Land-sea Breeze System Research Articles

Influence of Regional Pollution Outflow on Particle Number Concentration and Particle Size in Airshed of Guangzhou, South China

A measurement campaign of particle number concentration and size distribution was conducted at urban (SYSU) and suburban (Panyu) areas of Guangzhou, South China, during 16 January to 3 February 2020 before and during the Chinese New Year (CNY) holiday. Average particle number concentration (PNC) was 6.3 × 103 cm-3 and 9.7 × 103 cm-3, respectively, at urban and suburban sites, indicating the severe particulate matter (PM) pollution. The PNC in the region was influenced by monsoon and the land–sea breeze systems. During monsoon seasons, PM pollution occurred at a regional scale affecting the urban and suburban areas as indicated by the high PNC correlation (r value = 0.70), but the PNCs were lower than that during land-sea breeze period (with lower PNCM1/PNCLSB1 and PNCM2/PNCLSB2 ratios) due to the higher atmospheric dispersion. There is a strong local emission (mainly vehicular emissions) in both urban and suburban areas which was significantly lowered during the CNY period due to reduced human activities. The PM pollution was found to be significantly influenced by local emissions (dominated by Aitken mode particles) and new particle formation (NPF) process (dominated by nucleation mode particles). NPF event was found to be associated with a higher N10-25/H2SO4 proxy ratio during the low wind speed condition.

Dynamics of the Land–Sea Breeze System and the Surface Current Response in South-West Australia

The land–sea breeze (LSB) system, driven by the thermal contrast between the land and the adjacent ocean is a widely known atmospheric phenomenon, which occurs in coastal regions globally. South-west Australia experiences a persistent and one of the strongest LSB systems globally with maximum wind speeds associated with the LSB system often exceeding 15 ms−1. In this paper, using field measurements and numerical simulations, we examine: (1) the local winds associated with the land–sea breeze with an emphasis on the ocean; and, (2) the response of the surface currents to the diurnal wind forcing. The measurements indicated that the wind speeds decreased between midnight and 0400 and increased rapidly after 1100, reaching maxima >10 ms−1 around 1800) associated with the sea breeze and decreased to midnight. Wind directions were such that they were blowing from south-east (120°) in the morning and changed to almost southerly (~200°) in the afternoon. Decomposition of the wind record to the diurnal and synoptic components indicated that the diurnal component of winds (i.e., LSB) was oriented along the south-west to north-east axis. However, the stronger synoptic winds were from the south-east to south quadrant and in combination with the LSB, the winds consisted of a strong southerly component. We examined the evolution, horizontal extent, and propagation properties of sea breeze fronts for characteristic LSB cycles and the sea breeze cell propagating offshore and inland. The results indicated that the sea breeze cell was initiated in the morning in a small area, close to 33° S, 115.5° E, with a width of ~25 km and expanded onshore, offshore and alongshore. The sea breeze cell expanded faster (30 kmh−1) and farther (120 km) in the offshore direction than in the onshore direction (10 kmh−1 and 30–40 km). Winds during the LSB cycle followed a counterclockwise rotation that was also reflected in the surface currents. The winds and surface currents rotated anticlockwise with the surface currents responding almost instantaneously to changes in wind forcing but were modified by topography. The diurnal surface currents were enhanced due to the resonance between the LSB forcing and the inertial response.

Aerosol distribution in the northern Gulf of Guinea: local anthropogenic sources, long-range transport, and the role of coastal shallow circulations

Abstract. The complex vertical distribution of aerosols over coastal southern West Africa (SWA) is investigated using airborne observations and numerical simulations. Observations were gathered on 2 July 2016 offshore of Ghana and Togo, during the field phase of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa project. This was the only flight conducted over the ocean during which a downward-looking lidar was operational. The aerosol loading in the lower troposphere includes emissions from coastal cities (Accra, Lomé, Cotonou, and Lagos) as well as biomass burning aerosol and dust associated with long-range transport from central Africa and the Sahara, respectively. Our results indicate that the aerosol distribution on this day is impacted by subsidence associated with zonal and meridional regional-scale overturning circulations associated with the land–sea surface temperature contrast and orography over Ghana and Togo, as typically observed on hot, cloud-free summer days such as 2 July 2016. Furthermore, we show that the zonal circulation evidenced on 2 July is a persistent feature over the Gulf of Guinea during July 2016. Numerical tracer release experiments highlight the dominance of aged emissions from Accra on the observed pollution plume loadings over the ocean, in the area of aircraft operation. The contribution of aged emission from Lomé and Cotonou is also evident above the marine boundary layer. Given the general direction of the monsoon flow, the tracer experiments indicate no contribution from Lagos emissions to the atmospheric composition of the area west of Cotonou, where our airborne observations were gathered. The tracer plume does not extend very far south over the ocean (i.e. less than 100 km from Accra), mostly because emissions are transported northeastward near the surface over land and westward above the marine atmospheric boundary layer. The latter is possible due to interactions between the monsoon flow, complex terrain, and land–sea breeze systems, which support the vertical mixing of the urban pollution. This work sheds light on the complex – and to date undocumented – mechanisms by which coastal shallow circulations can distribute atmospheric pollutants over the densely populated SWA region.

Diurnal Sea Breezes Force Near-Inertial Waves along Rottnest Continental Shelf, Southwestern Australia

AbstractObservations of upper-ocean dynamics close to the critical latitude (ratio of the local inertial to diurnal frequency is 0.94) from a range of platforms (surface currents using HF radar, moored instruments, and satellite remote sensing data) off southwest Australia indicated the presence of energetic, near-inertial waves generated through the diurnal–inertial resonance. During the austral summer, when southerly winds and land–sea breeze (LSB) system dominated the wind regime, strong counterclockwise diurnal motions (amplitudes surpassing 0.3 m s−1) penetrated to 300-m depth with diurnal vertical isotherm fluctuations up to 60 m. The upward phase propagation speed of ~140 m day−1, deep penetration of diurnal currents below the mixed layer, and the ~180° phase difference between the upper and lower water column suggested that the local LSB system caused the resonant diurnal motions. Relative vorticity fluctuations along two cross-shore transects indicated changes to the local effective Coriolis frequency by more than 50% (±0.5f). In the presence of strong and relatively consistent cross-shore diurnal wind forcing in the study area, the main factors that controlled the observed energetic but sporadic near-inertial oscillations were the Leeuwin Current strength and spatial–temporal variations. These variations controlled the effective Coriolis frequency and enabled the effective pumping of diurnal wind energy into the ocean particularly when the effective Coriolis frequency was ~24 h.

Impact of land–sea breezes at different scales on the diurnal rainfall in Taiwan

The formation mechanism of diurnal rainfall in Taiwan is commonly recognized as a result of local forcings involving solar thermal heating and island-scale land–sea breeze (LSB) interacting with orography. This study found that the diurnal variation of the large-scale circulation over the East Asia-Western North Pacific (EAWNP) modulates considerably the diurnal rainfall in Taiwan. It is shown that the interaction between the two LSB systems—the island-scale LSB and the large-scale LSB over EAWNP—facilitates the formation of the early morning rainfall in western Taiwan, afternoon rainfall in central Taiwan, and nighttime rainfall in eastern Taiwan. Moreover, the post-1998 strengthening of a shallow, low-level southerly wind belt along the coast of Southeast China appears to intensify the diurnal rainfall activity in Taiwan. These findings reveal the role of the large-scale LSB and its long-term variation in the modulation of local diurnal rainfall.

Impact of Diurnal Variation of Winds on Coastal Waves off South East Coast of India

The land-sea breeze systems, the most interesting phenomena observed at coastal regions, have significant impact on the costal wave characteristics. Present study also focuses the diurnal variations of winds and its impact on wave parameters like significant wave height and mean wave period off Ennore port located in the south east coast of India The impact of the diurnal variation of winds on complex wave patterns in the coastal regions of Indian Ocean have been addressed earlier also. In the present study an attempt has been made to explore the impact of diurnal variation of winds on coastal waves using numerical model forced with European Centre for Medium Range Weather Forecasts (ECMWF) and National Centre for Environmental Prediction (NCEP) blended wind fields. It has been observed that most of the time, the ECMWF blended wind field reproduces the diurnal variation.

Recent Studies on Wind Seas and Swells in the Indian Ocean: A Review

Winds and waves data measured at a few locations along the west coast of India as well as wave model results were analysed to study the wave characteristics in the deep as well as nearshore regions during different seasons. During fair weather season, we can observe distinct and systematic diurnal variation in wave height and wave period, especially, simultaneous increase in wave height and decrease in wave period with increase in local wind speed. This is due to superimposition of wind seas generated by the sea breeze system with the pre-existing swells. The swells observed in the Arabian Sea are from SW direction during SW monsoon season and from SW/SSW and NW directions during both pre-monsoon and post-monsoon seasons. The NW swells which occur during Shamal events show distinct characteristics such as an increase in wave height, decrease in swell period and a common propagation direction (northwest) for wind seas and swells. Also the swells generated at 400 S, propagate in the north Indian Ocean, and enter the Bay of Bengal without affecting the Arabian Sea. Fine resolution winds (say, MM5 or WRF) are necessary to understand the effect of land-sea breeze system on wind-sea generation in the coastal regions. Here, we review the recent work on wind seas and swells in the Indian Ocean and coastal regions that we have carried out using measured wave data and model results.

Identification and origin of nocturnal ozone maxima at urban and rural areas of Northern Portugal – Influence of horizontal transport

Nocturnal ozone maxima have been reported by several authors in different environments. The aim of this study was to evaluate the influence of horizontal transport on the occurrence of nocturnal ozone maxima. Frequency, concentration and time of the nocturnal ozone maxima were investigated during three consecutive years (2005–2007) in two urban and two rural sites of Northern Portugal using hourly ozone concentrations. These concentrations were further related with temperature, relative humidity, wind direction and speed. Results showed that urban sites had frequencies of nocturnal ozone maxima between 40% and 50% and rural sites of 15%. Rural sites were those with higher concentrations being in average 117 ± 35 μg m −3 , with minimum and maximum concentrations of, respectively, 51 μg m −3 and 283 μg m −3 ; information and alert thresholds were exceeded ( Directive 2002/3/EC ). At urban sites, the nocturnal ozone maxima occurred most at 5 h and at rural sites at 23 h/24 h. Among the meteorological parameters analysed, wind direction was the only one that had influence on the occurrence of nocturnal O 3 maxima. When nocturnal O 3 maxima occurred it was possible to observe at the urban sites, prevailing winds from ESE, related with land–sea breezes, meaning that land–sea breeze influenced the occurrence of nocturnal O 3 maxima trough the transport of O 3 from places where its concentrations were higher. At rural sites similar behaviour has been observed, nevertheless with different wind directions according to meteorological characteristics of the sites; frequent wind directions were from WNW for LO and from the North quadrant for SM. Additionally, between 70% and 99% of the nocturnal O 3 maxima for urban and rural sites occurred when there was a shift in the wind direction. Results allowed concluding that horizontal transport might have high importance on the occurrence of those maxima. Nevertheless, other factors, such as vertical transport, may also be important, as have been suggested by other authors. ► Monthly frequency of nocturnal O 3 maxima is lower at rural than at urban ones. ► Nocturnal O 3 maxima occurred in general at 5 h in urban costal sites. ► Nocturnal O 3 maxima occurred in general at 23 h in rural sites. ► Nocturnal O 3 maxima were related with land–sea breeze system at urban coastal sites. ► Nocturnal O 3 maxima were related with regional wind system at rural sites.

Measurements of nitrogen oxides, ozone, and carbon monoxide during the FIELDVOC'94 campaign in Tábua

Measurements of NO,NOx,NOy,CO,O3,JNO2 (the photolysis frequency of NO2) and of meteorological parameters made during the FIELDVOC'94 campaign in Tábua, Portugal, are described. The situation at the site was dominated by a land–sea breeze system coupled with thermal up-slope on the mountain ridge, which caused northwesterly flow from the coastal area during daytime and southeasterly flow at night. O3 up to 100 ppbv was observed in photochemically aged air masses that reached the site in the afternoon from the coastal areas. At night, O3 mixing ratios at the surface dropped below 10 ppbv. NOx mixing ratios varied between 200 pptv during daytime and 3 ppbv at night with peaks up to 6 ppbv. NOy (the sum of NOx and its oxidation products) varied between 1 and 6 ppbv and CO between 100 and 280 ppbv. Simultaneous measurements of NO2 by photolytic conversion/NO-chemiluminescence and by tunable diode laser spectroscopy showed a tight correlation, in particular at night. Taking the identified difference in calibration between the two methods (8%) into account, the disagreement in the atmosphere, was ⩽2%, on average. Photochemical ozone formation in air transported from the coast was identified by a positive correlation between O3 and the oxidation products of NOx referenced as NOz. The slope of 16 represents an upper limit to the efficiency of NOx in catalyzing ozone formation in air. The relatively high ozone formation efficiency is supported by the large O3/CO ratio of about 0.4. The correlation between CO and NOy indicates an upper limit for the CO/NOx emission ratio of 28.

Precipitation dynamics in Ecuador and northern Peru during the 1991/92 El Nino: A remote sensing perspective

The formation, dynamics and spatial distribution of heavy precipitation during the 1991/92 El Nino in Ecuador and northern Peru were examined by means of Meteosat-3 imagery, NOAA-AVHRR-based multichannel sea surface temperatures (MCSST) and additional meteorological observations. The Convective and Stratiform Technique (CST) was used for rain retrieval by means of Meteosat IR data and a cross-correlation approach was applied to Meteosat image sequences to derive cloud motion winds (CMW) which are essential for the analysis of circulation patterns leading to severe precipitation. From an analysis of 45 days with severe precipitation it is proven that three mechanisms were responsible for the formation of heavy rains. Each mechanism reveals a specific localized impact. (1) The most frequent mechanism (frequency of ∼61%) represents an extended land-sea breeze system. During such weather conditions, predominantly locally confined precipitation patterns occured. Areas affected by the sea wind front during the day were the coastal plains up to the 1000 m contour line on the western Andean slope. Local maxima in the frequency of cloudiness leading to precipitation could be found at isolated peaks of a lower coastal cordillera. At night the highest frequency of precipitation was found over the warm water surface of the Gulf of Guayaquil, mainly due to its coastal shape which significantly favours convergence of the nocturnal land breeze. (2) Convection, initiated in the coastal plain and on the western Andean slopes during the afternoon, was significantly intensified by an entrainment of remainders of cirrus shields from the Amazon basin. These cloud fragments spilled over the Andes with well-developed trades in the mid/upper troposphere which blew in the opposite direction to the daily sea/up-slope breeze. The spill over points were characterized by areas of deep convection on the western Andean slopes and were frequently valley axes perpendicular to the mountain chain as well as the Andean depression in southern Ecuador. (3) During the main El Nino phase (March-April), heavy and persistent precipitation was extended over wide areas of the coastal plain showing neither a distinct diurnal cycle nor preferential areas. Deep convection was frequently organized in mesoscale convective complexes (MCC) and was spatially correlated with MCSST > 27 . The extensive instability of the troposphere during these weather conditions was marked by convective cloud streets and an intensification of the meridional Hadley circulation off the coast of southern Ecuador and Peru.

The Impact of Secondary Flow Systems on Air Pollution in the Area of São Paulo

AbstractThe area between the Atlantic Ocean and São Paulo is highly polluted due to high emission rates at Cubatão, a city situated 15 km inland at a steep slope. It was expected that secondary circulations would develop caused by the land–sea contrast and strong orographic changes, which influence the transport and diffusion of air pollutants. In 1994–95, surface stations were operated and radiosonde ascents were performed to analyze the characteristic features of the land–sea-breeze circulation.The stations make evident a land–sea-breeze system that arrived in the suburbs of São Paulo in the early afternoon. The upslope winds favor the propagation of the sea breeze at the steep slope. During the measurement period, large-scale northwesterly winds prevailed that advected warm air from the plateau to the coastal area in the afternoon and resulted in a limitation of the boundary layer growth. The data were used to initialize a three-dimensional mesoscale model for calculation of the transport and deposition of SO2 emitted at Cubatão. The boundary layer height was found to be a limitation for vertical mixing of the air pollutants. However, a step between the coastal boundary layer and the boundary layer over the plateau causes SO2 to be vented into the free atmosphere at the slope and then transported toward the Atlantic Ocean with the large-scale northwesterly winds. Thus, over the coastal area, the SO2 concentrations in the free atmosphere were even higher than within the mixed layer. The deposition, summed up over a day, was calculated and found to be strongest at the slope and over the Atlantic Ocean.

A numerical simulation of the transport of surface ozone along a Mediterranean coastal area

A simple numerical simulation of daily summer circulation in W-NW and E-SE directions was applied to a coastal area of Greater Ravenna in Northern Italy to assess the influence on ozone levels of the transport mechanism in the land-sea breeze system. Field measurements of ozone concentration and micrometeorological data collected over a ten-year period (1978–1989) by a coastal monitoring station were processed to identify meteorological and climatic situations. The diurnal pattern of a «pure» off-shore-in-shore circulation system was reconstructed by applying a simple model in which data were considered or rejected as a function of the prevailing wind direction. Only data collected under W-NW and E-SE wind direction conditions became part of the «reconstructed wind circulation system». The findings show that the relative concentration values and the time duration of the simulated transport phenomenon are in good agreement with the recorded episodes of transport and of severe continuous fumigation, occurring during the night, at the polluted area.

Flux aggregation at large scales: on the limits of validity of the concept of blending height

The concept of blending height is known to be applicable to estimate areally averaged surface heat and momentum fluxes over heterogeneous terrain with horizontal scales of surface variations much smaller than 10 km. Here, the performance of this concept is explored beyond this limit of validity. This is accomplished by analysing a three-dimensional meso-scale simulation of land-sea breeze systems which originate at large-scale temperature differences between water and land surfaces and by employing a one-dimensional version of the meso-scale model as a vertical column of a hypothesized macro-scale model. It appears that areally averaged surface fluxes can be reproduced reasonably well. This is valid for area-averaged fluxes obtained by the average of surface fluxes on each land type, i.e. for so-called flux aggregation, and for a combination of flux aggregation and so-called parameter aggregation where similar land types are combined into an aggregated land surface. The weak dependence of averaged surface fluxes on secondary, meso-scale motions agrees with earlier theoretical considerations.

Mesoscale flow analysis

Various methods are outlined for the analysis of mesoscale flow patterns and these are illustrated by application to the Adelaide (South Australia) urban area. They illustrate locally induced variations within the flow and also the seasonal transference of energy from synoptic sources in winter to the diurnal land-sea breeze system in the summer.