of a surface low and is transported out of the boundary layer by a cyclonic conveyor belt flow. The result is an anomaly of increased static stability in the lower troposphere, which restricts the growth of the baroclinic wave. The reduced coupling between lower and upper levels can be sufficient to change the character of the upper-level evolution of the mature wave. The basic features of the baroclinic damping mechanism are robust for different frontal structures, with and without turbulent heat fluxes, and for the range of surface roughness found over the oceans.

scholarly journals Using Frontogenesis to Identify Sting Jets in Extratropical Cyclones

2013 ◽  
Vol 28 (3) ◽  
pp. 603-613 ◽  
Author(s):  
David M. Schultz ◽  
Joseph M. Sienkiewicz
Keyword(s):  
Boundary Layer ◽  
North Atlantic Ocean ◽  
Surface Wind ◽  
Conveyor Belt ◽  
Static Stability ◽  
Heat Fluxes ◽  
Extratropical Cyclones ◽  
The North ◽  
Neutral Boundary Layer ◽  
Strong Winds

AbstractSting jets, or surface wind maxima at the end of bent-back fronts in Shapiro–Keyser cyclones, are one cause of strong winds in extratropical cyclones. Although previous studies identified the release of conditional symmetric instability as a cause of sting jets, the mechanism to initiate its release remains unidentified. To identify this mechanism, a case study was selected of an intense cyclone over the North Atlantic Ocean during 7–8 December 2005 that possessed a sting jet detected from the NASA Quick Scatterometer (QuikSCAT). A couplet of Petterssen frontogenesis and frontolysis occurred along the bent-back front. The direct circulation associated with the frontogenesis led to ascent within the cyclonically turning portion of the warm conveyor belt, contributing to the comma-cloud head. When the bent-back front became frontolytic, an indirect circulation associated with the frontolysis, in conjunction with alongfront cold advection, led to descent within and on the warm side of the front, bringing higher-momentum air down toward the boundary layer. Sensible heat fluxes from the ocean surface and cold-air advection destabilized the boundary layer, resulting in near-neutral static stability facilitating downward mixing. Thus, descent associated with the frontolysis reaching a near-neutral boundary layer provides a physical mechanism for sting jets, is consistent with previous studies, and synthesizes existing knowledge. Specifically, this couplet of frontogenesis and frontolysis could explain why sting jets occur at the end of the bent-back front and emerge from the cloud head, why sting jets are mesoscale phenomena, and why they only occur within Shapiro–Keyser cyclones. A larger dataset of cases is necessary to test this hypothesis.

Estimation of turbulent heat fluxes and exchange coefficients for heat at Dumont d'Urville, East Antarctica

1999 ◽  
Vol 11 (1) ◽  
pp. 93-99 ◽  
Author(s):  
S. Argentini ◽  
G. Mastrantonio ◽  
A. Viola
Keyword(s):  
Boundary Layer ◽  
Heat Fluxes ◽  
East Antarctica ◽  
Turbulent Heat ◽  
Convective Boundary ◽  
Doppler Sodar ◽  
Turbulent Heat Fluxes ◽  
Unstable Boundary Layer ◽  
Sodar Measurements ◽  
The Antarctic

Simultaneous acoustic Doppler sodar and tethersonde measurements were used to study some of the characteristics of the unstable boundary layer at Dumont d'Urville, Adélie Land, East Antarctica during the summer 1993–94. A description of the convective boundary layer and its behaviour in connection with the wind regime is given along with the frequency distribution of free convection episodes. The surface heat flux has been evaluated using the vertical velocity variance derived from sodar measurements. The turbulent exchange coefficients, estimated by coupling sodar and tethered balloon measurements, are in strong agreement with those present in literature for the Antarctic regions.

scholarly journals Challenges in Modeling Turbulent Heat Fluxes to Snowpacks in Forest Clearings

2018 ◽  
Vol 19 (10) ◽  
pp. 1599-1616 ◽  
Author(s):  
Jonathan P. Conway ◽  
John W. Pomeroy ◽  
Warren D. Helgason ◽  
Nicholas J. Kinar
Keyword(s):  
Heat Exchange ◽  
Surface Temperature ◽  
Latent Heat ◽  
Turbulent Fluxes ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Snow Surface ◽  
Turbulent Heat Exchange ◽  
Uncertainty Estimates ◽  
Model Tuning

Abstract Forest clearings are common features of evergreen forests and produce snowpack accumulation and melt differing from that in adjacent forests and open terrain. This study has investigated the challenges in specifying the turbulent fluxes of sensible and latent heat to snowpacks in forest clearings. The snowpack in two forest clearings in the Canadian Rockies was simulated using a one-dimensional (1D) snowpack model. A trade-off was found between optimizing against measured snow surface temperature or snowmelt when choosing how to specify the turbulent fluxes. Schemes using the Monin–Obukhov similarity theory tended to produce negatively biased surface temperature, while schemes that enhanced turbulent fluxes, to reduce the surface temperature bias, resulted in too much melt. Uncertainty estimates from Monte Carlo experiments showed that no realistic parameter set could successfully remove biases in both surface temperature and melt. A simple scheme that excludes atmospheric stability correction was required to successfully simulate surface temperature under low wind speed conditions. Nonturbulent advective fluxes and/or nonlocal sources of turbulence are thought to account for the maintenance of heat exchange in low-wind conditions. The simulation of snowmelt was improved by allowing enhanced latent heat fluxes during low-wind conditions. Caution is warranted when snowpack models are optimized on surface temperature, as model tuning may compensate for deficiencies in conceptual and numerical models of radiative, conductive, and turbulent heat exchange at the snow surface and within the snowpack. Such model tuning could have large impacts on the melt rate and timing of the snow-free transition in simulations of forest clearings within hydrological and meteorological models.

scholarly journals Areal melt and discharge modelling of Storglaciären, Sweden

1997 ◽  
Vol 24 ◽  
pp. 211-216 ◽  
Author(s):  
Regine Hock ◽  
Christian Noetzli
Keyword(s):  
Meteorological Data ◽  
Grid Point ◽  
Digital Terrain Model ◽  
Turbulent Fluxes ◽  
Heat Fluxes ◽  
Balance Model ◽  
Turbulent Heat ◽  
Terrain Model ◽  
Digital Terrain ◽  
One Year

A grid-based glacier melt-and-discharge model was applied to Storglaciären, a small valley glacier (3 km2) in northern Sweden, for the melt seasons of 1993 and 1994. The energy available for melt was estimated from a surface energy-balance model using meteorological data collected by automatic weather stations on the glacier. Net radiation and the turbulent heat fluxes were calculated hourly for every grid point of a 30 m resolution digital terrain model, using the measurements of temperature, humidity, wind speed and radiative fluxes on the glacier. Two different bulk approaches were used to calculate the turbulent fluxes and compared with respect to their impact on discharge simulations. Discharge of Storglaciären was simulated from calculated meltwater production and precipitation by three parallel linear reservoirs corresponding to the different storage properties of firn, snow and ice. The performance of the model was validated by comparing simulated discharge to measured discharge at the glacier snout. Depending on which parameterization of the turbulent fluxes was used, the timing and magnitude of simulated discharge was in good agreement with observed discharge, or simulated discharge was considerably underestimated in one year.

The Importance of Relative Wind Speed in Estimating Air–Sea Turbulent Heat Fluxes in Bulk Formulas: Examples in the Bohai Sea

2020 ◽  
Vol 37 (4) ◽  
pp. 589-603 ◽  
Author(s):  
Xiangzhou Song
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Wind Speed ◽  
Bohai Sea ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Surface Currents ◽  
The Bohai Sea ◽  
Turbulent Heat Fluxes ◽  
Relative Wind

AbstractSea surface currents are commonly neglected when estimating the air–sea turbulent heat fluxes in bulk formulas. Using buoy observations in the Bohai Sea, this paper investigated the effects of near-coast multiscale currents on the quantification of turbulent heat fluxes, namely, latent heat flux (LH) and sensible heat flux (SH). The maximum current reached 1 m s−1 in magnitude, and a steady northeastward current of 0.16 m s−1 appeared in the southern Bohai Strait. The predominant tidal signal was the semidiurnal current, followed by diurnal components. The mean absolute surface wind was from the northeast with a speed of approximately 3 m s−1. The surface winds at a height of 11 m were dominated by the East Asian monsoon. As a result of upwind flow, the monthly mean differences in LH and SH between the estimates with and without surface currents ranged from 1 to 2 W m−2 in July (stable boundary layer) and November (unstable boundary layer). The hourly differences were on average 10 W m−2 and ranged from 0 to 24 W m−2 due to changes in the relative wind speed by high-frequency rotating surface tidal currents. The diurnal variability in LH/SH was demonstrated under stable and unstable boundary conditions. Observations provided an accurate benchmark for flux comparisons. The newly updated atmospheric reanalysis products MERRA-2 and ERA5 were superior to the 1° OAFlux data at this buoy location. However, future efforts in heat flux computation are still needed to, for example, consider surface currents and resolve diurnal variations.

scholarly journals Vertical Velocities and Available Potential Energy Generated by Landscape Variability—Theory

2008 ◽  
Vol 47 (2) ◽  
pp. 397-410 ◽  
Author(s):  
M. Baldi ◽  
G. A. Dalu ◽  
R. A. Pielke
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Potential Energy ◽  
Sensible Heat Flux ◽  
Lower Troposphere ◽  
Static Stability ◽  
Ambient Wind ◽  
Available Potential Energy ◽  
Landscape Variability ◽  
Thermal Perturbations

Abstract It is shown that landscape variability decreases the temperature in the surface layer when, through mesoscale flow, cool air intrudes over warm patches, lifting warm air and weakening the static stability of the upper part of the planetary boundary layer. This mechanism generates regions of upward vertical motion and a sizable amount of available potential energy and can make the environment of the lower troposphere more favorable to cloud formation. This process is enhanced by light ambient wind through the generation of trapped propagating waves, which penetrate into the midtropospheric levels, transporting upward the thermal perturbations and weakening the static stability around the top of the boundary layer. At moderate ambient wind speeds, the presence of surface roughness changes strengthens the wave activity, further favoring the vertical transport of the thermal perturbations. When the intensity of the ambient wind is larger than 5 m s−1, the vertical velocities induced by the surface roughness changes prevail over those induced by the diabatic flux changes. The analysis is performed using a linear theory in which the mesoscale dynamics are forced by the diurnal diabatic sensible heat flux and by the surface stress. Results are shown as a function of ambient flow intensity and of the wavelength of a sinusoidal landscape variability.

scholarly journals The use of bulk and profile methods for determining surface heat fluxes in the presence of glacier winds

2000 ◽  
Vol 46 (154) ◽  
pp. 445-452 ◽  
Author(s):  
Bruce Denby ◽  
W. Greuell
Keyword(s):  
Wind Speed ◽  
Turbulent Fluxes ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Katabatic Wind ◽  
Turbulent Heat ◽  
Glacier Surface ◽  
Surface Heat Fluxes ◽  
Wind Speed Maximum

AbstractA one-dimensional second-order closure model and in situ observations on a melting glacier surface are used to investigate the suitability of bulk and profile methods for determining turbulent fluxes in the presence of the katabatic wind-speed maximum associated with glacier winds. The results show that profile methods severely underestimate turbulent fluxes when a wind-speed maximum is present. The bulk method, on the other hand, only slightly overestimates the turbulent heat flux in the entire region below the wind-speed maximum and is thus much more appropriate for use on sloping glacier surfaces where katabatic winds dominate and wind-speed maxima are just a few meters above the surface.

scholarly journals Origin of Strong Winds in an Explosive Mediterranean Extratropical Cyclone

2019 ◽  
Vol 147 (10) ◽  
pp. 3649-3671 ◽  
Author(s):  
Mihaela Brâncuş ◽  
David M. Schultz ◽  
Bogdan Antonescu ◽  
Christopher Dearden ◽  
Sabina Ştefan
Keyword(s):  
Boundary Layer ◽  
Conveyor Belt ◽  
Heat Fluxes ◽  
The Black Sea ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Strong Wind ◽  
Near Surface ◽  
Cyclone Center ◽  
Strong Winds

Abstract During 2–3 December 2012, the Black Sea and east coast of Romania were affected by a rapidly deepening Mediterranean cyclone. The cyclone developed a bent-back front along which short-lived (2–4 h) strong winds up to 38 m s−1 were recorded equatorward of the cyclone center. A mesoscale model simulation was used to analyze the evolution of the wind field, to investigate the physical processes that were responsible for the strong winds and their acceleration, and to investigate the relative importance of the stability of the boundary layer to those strong winds. The origin of the air in the wind maximum equatorward of the cyclone center was twofold. The first was associated with a sting jet, a descending airstream from the midlevels of the cloud head and the lower part of the cyclonic branch of the warm conveyor belt. The sting jet started to descend west of the cyclone center, ending at the frontolytic tip of the bent-back front. The second was a low-level airstream associated with the cold conveyor belt that originated northeast of the cyclone center and traveled below 900 hPa along the cold side of the bent-back front, ending behind the cold front. Both airstreams were accelerated by the along-flow pressure gradient force, with the largest accelerations acting on the sting-jet air before entering into the near-surface strong-wind area. The sensible heat fluxes destabilized the boundary layer to near-neutral conditions south of the cyclone center, facilitating downward mixing and allowing the descending air to reach the surface. Mesoscale instabilities appeared to be unimportant in the sting-jet formation.

Effects of spatial resolution and temporal offset of air-sea boundary-layer variables on turbulent heat flux estimates

2021 ◽  
Author(s):  
Tong Lee ◽  
Chelle Centemann ◽  
Carol Anne Clayson ◽  
Mark Bourassa ◽  
Shannon Brown ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
High Resolution ◽  
Surface Wind ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Turbulent Heat ◽  
Input Variables ◽  
Temporal Offset

<p>Air-sea turbulent heat fluxes and their spatial gradients are important to the ocean, climate, weather, and their interactions. Satellite-based estimation of air-sea latent and sensible fluxes, providing broad coverage, require measurements of sea surface temperature, ocean-surface wind speed, and air temperature and humidity above sea surface. Because no single satellite has been able to provide simultaneous measurements of these input variables, they typically come from various satellites with different spatial resolutions and sampling times that can be offset by hours. These factors introduce errors in the estimated heat fluxes and their gradients that are not well documented. As a model-based assessment of these errors, we performed a simulation using a Weather Research and Forecasting (WRF) model forced by high-resolution blended satellite SST for the Gulf Stream extension region with a 3-km resolution and with 30-minute output. Latent and sensible heat fluxes were first computed from input variables with the original model resolutions and at coincident times. We then computed the heat fluxes by (1) decimating the input variables to various resolutions from 12.5 to 50 km, and (2) offsetting the “sampling” times of some input variables from others by 3 hours. The resultant estimations of heat fluxes and their gradients from (1) and (2) were compared with the counterparts without reducing resolution and without temporal offset of the input variables. The results show that reducing input-variable resolutions from 12.5 to 50 km weakened the magnitudes of the time-mean and instantaneous heat fluxes and their gradients substantially, for example, by a factor of two for the time-mean gradients. The temporal offset of input variables substantially impacted the instantaneous fluxes and their gradients, although not their time-mean values. The implications of these effects on scientific and operational applications of heat flux products will be discussed. Finally, we highlight a mission concept for providing simultaneous, high-resolution measurements of boundary-layer variables from a single satellite to improve air-sea turbulent heat flux estimation.</p>

Dynamics of a Diabatic Layer in the quasi-geostrophic framework

2021 ◽  
Author(s):  
Rupert Klein ◽  
Lisa Schielicke ◽  
Stephan Pfahl ◽  
Boualem Khouider
Keyword(s):  
Potential Temperature ◽  
Internal Gravity Waves ◽  
Ekman Layer ◽  
Heat Fluxes ◽  
Cloud Formation ◽  
Turbulent Heat ◽  
Ekman Pumping ◽  
Turbulent Friction ◽  
Geostrophic Balance ◽  
Diabatic Processes

<p>Quasi-geostrophic (QG) theory describes the dynamics of synoptic scale flows in the trophosphere that are balanced with respect to both acoustic and internal gravity waves. Within this framework, effects of (turbulent) friction near the ground are usually represented by invoking Ekman Layer theory. The troposphere covers roughly the lowest ten kilometers of the atmosphere while Ekman layer heights are typically just a few hundred meters. However, this two-layer asymptotic theory does not explicitly account for substantial changes of the potential temperature stratification due to diabatic heating associated with cloud formation or with radiative or turbulent heat fluxes, which, in the middle latitudes, can be particularly important in roughly the lowest three kilometers. To alleviate this constraint, this work extends the classical QG plus Ekman layer model by introducing an intermediate, dynamically and thermodynamically active layer, called the "Diabatic Layer" here. The flow in this layer is also in acoustic, hydrostatic, and geostrophic balance but, in contrast to QG flow, variations of potential temperature are not restricted to small deviations from a stable and time independent background stratification. Instead, within this layer, diabatic processes are allowed to affect the leading-order stratification. As a consequence, the Diabatic Layer modifies the pressure field at the top of the Ekman layer, and with it the intensity of Ekman pumping seen by the quasi-geostrophic bulk flow. This leads to a new model for the coupled dynamics of the bulk troposphere, the diabatic layer, and the Ekman layer when strong diabatic processes substantially change the stratification in the lower part of the atmosphere. </p>

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