Sea-breeze Front Research Articles

Meteorological and Land Surface Properties Impacting Sea Breeze Extent and Aerosol Distribution in a Dry Environment

AbstractThe properties of sea breeze circulations are influenced by a variety of meteorological and geophysical factors that interact with one another. These circulations can redistribute aerosol particles and pollution and therefore can play an important role in local air quality, as well as impact remote sensing. In this study, we select 11 factors that have the potential to impact either the sea breeze circulation properties and/or the spatial distribution of aerosols. Simulations are run to identify which of the 11 factors have the largest influence on the sea breeze properties and aerosol concentrations and to subsequently understand the mean response of these variables to the selected factors. All simulations are designed to be representative of conditions in coastal sub tropical environments and are thus relatively dry, as such they do not support deep convection associated with the sea breeze front. For this dry sea breeze regime, we find that the background wind speed was the most influential factor for the sea breeze propagation, with the soil saturation fraction also being important. For the spatial aerosol distribution, the most important factors were the soil moisture, sea‐air temperature difference, and the initial boundary layer height. The importance of these factors seems to be strongly tied to the development of the surface‐based mixed layer both ahead of and behind the sea breeze front. This study highlights potential avenues for further research regarding sea breeze dynamics and the impact of sea breeze circulations on pollution dispersion and remote sensing algorithms.

Diurnal Course of Evaporation From the Dead Sea in Summer: A Distinct Double Peak Induced by Solar Radiation and Night Sea Breeze

AbstractPartitioning between the relative effects of the radiative and aerodynamic components of the atmospheric forcing on evaporation is challenging since diurnal distributions of wind speed and solar radiation typically overlap. The Dead Sea is located about a 100 km off the Eastern Mediterranean coast, where and the Mediterranean Sea breeze front reaches it after sunset. Therefore, in the Dead Sea the peaks of solar radiation and wind speed diurnal cycles in the Dead Sea are distinctly separated in time, offering a unique opportunity to distinguish between their relative impacts on evaporation. We present mid‐summer eddy covariance and meteorological measurements of evaporation rate and surface energy fluxes over the Dead Sea. The evaporation rate is characterized by a clear diurnal cycle with a daytime peak, few hours after solar radiation peak, and a nighttime peak coincident with wind speed peak. Evaporation rate is minimum during sunrise and sunset. Measurements of evaporation rate from two other water bodies that are closer to the Mediterranean coast, Eshkol Reservoir, and Lake Kinneret, present a single afternoon peak, synchronous with the sea breeze. The inland diurnal evaporation rate cycle varies with the distance from the Mediterranean coast, following the propagation of sea breeze front: near the coast, wind speed, and radiation peaks are close and consequently a single daily evaporation peak appears in the afternoon; at the Dead Sea, about a 100 km inland, the sea breeze front arrives at sunset, resulting in a diurnal evaporation cycle characterized by a distinct double peak.

The influence of sea- and land-breeze circulations on the diurnal variability in precipitation over a tropical island

Abstract. This study examines the diurnal variation in precipitation over Hainan Island in the South China Sea using gauge observations from 1951 to 2012 and Climate Prediction Center MORPHing technique (CMORPH) satellite estimates from 2006 to 2015, as well as numerical simulations. The simulations are the first to use climatological mean initial and lateral boundary conditions to study the dynamic and thermodynamic processes (and the impacts of land–sea breeze circulations) that control the rainfall distribution and climatology. Precipitation is most significant from April to October and exhibits a strong diurnal cycle resulting from land–sea breeze circulations. More than 60 % of the total annual precipitation over the island is attributable to the diurnal cycle with a significant monthly variability. The CMORPH and gauge datasets agree well, except that the CMORPH data underestimate precipitation and have a 1 h peak delay. The diurnal cycle of the rainfall and the related land–sea breeze circulations during May and June were well captured by convection-permitting numerical simulations with the Weather Research and Forecasting (WRF) model, which were initiated from a 10-year average ERA-Interim reanalysis. The simulations have a slight overestimation of rainfall amounts and a 1 h delay in peak rainfall time. The diurnal cycle of precipitation is driven by the occurrence of moist convection around noontime owing to low-level convergence associated with the sea-breeze circulations. The precipitation intensifies rapidly thereafter and peaks in the afternoon with the collisions of sea-breeze fronts from different sides of the island. Cold pools of the convective storms contribute to the inland propagation of the sea breeze. Generally, precipitation dissipates quickly in the evening due to the cooling and stabilization of the lower troposphere and decrease of boundary layer moisture. Interestingly, the rather high island orography is not a dominant factor in the diurnal variation in precipitation over the island.

Interaction of Urban Heating and Local Winds During the Calm Intermonsoon Seasons in the Tropics

AbstractRapid urbanization of cities has greatly modified the thermal and dynamic profile in the urban boundary layer. This paper attempts to study the interaction of urban heating and the local topographic‐induced flow circulation for a tropical coastal city, Greater Kuala Lumpur, in Malaysia. The role of sea‐and‐valley‐breeze‐orientated synoptic flow (SBOS) on the interaction is determined by comparing two intermonsoon periods. A state‐of‐the‐art numerical model, Advanced Research Weather Research and Forecasting model, is used to identify the influence of urbanization through modification of urban surfaces. The model reasonably reproduces the vertical sounding data and near‐surface weather parameters. The diurnal urban heating pattern is attributed to three predominant factors: (i) weak under calm and clear‐sky condition (morning heating), (ii) weak under larger atmospheric moisture content (late afternoon convection), and (iii) largest (1.4°C) due to differential cooling rate of urban and rural surface at night. The interaction of urban thermals and upper level SBOS affects the effect of urbanization on local circulation during the day. The urban thermals reduce the weak opposing SBOS (<2 m s−1) and enhance the inflow of moisture‐rich sea breeze passage. This increases the intensity of downwind convective precipitation during late afternoon. On contrary, the strong opposing SBOS (>2 m s−1) suppresses the vertical lifting of urban thermals and decelerates the sea breeze front. It is discovered that the interaction of urban heating and topographic‐induced flow is interdependent while the synoptic flow plays a critical role in modifying both factors, respectively.

Influence of Monsoonal Wind Speed and Moisture Content on Intensity and Diurnal Variations of the Mei-Yu Season Coastal Rainfall over South China

AbstractConvection-permitting numerical experiments using the Weather Research and Forecasting (WRF) Model are performed to explore the influence of monsoonal onshore wind speed and moisture content on the intensity and diurnal variations of coastal rainfall over south China during the mei-yu seasons. The focus of the analyses is on a pair of 10-day WRF simulations with diurnally cyclic-in-time lateral boundary conditions averaged over the high versus low onshore wind speed days of the 2007–09 mei-yu seasons. Despite differences in the rainfall intensity, the spatial distributions and diurnal variations of rainfall in both simulations verified qualitatively well against the mean estimates derived from ground-based radar observations, averaged respectively over either the high-wind or low-wind days.Sensitivity experiments show that the pattern of coastal rainfall spatial distribution is mostly controlled by the ambient onshore wind speed. During the high-wind days, strong coastal rainfall is concentrated along the coastline and reaches its maximum in the early morning. The coastal lifting induced by the differential surface friction and small hills is the primary cause for the strong coastal rainfall, while land breeze enhances coastal lifting and precipitation from evening to early morning. In the low-wind days, on the other hand, coastal rainfall is mainly induced by the land–sea-breeze fronts, which has apparent diurnal propagation perpendicular to the coastline. With stronger land–sea temperature contrast, the land–sea breeze is stronger during the low-wind days. Both in the high-wind and low-wind days, the coastal rainfall intensity is sensitive to the incoming moisture in the upstream oceanic airflow, especially to the moisture content in the boundary layer.

Sea breeze effect mapping for mitigating summer urban warming: For making urban environmental climate map of Yokohama and its surrounding area

Temperatures in urban area rise; therefore, it is necessary to consider the mitigation of urban warming. Considering that, in coastal areas such as Yokohama, an effective mitigation could be based on sea breeze patterns during summer daytime, this study aims at analyzing the relationship between temperature distributions and sea breeze fronts. Eventually, the target of this paper could be the creation of an Urban Environmental Climate Map (UECM) showing mitigation measures for each zone. Firstly, fine weather days are extracted and the sea breeze blowing patterns are classified in the target area. Secondly, the wind and temperature distribution patterns are mapped using WRF. The results obtained enable the mapping of the sea breeze fronts and the analysis of the relationship between temperature distributions and sea breeze fronts. Finally, a spatial zoning is performed. The results are presented as follows: 1) Sea breeze blowing patterns are classified into three patterns; 2) Based on the numerical simulation results and the temperature data observed in coastal areas, southwest wind has a cooling effect; 3) The target area is classified into “Sea breeze arrival zone” and others based on the presence of a see breeze; 4) An UECM showing proper mitigation measures is proposed.

Relay transport of aerosols to Beijing-Tianjin-Hebei region by multi-scale atmospheric circulations

The Beijing-Tianjin-Hebei (BTH) region experiences heavy aerosol pollution, which is found to have close relationships with the synoptic- and local-scale atmospheric circulations. However, how and to what extent these multi-scale circulations interplay to modulate aerosol transport have not been fully understood. To this end, this study comprehensively investigated the impacts of these circulations on aerosol transport in BTH by focusing on an episode occurred on 1 June 2013 through combining both observations and three-dimensional simulations. It was found that during this episode, the Bohai Sea acted as a transfer station, and the high-pressure system over the Yellow Sea and sea-breeze in BTH took turns to affect the transport of aerosols. In the morning, influenced by the high-pressure system, lots of aerosols emitted from Shandong and Jiangsu provinces were first transported to the Bohai Sea. After then, these aerosols were brought to the BTH region in the afternoon through the inland penetration of sea-breeze, significantly exacerbating the air quality in BTH. The inland penetration of sea-breeze could be identified by the sharp changes in ground-based observed temperature, humidity, and wind when the sea-breeze front (SBF) passed by. Combining observations with model outputs, the SBF was found to be able to advance inland more than ∼150 km till reaching Beijing. This study has important implications for better understanding the aerosol transport in BTH, and improving the forecast of such aerosol pollution.

Impact of sea breeze penetration over urban areas on midsummer temperature distributions in the Tokyo Metropolitan area

We investigate the relationship between sea breeze penetration and anomalously high summer daytime temperatures in the inland suburbs of Tokyo, with consideration of the relative position of sea breeze fronts. We use data from the high‐density temperature observation network (Extended‐METROS) established by our research group to clarify in detail the mechanisms creating these high‐temperature areas.We conducted case and composite analyses of the temperature and wind distributions when typical sea breeze fronts developed. The observations showed that the highest temperatures were near Kawagoe City in the early afternoon, which is 40 km leeward of central Tokyo, on all days analysed, and that the high‐temperature inland areas were located leeward of Tokyo. The high temperatures near Kawagoe City were not observed by the Automated Meteorological Data Acquisition System (AMeDAS). From the case and composite analyses, there was a 2‐h time lag between the sea breeze front penetrating the area leeward of Tokyo and extreme high temperatures being observed around Kawagoe City. Significant divergence was also observed during this time, and a decrease in the dew point temperature was also observed around Kawagoe City on the case study day; these conditions persisted for approximately 2 h. These results suggest the existence of a subsidence flow near Kawagoe City, and the resulting adiabatic compression heating contributed to the observed extreme high temperatures. In the late evening, a high‐temperature band was observed leeward of the northern suburbs of Tokyo, even though southerly sea breezes were recorded over the entire Kanto Plain, and cold advection dominated at that time. However, the cold advection in this high‐temperature band was less notable than in the periphery. Thus, temperature declines were small in this area, and overall temperatures remained relatively high.

Interaction between turbulent flow and sea breeze front over urban‐like coast in large‐eddy simulation

AbstractTurbulent flow and its interaction with a sea breeze front (SBF) over an urban‐like coast with a regular block array were investigated using a building‐resolving computational fluid dynamics model. It was found that during daytime with an offshore ambient flow, streaky turbulent structures tended to grow within the convective boundary layer (CBL) over a warm urban surface ahead of the SBF. The structures were organized as streamwise streaks at an interval of a few hundred meters, which initiated at the rooftop level with strong wind shear and strengthens in the CBL with moderate buoyancy. The streaks then interacted with the onshore‐propagating SBF as it made landfall. The SBF, which was initially characterized as a shallow and quasi‐linear feature over the sea, developed three‐dimensional structures with intensified updrafts at an elevated frontal head after landfall. Frontal updrafts were locally enhanced at intersections where the streaks merged with the SBF, which greatly increased turbulent fluxes at the front. The frontal line was irregular because of merging, tilting, and transformation effects of vorticity associated with streaky structures. Inland penetration of the SBF was slowed by the frictional effect of urban‐like surfaces and turbulent flow on land. The overall SBF intensity weakened after the interaction with turbulent flow. These findings aid understanding of local weather over coastal cities during typical sea breeze conditions.

Lobe‐cleft instability in the buoyant gravity current generated by estuarine outflow

AbstractGravity currents represent a broad class of geophysical flows including turbidity currents, powder avalanches, pyroclastic flows, sea breeze fronts, haboobs, and river plumes. A defining feature in many gravity currents is the formation of three‐dimensional lobes and clefts along the front and researchers have sought to understand these ubiquitous geophysical structures for decades. The prevailing explanation is based largely on early laboratory and numerical model experiments at much smaller scales, which concluded that lobes and clefts are generated due to hydrostatic instability exclusively in currents propagating over a nonslip boundary. Recent studies suggest that frontal dynamics change as the flow scale increases, but no measurements have been made that sufficiently resolve the flow structure in full‐scale geophysical flows. Here we use thermal infrared and acoustic imaging of a river plume to reveal the three‐dimensional structure of lobes and clefts formed in a geophysical gravity current front. The observed lobes and clefts are generated at the front in the absence of a nonslip boundary, contradicting the prevailing explanation. The observed flow structure is consistent with an alternative formation mechanism, which predicts that the lobe scale is inherited from subsurface vortex structures.

Idealized simulations of sea breezes over mountainous islands

High‐resolution simulations of diurnally heated airflows over mountainous islands are conducted to study the impacts of island terrain on sea‐breeze circulations. As the simulated island terrain height increases, the sea‐breeze front (SBF) propagates inland faster but its frontal circulation weakens dramatically, as does the sea‐breeze flow behind it. This sensitivity is interpreted through the frontogenesis budget in a SBF‐relative reference frame. The dominant frontogenetic term (the cross‐frontal convergence) weakens over taller islands, but this trend is offset by a similar weakening of the dominant frontolytic term (the front‐relative advection) to yield minimal sensitivity of the SBF baroclinicity to island terrain height. At first glimpse, this finding appears inconsistent with the systematic reduction in SBF circulation strength over taller islands. However, as the island height is increased, baroclinicity at the SBF becomes increasingly associated not with the front itself but with the ‘background’ baroclinicity associated with elevated heating over steeper island slopes. When this latter contribution is removed, a clear weakening of the SBF baroclinicity is recovered over the taller islands. Analysis of the cross‐frontal convergence budget indicates that this reduction in cross‐frontal convergence, and hence the SBF baroclinicity, is caused by an increased slope‐parallel buoyancy gradient. As the SBF migrates inland, this gradient preferentially accelerates the air ahead of the SBF up the slope, which weakens the convergence along the front. Finally, as shown by a simple scaling of the slope‐parallel momentum equation, the weakened sea‐breeze flow over taller islands is associated with a weaker onshore perturbation pressure gradient force, owing to the protrusion of the convective boundary layer into the stable free troposphere.

Sea breeze and precipitation over Hainan Island

The relationship between sea breeze and precipitation over Hainan Island is investigated, along with consideration of the effects of orography, synoptic wind and environmental conditions, using a period (2011–2015) of high‐resolution observational data, ERA‐Interim reanalysis data, and numerical experiments. The results show that the combined effect of mountain–plain and plain–sea thermal contrasts plays a major role in promoting sea‐breeze occurrence on Hainan Island. Although the sea‐breeze convergence tendency favours precipitation formation during October to March, the low humidity and convective available potential energy (CAPE), stable atmospheric stratification, high free convection level (LFC) and strong vertical wind shear in northeasterlies during this period notably inhibit the occurrence of heavy precipitation on Hainan Island. On the contrary, large amounts of precipitation over Hainan Island during April to September arise from high humidity and CAPE, unstable atmospheric stratification, low LFC and weak vertical wind shear associated with southwesterlies; additionally, the sea‐breeze front (SBF) has a convergence tendency under weak synoptic winds. In the context of the synoptic wind, these strong precipitations are prone to occur on the mountain leeward side, likely because of a strong leeward‐side SBF, which could be enhanced considerably by the orography; however, heavy precipitation seldom occurs on the mountain windward side, likely due to weak lifting motion associated with weak synoptic winds and a less significant windward‐side SBF. Because of these effects, pronounced precipitation is apt to occur over the east and west coasts, and propagates northeastward or northward with the steering of the southwesterlies and the development of SBFs in the north part of the island, during April to August. Whereas in September similar amounts of precipitation tend to occur over the southeast and southwest coasts, and expand locally with the influence of the easterlies and the inland penetration of SBFs.

An Israeli haboob: Sea breeze activating local anthropogenic dust sources in the Negev loess

Abstract Meso-scale weather systems, such as convective haboobs, are considered to be an important dust generation mechanism. In Israel, however, rather than of meso-scale weather systems, most dust storms are generated by synoptic-scale systems, originating from Sahara and Arabia. Consequently, only distal sources of suspended and deposited dust in Israel are currently reported. Here we report the first detailed study on the merging of synoptic- and meso-scale weather systems leading to a prominent dust outbreak over the Negev, Israel. During the afternoon of May 2 nd , 2007, a massive dust storm covered the northern Negev, forming a one kilometer high wall of dust. The haboob was associated with PM 10 concentrations of 1000–1500 μg m −3 that advanced at a speed of 10–15 m s −1 and caused temporary closure of local airports. In contrast to most reported haboobs, this one was generated by a sea breeze front acting as a weak cold front enhanced by a cold core cyclone positioned over Libya and Egypt. The sea breeze that brought cold and moist marine air acted as a gravity current with strong surface winds. The sources for the haboob were the loessial soils of the northwestern Negev, especially agricultural fields that were highly disturbed in late spring to early summer. Such surface disturbance is caused by agricultural and/or intensive grazing practices. Our study emphasizes the importance of local dust sources in the Negev and stresses loess recycling as an important process in contemporary dust storms over Israel.

Evaluation of weather research and forecasting model parameterizations under sea-breeze conditions in a North Sea coastal environment

Three atmospheric boundary layer (ABL) schemes and two land surface models that are used in the Weather Research and Forecasting (WRF) model, version 3.4.1, were evaluated with numerical simulations by using data from the north coast of France (Dunkerque). The ABL schemes YSU (Yonsei University), ACM2 (Asymmetric Convective Model version 2), and MYJ (Mellor–Yamada–Janjic) were combined with two land surface models, Noah and RUC (Rapid Update Cycle), in order to determine the performances under sea-breeze conditions. Particular attention is given in the determination of the thermal internal boundary layer (TIBL), which is very important in air pollution scenarios. The other physics parameterizations used in the model were consistent for all simulations. The predictions of the sea-breeze dynamics output from the WRF model were compared with observations taken from sonic detection and ranging, light detection and ranging systems and a meteorological surface station to verify that the model had reasonable accuracy in predicting the behavior of local circulations. The temporal comparisons of the vertical and horizontal wind speeds and wind directions predicted by the WRF model showed that all runs detected the passage of the sea-breeze front. However, except for the combination of MYJ and Noah, all runs had a time delay compared with the frontal passage measured by the instruments. The proposed study shows that the synoptic wind attenuated the intensity and penetration of the sea breeze. This provided changes in the vertical mixing in a short period of time and on soil temperature that could not be detected by the WRF model simulations with the computational grid used. Additionally, among the tested schemes, the combination of the localclosure MYJ scheme with the land surface Noah scheme was able to produce the most accurate ABL height compared with observations, and it was also able to capture the TIBL.

Diurnal Variations of the Land–Sea Breeze and Its Related Precipitation over South China

Abstract Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) Model are performed to examine the diurnal cycles of land and sea breeze and its related precipitation over the south China coastal region during the mei-yu season. The focus of the analyses is a 10-day simulation initialized with the average of the 0000 UTC gridded global analyses during the 2007–09 mei-yu seasons (11 May–24 June) with diurnally varying cyclic lateral boundary conditions. Despite differences in the rainfall intensity and locations, the simulation verified well against averages of 3-yr ground-based radar, surface, and CMORPH observations and successfully simulated the diurnal variation and propagation of rainfall associated with the land and sea breeze over the south China coastal region. The nocturnal offshore rainfall in this region is found to be induced by the convergence line between the prevailing low-level monsoonal wind and the land breeze. Inhomogeneity of rainfall intensity can be found along the coastline, with heavier rainfall occurring in the region with coastal orography. In the night, the mountain–plain solenoid produced by the coastal terrain can combine with the land breeze to enhance offshore convergence. In the daytime, rainfall propagates inland with the inland penetration of the sea breeze, which can be slowed by the coastal mountains. The cold pool dynamics also plays an essential role in the inland penetration of precipitation and the sea breeze. Dynamic lifting produced by the sea-breeze front is strong enough to produce precipitation, while the intensity of precipitation can be dramatically increased with the latent heating effect.

A sea breeze induced thunderstorm over an inland station over Indian South Peninsula – A case study

The dynamic interaction of sea breeze with the prevailing synoptic flows can give rise to meteorological conditions conducive for the occurrence to the thunderstorms over coastal and adjoining regions. Here, we present a rare case study of the genesis of the thunderstorm that occurred on 4th May 2011 at 1500Z over Gadanki (13.5°N, 79.2°E), one of the tropical inland stations (100km) near to the east coast of the Indian peninsula. The objective of present work is to understand the underlying physical mechanism of initiation of such convection over this region. A set of meteorological observations obtained from microwave radiometer profiler, eddy covariance flux tower system, and Doppler weather radar, are used for investigating the convection genesis characteristics. In conjunction with observations, to bridge the gap of lack of high resolutional spatial observations, the high-resolution (2km) model analysis is developed using Weather Research and Forecasting (WRF) model and four-dimensional data assimilation technique. The analysis of thermodynamical and dynamical indices carried out from the model analysis as well as observations. Results obtained from this study indicated the presence of a wind discontinuity line and a warm air advection from the north Indian region towards Gadanki caused this area hot dry and convectively active. The sea breeze front propagated over hot and dry area few hours before the genesis of the thunderstorm. The moisture flux convergence increased with the inland propagation of sea breeze front. We found that the inland penetration of sea-breeze front caused advection of moist and cold air over warm and dry region; reduction in dew point depression causing bulging of dry line and lowering of lifting condensation level; development of shear in wind direction and speed; increase in low level convergence and vertical velocity, upward transport of moist air and finally increase in helicity of the environment. The wind shear instability and thermodynamic instability generated because of intrusion of sea breeze front at Gadanki region and reduction in dew point depression because of the incursion of moisture respectively. Convective instability developed due to the lifting of the dry and stable parcel under the influence of wind shear instability and its moistening due to advected moisture over this region are responsible for thunderstorm initiation over this region.

Ceilometer evaluation of the eastern Mediterranean summer boundary layer height – first study of two Israeli sites

Abstract. Active remote-sensing instruments, such as ceilometers, have been shown to be potentially useful for the investigation of the behavior of the atmospheric mixing layer height (MLH). For the first time ever, high-resolution measurements of backscatter intensity, taken from two CL31 ceilometers situated inland and onshore of Israel, have enabled evaluation of the mean diurnal cycle of the MLH in the eastern Mediterranean region. Although the Israeli summer synoptic conditions are considered to be quite stable, results for the summer season (July–August 2014) showed the inland MLH to be about 200 m higher than the MLH at the onshore site, situated only 7.5 km away. The prevailing influence of the sea breeze front (SBF), as it progresses inland, is presented by the ceilometer plots. Complementing results were found between the radiosonde profiles and the adjacent ceilometer at the inland site of Beit Dagan. In contrast to the expected regularity of clear skies during the Israeli summer, the ceilometers revealed significant cloud cover throughout the day, with higher presence onshore. Assessment of cloud thickness in further research would serve to improve the evaluation of the MLH evolution.

Modeling of urban heat island and its impacts on thermal circulations in the Beijing–Tianjin–Hebei region, China

Through regulating the land–atmosphere energy balance, urbanization plays an important role in modifying local circulations and cross-border transport of air pollutants. The Beijing–Tianjin–Hebei (BTH) metropolitan area in northern China is frequently influenced by complex atmospheric thermal circulations due to its special topography and geographic position. In this study, the Weather Research and Forecasting (WRF) model combined with remote sensing is used to explore the urbanization impacts on local circulations in the BTH region. The urban heat island (UHI) effect generated around Beijing and Tianjin shows complex interactions with local thermal circulations. Due to the combined effects of UHI and topography, the UHI circulation around Beijing and valley breeze at the southern slopes of Yan Mountain are coupled together to reinforce each other. At the coastal cities, the increased land/sea temperature gradient considerably accelerates the sea breeze along Bohai Bay and moves the sea breeze front further inland to reach as far as Beijing. This study may lay a foundation for the better understanding of air pollutant dispersion on complex terrain.

Intrusive gravity currents propagating into two-layer stratified ambients: Vorticity modeling

Intrusive gravity currents propagating into two-layer stratified ambients: Vorticity modeling M. A. Khodkar, M. M. Nasr-Azadani and E. Meiburg Department of Mechanical Engineering University of California at Santa Barbara Santa Barbara, CA 93106. Email: mkhodkar@umail.ucsb.edu Abstract A simplified model is developed for intrusive gravity currents propagating along the interface of a two- layer stratified ambient. The model is based on the conservation of mass and vorticity, and it does not require any empirical closure assumptions. A parametric study conducted with this model reproduces the correct behavior in various limits, and is consistent with previously reported experimental observations. Specifically, it predicts the formation of equilibrium intrusions when the intrusion density equals the depth-weighted mean density of the two ambient layers. It furthermore demonstrates the existence of non- smooth limits under certain conditions. An energy analysis shows that under non-equilibrium conditions the intrusion gains energy. The predictions by the parametric study are furthermore compared to two- dimensional DNS results, and very good agreement is observed with regard to all flow properties. 1. Introduction Intrusions represent a special class of gravity currents that propagate horizontally into a stratified ambient at intermediate depths. They occur in a variety of atmospheric and oceanic situations, where they can influence the dynamics of such flows as sea breeze fronts, river plumes and powder snow avalanches. Fig1. Schematic of an intrusion produced via a lock-release process: a) at the initial state, where all the fluids are at rest, b) when the gate is removed and a quasisteady, symmetric intrusion forms and c) when (𝜌 𝑙 𝑑 𝑙 + 𝜌 𝑢 𝑑 𝑢 )/𝐻 < 𝜌 𝑐 and a quasisteady, non-symmetric intrusion emerges.

A modeling study of convection initiation prior to the merger of a sea-breeze front and a gust front

Convection initiation (CI) prior to the merger of a sea-breeze front (SBF) with a gust front (GF) in North China is investigated using a real-data Weather Research and Forecasting (WRF) simulation with a high resolution of 444.4m. The overall evolution of the GF and SBF is well reproduced by the simulation. The GF was produced by the decaying convective storm over northern Beijing, while the SBF came from the Bohai Sea. Several convective cells were generated between the two fronts even though they were still about 25–30km far away from each other. During the development of these cells, the low-level convergence and conditional instability averaged within the intermediate area between the two fronts were enhanced significantly, both of which favored the initiation of convection.Vertical momentum budgets were conducted in the intermediate area as well as along the backward trajectories of parcels within a selected convective cell. The vertical acceleration was decomposed into dynamic and buoyant components, respectively. The diagnostic results showed that the dynamic acceleration dominated in the low level, while buoyant acceleration became evident only when the parcel reached a high altitude above 2km. Therefore the dynamic forcing appeared to be more relevant to CI. The dynamic acceleration was further decomposed into four terms based on anelastic approximation. The positive dynamic acceleration was mainly caused by fluid extension associated with the low-level convergence, while fluid twisting in the vertical contributed negatively to the dynamic acceleration. The other two terms related to horizontal curvature and height variation of density were negligibly small.