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

AbstractUnderstanding 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.

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Seamless Stratocumulus Simulation across the Turbulent Gray Zone

Monthly Weather Review ◽

10.1175/mwr-d-13-00229.1 ◽

2014 ◽

Vol 142 (4) ◽

pp. 1655-1668 ◽

Cited By ~ 98

Author(s):

I. A. Boutle ◽

J. E. J. Eyre ◽

A. P. Lock

Keyword(s):

Boundary Layer ◽

Prediction Models ◽

Weather Prediction ◽

Eddy Simulation ◽

Large Eddy ◽

Aircraft Observations ◽

Microphysical Parameterization ◽

Grid Length ◽

Numerical Weather Prediction Models

Abstract A pragmatic approach for representing partially resolved turbulence in numerical weather prediction models is introduced and tested. The method blends a conventional boundary layer parameterization, suitable for large grid lengths, with a subgrid turbulence scheme suitable for large-eddy simulation. The key parameter for blending the schemes is the ratio of grid length to boundary layer depth. The new parameterization is combined with a scale-aware microphysical parameterization and tested on a case study forecast of stratocumulus evolution. Simulations at a range of model grid lengths between 1 km and 100 m are compared to aircraft observations. The improved microphysical representation removes the correlation between precipitation rate and model grid length, while the new turbulence parameterization improves the transition from unresolved to resolved turbulence as grid length is reduced.

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The role of urban boundary layer investigated by high resolution models and ground based observations in Rome area: a step for understanding parameterizations potentialities

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-6-5297-2013 ◽

2013 ◽

Vol 6 (3) ◽

pp. 5297-5344

Author(s):

E. Pichelli ◽

R. Ferretti ◽

M. Cacciani ◽

A. M. Siani ◽

V. Ciardini ◽

...

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Urban Areas ◽

Prediction Models ◽

Weather Prediction ◽

Sonic Anemometer ◽

Lower Atmosphere ◽

Area Of Interest ◽

The Impact

Abstract. The urban forcing on thermo-dynamical conditions can largely influences local evolution of the atmospheric boundary layer. Urban heat storage can produce noteworthy mesoscale perturbations of the lower atmosphere. The new generations of high-resolution numerical weather prediction models (NWP) is nowadays largely applied also to urban areas. It is therefore critical to reproduce correctly the urban forcing which turns in variations of wind, temperature and water vapor content of the planetary boundary layer (PBL). WRF-ARW, a new model generation, has been used to reproduce the circulation in the urban area of Rome. A sensitivity study is performed using different PBL and surface schemes. The significant role of the surface forcing in the PBL evolution has been verified by comparing model results with observations coming from many instruments (LiDAR, SODAR, sonic anemometer and surface stations). The crucial role of a correct urban representation has been demonstrated by testing the impact of different urban canopy models (UCM) on the forecast. Only one of three meteorological events studied will be presented, chosen as statistically relevant for the area of interest. The WRF-ARW model shows a tendency to overestimate vertical transmission of horizontal momentum from upper levels to low atmosphere, that is partially corrected by local PBL scheme coupled with an advanced UCM. Depending on background meteorological scenario, WRF-ARW shows an opposite behavior in correctly representing canopy layer and upper levels when local and non local PBL are compared. Moreover a tendency of the model in largely underestimating vertical motions has been verified.

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Screen-level non-GTS data assimilation in a limited-area mesoscale model

Natural Hazards and Earth System Science ◽

10.5194/nhess-10-1129-2010 ◽

2010 ◽

Vol 10 (6) ◽

pp. 1129-1149 ◽

Cited By ~ 2

Author(s):

M. Milelli ◽

M. Turco ◽

E. Oberto

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Data Assimilation ◽

Planetary Boundary Layer ◽

Prediction Models ◽

Mesoscale Model ◽

Positive Impact ◽

Weather Prediction ◽

Limited Area ◽

Very High

Abstract. The forecast in areas of very complex topography, as for instance the Alpine region, is still a challenge even for the new generation of numerical weather prediction models which aim at reaching the km-scale. The problem is enhanced by a general lack of standard observations, which is even more evident over the southern side of the Alps. For this reason, it would be useful to increase the performance of the mathematical models by locally assimilating non-conventional data. Since in ARPA Piemonte there is the availability of a great number of non-GTS stations, it has been decided to assimilate the 2 m temperature, coming from this dataset, in the very-high resolution version of the COSMO model, which has a horizontal resolution of about 3 km, more similar to the average resolution of the thermometers. Four different weather situations have been considered, ranging from spring to winter, from cloudy to clear sky. The aim of the work is to investigate the effects of the assimilation of non-GTS data in order to create an operational very high-resolution analysis, but also to test the option of running in the future a very short-range forecast starting from these analyses (RUC or Rapid Update Cycle). The results, in terms of Root Mean Square Error, Mean Error and diurnal cycle of some surface variables such as 2 m temperature, 2 m relative humidity and 10 m wind intensity show a positive impact during the assimilation cycle which tends to dissipate a few hours after the end of it. Moreover, the 2 m temperature assimilation has a slightly positive or neutral impact on the vertical profiles of temperature, eventhough some calibration is needed for the precipitation field which is too much perturbed during the assimilation cycle, while it is unaffected in the forecast period. So the stability of the planetary boundary layer, on the one hand, has not been particularly improved by the new-data assimilation, but, on the other hand, it has not been destroyed. It has to be pointed out that a correct description of the planetary boundary layer, even only the lowest part of it, could be helpful to the forecasters and, in general, to the users, in order to deal with meteorological hazards such as snow (in particular snow/rain limit definition), or fog (description of temperature inversions).

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Predicting Small-Scale, Short-Lived Downbursts: Case Study with the NWP Limited-Area ALARO Model for the Pukkelpop Thunderstorm

Monthly Weather Review ◽

10.1175/mwr-d-14-00290.1 ◽

2015 ◽

Vol 143 (3) ◽

pp. 742-756 ◽

Cited By ~ 14

Author(s):

Pieter De Meutter ◽

Luc Gerard ◽

Geert Smet ◽

Karim Hamid ◽

Rafiq Hamdi ◽

...

Keyword(s):

Prediction Models ◽

Deep Convection ◽

Weather Prediction ◽

Parameterization Scheme ◽

Small Scale ◽

Limited Area ◽

Operational Model ◽

Music Festival ◽

Forecast Models

Abstract The authors consider a thunderstorm event in 2011 during a music festival in Belgium that produced a short-lived downburst of a diameter of less than 100 m. This is far too small to be resolved by the kilometric resolutions of today’s operational numerical weather prediction models. Operational forecast models will not run at hectometric resolutions in the foreseeable future. The storm caused five casualties and raised strong societal questions regarding the predictability of such a traumatic weather event. In this paper it is investigated whether the downdrafts of a parameterization scheme of deep convection can be used as proxies for the unresolved downbursts. To this end the operational model ALARO [a version of the Action de Recherche Petite Echelle Grande Echelle-Aire Limitée Adaptation Dynamique Développement International (ARPEGE-ALADIN) operational limited area model with a revised and modular structure of the physical parameterizations] of the Royal Meteorological Institute of Belgium is used. While the model in its operational configuration at the time of the event did not give a clear hint of a downburst event, it has been found that (i) the use of unsaturated downdrafts and (ii) some adaptations of the features of this downdraft parameterization scheme, specifically the sensitivity to the entrainment and friction, can make the downdrafts sensitive enough to the surrounding resolved-scale conditions to make them useful as indicators of the possibility of such downbursts.

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Boundary Layer and Surface Verification of the High-Resolution Rapid Refresh, Version 3

Weather and Forecasting ◽

10.1175/waf-d-20-0101.1 ◽

2020 ◽

Vol 35 (6) ◽

pp. 2255-2278

Author(s):

Robert G. Fovell ◽

Alex Gallagher

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Wind Speed ◽

Prediction Models ◽

Weather Prediction ◽

Potential Temperature ◽

Ground Level ◽

Systematic Errors ◽

Radiosonde Data ◽

Near Surface

AbstractWhile numerical weather prediction models have made considerable progress regarding forecast skill, less attention has been paid to the planetary boundary layer. This study leverages High-Resolution Rapid Refresh (HRRR) forecasts on native levels, 1-s radiosonde data, and (primarily airport) surface observations across the conterminous United States. We construct temporally and spatially averaged composites of wind speed and potential temperature in the lowest 1 km for selected months to identify systematic errors in both forecasts and observations in this critical layer. We find near-surface temperature and wind speed predictions to be skillful, although wind biases were negatively correlated with observed speed and temperature biases revealed a robust relationship with station elevation. Above ≈250 m above ground level, below which radiosonde wind data were apparently contaminated by processing, biases were small for wind speed and potential temperature at the analysis time (which incorporates sonde data) but became substantial by the 24-h forecast. Wind biases were positive through the layer for both 0000 and 1200 UTC, and morning potential temperature profiles were marked by excessively steep lapse rates that persisted across seasons and (again) exaggerated at higher elevation sites. While the source or cause of these systematic errors are not fully understood, this analysis highlights areas for potential model improvement and the need for a continued and accessible archive of the data that make analyses like this possible.

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The Very Stable Boundary Layer on Nights with Weak Low-Level Jets

Journal of the Atmospheric Sciences ◽

10.1175/jas4002.1 ◽

2007 ◽

Vol 64 (9) ◽

pp. 3068-3090 ◽

Cited By ~ 71

Author(s):

Robert M. Banta ◽

Larry Mahrt ◽

Dean Vickers ◽

Jielun Sun ◽

Ben B. Balsley ◽

...

Keyword(s):

Boundary Layer ◽

Time Series ◽

Stable Boundary Layer ◽

Weather Prediction ◽

Density Currents ◽

Lower Boundary Condition ◽

Near Surface ◽

Stable Boundary ◽

Low Level ◽

Low Level Jets

Abstract The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very weak turbulence and negligible turbulent mixing. The focus of this paper is on the effects of this quiescent layer just above the shallow BL, and the impacts of this quiescent layer on turbulent transport and numerical modeling. High-frequency time series of temperature T on a 60-m tower showed that 1) the amplitudes of the T fluctuations were dramatically suppressed at levels above 30 m in contrast to the relatively larger intermittent T fluctuations in the shallow BL below, and 2) the temperature at 40- to 60-m height was nearly constant for several hours, indicating that the very cold air near the surface was not being mixed upward to those levels. The presence of this quiescent layer indicates that the atmosphere above the shallow BL was isolated and detached both from the surface and from the shallow BL. Although some of the nights studied had modestly stronger winds and traveling disturbances (density currents, gravity waves, shear instabilities), these disturbances seemed to pass through the region without having much effect on either the SBL structure or on the atmosphere–surface decoupling. The decoupling suggests that under very stable conditions, the surface-layer lower boundary condition for numerical weather prediction models should act to decouple and isolate the surface from the atmosphere, for example, as a free-slip, thermally insulated layer. A multiday time series of ozone from an air quality campaign in Tennessee, which exhibited nocturnal behavior typical of polluted air, showed the disappearance of ozone on weak low-level jets (LLJ) nights. This behavior is consistent with the two-stratum structure of the vSBL, and with the nearly complete isolation of the surface and the shallow BL from the rest of the atmosphere above, in contrast to cases with stronger LLJs, where such coupling was stronger.

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The role of urban boundary layer investigated with high-resolution models and ground-based observations in Rome area: a step towards understanding parameterization potentialities

Atmospheric Measurement Techniques ◽

10.5194/amt-7-315-2014 ◽

2014 ◽

Vol 7 (1) ◽

pp. 315-332 ◽

Cited By ~ 12

Author(s):

E. Pichelli ◽

R. Ferretti ◽

M. Cacciani ◽

A. M. Siani ◽

V. Ciardini ◽

...

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Urban Areas ◽

Prediction Models ◽

Weather Prediction ◽

Sonic Anemometer ◽

Lower Atmosphere ◽

Area Of Interest ◽

The Impact

Abstract. The urban forcing on thermodynamical conditions can greatly influence the local evolution of the atmospheric boundary layer. Heat stored in an urban environment can produce noteworthy mesoscale perturbations of the lower atmosphere. The new generation of high-resolution numerical weather prediction models (NWP) is nowadays often applied also to urban areas. An accurate representation of cities is key role because of the cities' influence on wind, temperature and water vapor content of the planetary boundary layer (PBL). The Advanced Weather Research and Forecasting model WRF (ARW) has been used to reproduce the circulation in the urban area of Rome. A sensitivity study is performed using different PBL and surface schemes. The significant role of the surface forcing in the PBL evolution has been investigated by comparing model results with observations coming from many instruments (lidar, sodar, sonic anemometer and surface stations). The impact of different urban canopy models (UCMs) on the forecast has also been investigated. One meteorological event will be presented, chosen as statistically relevant for the area of interest. The WRF-ARW model shows a tendency to overestimate the vertical transport of horizontal momentum from upper levels to low atmosphere if strong large-scale forcing occurs. This overestimation is partially corrected by a local PBL scheme coupled with an advanced UCM. Moreover, a general underestimation of vertical motions has been verified.

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Turbulence, Low-Level Jets, and Waves in the Tyrrhenian Coastal Zone as Shown by Sodar

Atmosphere ◽

10.3390/atmos11010028 ◽

2019 ◽

Vol 11 (1) ◽

pp. 28 ◽

Cited By ~ 1

Author(s):

Igor Petenko ◽

Giampietro Casasanta ◽

Simone Bucci ◽

Margarita Kallistratova ◽

Roberto Sozzi ◽

...

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Atmospheric Boundary Layer ◽

Vertical Wind ◽

Land Breeze ◽

Tyrrhenian Sea ◽

Turbulent Layer ◽

Low Level ◽

Doppler Sodar ◽

Low Level Jets

The characteristics of the vertical and temporal structure of the coastal atmospheric boundary layer are variable for different sites and are often not well known. Continuous monitoring of the atmospheric boundary layer was carried out close to the Tyrrhenian Sea, near Tarquinia (Italy), in 2015–2017. A ground-based remote sensing instrument (triaxial Doppler sodar) and in situ sensors (meteorological station, ultrasonic anemometer/thermometer, and net radiometer) were used to measure vertical wind velocity profiles, the thermal structure of the atmosphere, the height of the turbulent layer, turbulent heat and momentum fluxes in the surface layer, atmospheric radiation, and precipitation. Diurnal alternation of the atmospheric stability types governed by the solar cycle coupled with local sea/land breeze circulation processes is found to be variable and is classified into several main regimes. Low-level jets (LLJ) at heights of 100–300 m above the surface with maximum wind speed in the range of 5–18 m s−1 occur in land breezes, both during the night and early in the morning. Empirical relationships between the LLJ core wind speed characteristics and those near the surface are obtained. Two separated turbulent sub-layers, both below and above the LLJ core, are often observed, with the upper layer extending up to 400–600 m. Kelvin–Helmholtz billows associated with internal gravity–shear waves occurring in these layers present opposite slopes, in correspondence with the sign of vertical wind speed gradients. Our observational results provide a basis for the further development of theoretical and modelling approaches, taking into account the wave processes occurring in the atmospheric boundary layer at the land–sea interface.

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Model resolution dependence of convection initiation by orographically-induced thermal circulations

10.5194/egusphere-egu2020-18167 ◽

2020 ◽

Author(s):

Matthias Göbel ◽

Stefano Serafin ◽

Mathias Rotach

Keyword(s):

Boundary Layer ◽

Prediction Models ◽

Initial Conditions ◽

Weather Prediction ◽

Small Scale ◽

Limited Area ◽

Moist Convection ◽

Thermally Driven ◽

Deep Moist Convection ◽

Les Model

Thermally-driven circulations in mountainous terrain can play an essential role in the initiation of deep moist convection: They advect moisture at low levels and provide the necessary trigger mechanism to lift air parcels above the level of free convection. Current limited-area numerical weather prediction models with a horizontal grid spacing of around 1 km may adequately resolve the larger-scale thermal circulations, namely, valley winds and plain-to-mountain winds, but not the small-scale slope winds. In addition, the planetary boundary-layer parametrizations typically employed in these models are based on the assumption of horizontally homogeneous and flat terrain and assume none of the turbulent boundary-layer eddies are explicitly resolved. In this contribution, we investigate the problems that arise due to these deficiencies in the given context using idealized numerical simulations with the WRF model. We compare simulations at different horizontal resolutions in the turbulence gray zone with LES simulations. Previous idealized modeling studies have shown that simulations at kilometer-scale resolution may produce stronger moisture convergence due to thermally-driven circulations and thus earlier and more vigorous convection over the mountain ridges compared with an LES model. We focus on strongly-inhibited initial conditions that lead to deep moist convection with a kilometer-scale but not with an LES model and investigate the reasons for the different convective behavior. The benefits of scale-adaptive boundary-layer schemes for the studied process are evaluated.

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