Land-sea Breeze Research Articles

Interaction of Urban Heat Island Effects and Land–Sea Breezes during a New York City Heat Event

AbstractThe state of knowledge of the effects of urban heat islands is advanced through investigation of a heat event in the highly complex coastal environment of New York City (NYC) by using the Weather Research and Forecasting (WRF) Model and surface observations in the NYC metropolitan area to evaluate heat retention at high- and low-temperature times during 18–20 July 2013. Urban surface air temperatures are 1°–2°C higher than rural temperatures throughout the daytime and increase to 3°–5°C higher during the night. Lack of a land–sea temperature gradient prevents development of a land breeze during the night. A land–sea temperature difference approaching 20°C leads to sea-breeze effects during 18 July that reduce daytime skin temperatures, but higher winds greatly reduce the sea breeze during 19 July. WRF Model data are generated using three urban parameterization schemes. The most sophisticated multilayer urban parameterization proves to be most accurate when compared with surface observation data. Errors between WRF Model data and surface observations are attributed to assigned coastal sea surface temperatures, excessive building drag, and too little urban heat retention. Adjustments to the input parameters to the multilayer scheme improved accuracy to lead to the control simulation used for urban heat island effects and land–sea-breeze analysis. NYC building interaction with the synoptic flow generates urban drag and wake effects, although relatively high winds limit their extent. Urban flow results and identified model errors support the development and deployment of the best urban parameterization scheme.

Importance of Diurnal Forcing on the Summer Salinity Variability in the East China Sea

AbstractThe impacts of diurnal atmospheric forcing on the summer salinity change in the East China Sea are investigated using the Regional Ocean Modeling System, forced by the hourly and daily reanalysis of wind and insolation. The differences between the forcing of these two frequencies reveal a dipole pattern of salinity change with a positive salinity deviation (1–2 psu) offshore of the Yangtze River estuary, and a negative deviation (from −1 to −0.5 psu) along the Jiangsu Coast. Further dye tracking experiments confirm that diurnal forcing strengthened the northwestward longshore freshwater transport (NLFT) of the Yangtze River by 5.2 × 109m3and reduced the mean water age of 7 days. Sensitivity experiments using different forcing combinations suggest that the diurnal wind, that is, the land–sea breeze, is the key to developing the dipole pattern of salinity change and the NLFT. Through the experiment, the land–sea breeze induced a mean clockwise circulation offshore of the Yangtze River estuary. The above changes resulted from both the nonlinearity of wind stress averaging (i.e., the square nature of wind stress) and the baroclinic adjustment related to the diurnal salinity variation, which is directly connected to the diurnal swing of the Yangtze River front. The baroclinic adjustment generated a dipole pattern of vorticity changes offshore of the Yangtze River estuary and a coherent northwestward jet current strengthening the NLFT. These processes developed the summer dipole pattern of the salinity change.

Early morning peaks in the diurnal cycle of precipitation over the northern coast of West Java and possible influencing factors

Abstract. The diurnal cycles of precipitation over the northern coast of West Java have been studied using the Tropical Rainfall Measuring Mission (TRMM) Real Time Multi-satellite Precipitation Analyses (TMPA-RT) products with records spanning from 2000 to 2016, with emphasis on the occurrences of early morning precipitation peaks. Diurnal precipitation over the study area during November to March is basically characterized by precipitation peaks that occur in the afternoon to evening time (15:00–21:00 LT), but secondary peaks in the night to morning time (01:00–07:00 LT) are also pronounced in January and February. The harmonic analysis method was then applied to data of January and February to objectively determine the diurnal phase and classify the timing of precipitation for each day into three categories, i.e. afternoon-to-evening precipitation (AEP), early morning precipitation (EMP), and late morning precipitation (LMP) with peaks that occur in the time windows of 13:00–24:00, 01:00–04:00, and 05:00–12:00 LT, respectively. In terms of frequency of occurrence, AEP, EMP, and LMP constitute 55 %, 18.9 %, and 26.1 % of total samples of precipitation events. In spite of the smallest percentage, EMP events are characterized by seaward (as well as landward) propagation, flat phase distribution, and large mean amplitudes. The propagating characteristics of EMP are more prominent, with indications of stronger connectivity between precipitation systems over land and ocean, when data are composited by taking the 99th percentile values in each grid to represent extreme precipitation events. The flat phase distribution of EMP events suggests that the timing of coastal precipitation is not necessarily locked to the phase of land–sea breezes, thus allowing precipitation to occur more randomly. Furthermore, the role of the South China Sea cold tongue (SCS-CT) and cross-equatorial northerly surge (CENS) as influencing factors for the occurrences of the EMP event have also been investigated. In agreement with previous studies, we confirmed that the SCS-CT generally prevails in January and February, and morning precipitation events over the northern coast of West Java mainly occurred when there was more enhanced sea surface temperature (SST) cooling in the South China Sea. Additionally, we found that CENS is the most differential factor with regard to the phase of coastal precipitation. In this case, CENS is positively correlated with SCS-CT and, when associated with EMP events, concurrent enhancement of CENS and SCS-CT is connected to a narrow channelling of strong surface northerly wind anomalies just offshore of Indochina and the Malay Peninsula.

SO2 measurements in a clean coastal environment of the southwestern Europe: Sources, transport and influence in the formation of secondary aerosols.

Two years of SO2 measurements at El Arenosillo observatory located in the Gulf of Cadiz (Atlantic Ocean) were investigated. Annual hourly averages of 1.9±1.5μgm-3 and p95 between 3 and 4.4μgm-3 were recorded, showing clean and background environments. Monthly means vary between 1.5 and 2.4μgm-3, a monthly evolution was not found. SO2 fields from the MERRA2 model were used to identify SO2 sources and its transport, which could be affecting the studied region. Although SO2 records were low, major conductive for SO2 increases were observed in specific periods. A selection methodology was applied to extract these events, which showed a mean of ~11μgm-3. Surface meteorological observations and ERA5 meteorological fields from the ECMWF model were used to assess the weather conditions. SO2 increases, in cold months occurred under conditions governed by synoptic-scale. Two types of transport scenarios were identified: SO2 transport defined as direct impact, which is the sum of the plumes from Portugal and the Huelva area; and indirect impact, where SO2 and sulphate particle emissions from Portugal were transported and accumulated in the Gulf of Cadiz and then carried inland, where new particle formation were observed. Episodes with high SO2 concentrations were also reported in warm periods associated with pure sea-land breezes. The SO2 peaks under sea-land breezes were associated with the transport of SO2 from the south of Portugal to the Gulf of Cadiz, whereas SO2 from the east of the Iberian Peninsula and north of Africa reached the Mediterranean Sea and were then transported to the Atlantic Ocean following the Strait of Gibraltar. Blocking of the airflows from the Mediterranean Sea to the Atlantic Ocean turns the Gulf of Cadiz into a chemical reservoir, where chemical species such as SO2 can accumulate, triggering new particle formation processes.

The effects of synoptic and local meteorological condition on CO2, CH4, PM10 and PM2.5 at Bachok Marine Research Station (BMRS) in Peninsular Malaysia

This study is focused on Bachok Marine Research Station (BMRS) which is exposed to different scales of meteorological condition during the northeast (NE) and southwest (SW) monsoons. BMRS experiences both synoptic and land–sea breeze events during the NE monsoon, but only the land–sea breezes are dominant during the SW monsoon. Two case studies were investigated in which the first case study (CS1) was conducted from 25 to 27 January 2016 while the second one (CS2) from 3 to 5 June 2016. BMRS experienced strong synoptic effect during CS1 with strong northeasterly winds (wind speed > 10 ms−1) up to an altitude of 2500 m and vertical thermal gradient only up to 600 m. Using Concentrated Weighted Trajectory (CWT) analysis, BMRS was shown to be influenced by the regional sources of pollutants originated from the central region of China and areas along coastal Vietnam during CS1. Meanwhile, CS2 was associated with daily land–sea breeze event, strong diurnal effect and closely linked to the local source of pollutants from areas along the east coast of Peninsular Malaysia. The concentrations of PM10 and PM2.5 were two times higher when offshore winds dominated BMRS, suggesting its great relation to the local emissions. Overall, BMRS presents a valuable opportunity to study the influence of regional and local atmospheric flows to the variabilities of air pollutants, thus enabling better understanding and providing a key reference to formulate effective pollution abatement strategies.

Evaluation of urban heat islands using local climate zones and the influence of sea-land breeze

The Local Climate Zone (LCZ) scheme is an urban form and land cover/land use classification system used to study urban heat islands (UHIs). Many studies have evaluated the relationship between LCZs and air temperature. While the intensity and spatial pattern of an UHI can be influenced by the land-sea breeze, especially in coastal cities, few studies have been done to examine this using the LCZ scheme. In this study, Sendai, Japan, has been selected as a case study to evaluate whether the LCZ scheme can be used to study UHIs that are exposed to sea-land breeze since it has two urban-rural areas: mountain side and coastal side. UHI analysis was based on LCZ classes with GIS-derived geometric and land cover properties. By dividing Sendai into two regions along its urban center, the mitigating effects of sea-land breeze on the magnitudes of UHIs in each urban-rural area were demonstrated. Based on the results of this study, two specific UHI mitigation strategies were proposed for Sendai. This study confirmed that the LCZ scheme can be used by urban planners to assess both surface UHI (SUHI) and UHI effects, and proposed a feasible process for developing targeted UHI mitigation strategies.

Factors controlling surface ozone in the Seoul Metropolitan Area during the KORUS-AQ campaign.

To understand the characteristics of air quality in the Seoul Metropolitan Area, intensive measurements were conducted under the Korea-United States Air Quality (KORUS-AQ) campaign. Trace gases such as O3, NOx, NOy, SO2, CO, and volatile organic compounds (VOCs), photochemical byproducts such as H2O2 and HCHO, aerosol species, and meteorological variables including planetary boundary layer height were simultaneously measured at Olympic Park in Seoul. During the measurement period, high O3 episodes that exceeded the 90 ppbv hourly maximum occurred on 14 days under four distinct synoptic meteorological conditions. Furthermore, local circulation such as land–sea breeze and diurnal evolution of the boundary layer were crucial in determining the concentrations of precursor gases, including NOx and VOC as well as O3. During such episodes, the nighttime NOx and VOC and daytime UV levels were higher compared to non-episode days. The overall precursor levels and photochemical activity were represented fairly well by variations in the HCHO, which peaked in the morning during the high O3 episodes. This study revealed that toluene was the most abundant VOC in Seoul, and its concentration increased greatly with NOx due to the large local influence under stagnant conditions. When O3 was highly elevated concurrently with PM2.5 under dominant westerlies, NOx and VOCs were relatively lower and CO was noticeably higher than in other episodes. Additionally, the O3 production efficiency was the highest due to a low NOx with the highest NOz/NOy ratio among the four episodes. When westerlies were dominant in transport-south episode, the nighttime concentration of O 3 remained as high as 40~50 ppbv due to the minimum level of NOx titration. Overall, the Seoul Metropolitan Area is at NOx-saturated and VOC-limited conditions, which was diagnosed by indicator species and VOC/NOx ratios.

The Land-Sea Breeze of the Red Sea: Observations, Simulations, and Relationships to Regional Moisture Transport.

Unique in situ observations of atmospheric conditions over the Red Sea and the coastal Arabian Peninsula are examined to study the dynamics and regional impacts of the local land‐sea breeze cycle (LSBC). During a 26‐month data record spanning 2008–2011, observed LSBC events occurred year‐round, frequently exhibiting cross‐shore wind velocities in excess of 8 m/s. Observed onshore and offshore features of both the land‐ and sea‐breeze phases of the cycle are presented, and their seasonal modulation is considered. Weather Research and Forecasting climate downscaling simulations and satellite measurements are used to extend the analysis. In the model, the amplitude of the LSBC is significantly larger in the vicinity of the steeper terrain elements encircling the basin, suggesting an enhancement by the associated slope winds. Observed and simulated conditions also reflected distinct gravity‐current characteristics of the intrinsic moist marine air mass during both phases of the LSBC. Specifically, the advance and retreat of marine air mass was directly tied to the development of internal boundary layers onshore and offshore throughout the period of study. Convergence in the lateral moisture flux resulting from this air mass ascending the coastal topography (sea‐breeze phase) as well as colliding with air masses from the opposing coastline (land‐breeze phase) further resulted in cumulous cloud formation and precipitation.

Interações atmosfera-terra-oceano e suas repercussões na precipitação média registrada no leste da Região Nordeste do Brasil

O texto trata de explicar diferenciações climáticas existentes no leste da Região Nordeste do Brasil motivadas por fenômenos de circulação atmosférica local, influenciados pelas diferenças de pressão atmosférica no estado médio da atmosfera, atentando para as interações atmosfera-terra-oceano na área. Foram realizadas revisão de literatura, análise e interpretação de imagens de satélite e das normais climatológicas da área. Foi identificado que os fenômenos de brisa terra-mar e diferenças locais nos contrastes barométricos oceano-terra são responsáveis por importantes diferenciações climáticas na área. Esse trabalho de Geografia em escala Regional preencheu uma lacuna no conhecimento científico identificada desde a década de 1970.
 Palavras-chave: Geografia Regional, Climatologia Regional, Litoral do Nordeste, Costa dos Recifes.

A short-term solar radiation forecasting system for the Iberian Peninsula. Part 1: Models description and performance assessment

The ability of four models to provide short-term (up to 6 h ahead) GHI and DNI forecasts in the Iberian Peninsula is assessed based on two years of data collected at four stations. The models follow (mostly) independent approaches: one pure statistical model (Smart Persistence), one model based on CMV derived from satellite images (Satellite), one NWP model (WRF-Solar) and a hybrid satellite-NWP model (CIADCast). Overall, results show Smart Persistence to be the best at the first lead steps, advective models (Satellite and CIADCast) at intermediate ones and the WRF-Solar at the end of the forecasting period. The break-even point between the advective models and WRF-Solar varies between 1 and 3 h for GHI and 3 and 5 h for DNI. Nevertheless, a detailed analysis shows enormous differences between models performance related to 1) the local geographic and topographic conditions of the evaluation stations; 2) the evaluated variable (GHI vs. DNI); and 3) the sky and synoptic weather conditions over the study area. Depending on the station and lead time, rRMSE values range from 25% to 70% for GHI and from 35% to 100% for DNI. For the same stations and leading time, rRMSE values for DNI are between 50% and 100% higher than the corresponding GHI counterparts. Depending on the synoptic pattern, rRMSE values are about 10/20% for GHI/DNI (3 h lead time, during high pressure conditions) to about 80/180% for GHI/DNI (during low pressure conditions). All models show a poor performance at a coastal station, attributed to a lack of ability to forecast clouds associated with sea-land breezes. To conclude, no single model proves to be the best performing model and, therefore, results show that the four models are, somehow, complementary. The advantages attained by this complementarity are further explored in a companion paper (Part II).

Decadal long convection-permitting regional climate simulations over eastern China: evaluation of diurnal cycle of precipitation

With a decadal long period (1998–2010) climate simulation using the Weather Research and Forecasting model at convection-permitting resolution (4 km) (WRF_CPM), the diurnal cycles of precipitation amount (PA), frequency (PF) and intensity (PI) and their related large-scale atmospheric circulations over eastern China are analyzed. The simulations are further compared against the CN05.1, CMORPH v1.0 and the ECMWF Re-Analysis Interim (ERAIN). Results show that WRF_CPM can reasonably represent the observed seasonal rainfall and the atmospheric circulations. As for the features at a sub-daily scale, WRF_CPM is superior at reproducing the diurnal amplitude of PF that is similar to PA in terms of the spatial distribution. Moreover, the diurnal peak timing of summer PF and PA over the three sub-regions, i.e., North China (NC), Yangtze-Huaihe River basin (YHR) and South China (SC), can be properly reproduced by WRF_CPM. The observed precipitation systems exhibit obvious eastward propagation from the Plateau to its downstream, which may be due to the solenoid circulations associated with the low-level anomalous wind and moisture convergence. However, they are almost overestimated by WRF_CPM and in turn causing overestimated precipitation along YHR. The early morning precipitation in WRF_CPM has a larger fraction than CMORPH, which is related to the overestimated nocturnal low-level jet. Whereas, due to the solar heating and the land-sea breezes, the late-afternoon precipitation peak is mainly located along the coasts of eastern China, which matches well with the vertical motion in WRF_CPM.

Micro-scale modelling of the urban wind speed for air pollution applications

Modelling wind speeds in urban areas have many applications e.g. in relation to assessment of wind energy, modelling air pollution, and building design and engineering. Models for extrapolating the urban wind speed exist, but little attention has been paid to the influence of the upwind terrain and the foundations for the extrapolation schemes. To analyse the influence of the upwind terrain and the foundations for the extrapolation of the urban wind speed, measurements from six urban and non-urban stations were explored, and a model for the urban wind speed with and without upwind influence was developed and validated. The agreement between the wind directions at the stations is found to be good, and the influence of atmospheric stability, horizontal temperature gradients, land-sea breeze, temperature, global radiation and Monin-Obukhov Length is found to be small, although future work should explore if this is valid for other urban areas. Moreover, the model is found to perform reasonably well, but the upwind influence is overestimated. Areas of model improvement are thus identified. The upwind terrain thus influences the modelling of the urban wind speed to a large extent, and the fundamental assumptions for the extrapolation scheme are fulfilled for this specific case.

Impact of Vertical Wind Shear on Gravity Wave Propagation in the Land–Sea-Breeze Circulation at the Equator

Abstract The impact of vertical wind shear on the land–sea-breeze circulation at the equator is explored using idealized 2D numerical simulations and a simple 2D linear analytical model. Both the idealized and linear analytical models indicate Doppler shifting and attenuation effects coexist under the effect of vertical wind shear for the propagation of gravity waves that characterize the land–sea-breeze circulation. Without a background wind, the idealized sea breeze has two ray paths of gravity waves that extend outward and upward from the coast. A uniform background wind causes a tilting of the two ray paths due to Doppler shifting. With vertical shear in the background wind, the downstream ray path of wave propagation can be rapidly attenuated near a certain level, whereas the upstream ray path is not attenuated and the amplitudes even increase with height. The downstream attenuation level is found to descend with increasing linear wind shear. The present analytical model establishes that the attenuation level corresponds to the critical level where the background wind is equal to the horizontal gravity wave phase speed. The upstream gravity wave ray path can propagate upward without attenuation as there is no critical level there.

Evaluation of the impact of high-resolution winds on the coastal waves

This study discusses the impact of high-resolution winds on the coastal waves and analyses the effectiveness of the high-resolution winds in recreating the fine-scale features along the coastal regions during the pre-monsoon season (March–May). The influence of the diurnal variation of winds on waves is studied for the Tamil Nadu coastal region using wind fields from weather research and forecast (WRF) (3 km) and European Centre for Medium-Range Weather Forecasts (ECMWF) (27.5 km). The improvement in the coastal forecast is then quantified with wave rider buoy observations. The high-resolution wind fields simulated fine-scale features like land–sea breeze events and showed good agreement with observation results. The error in the wave height and period is reduced by 8% and 46%, respectively, with the use of high-resolution forcing winds WRF over ECMWF, although the overestimation of wave energy on high frequencies due to overestimated WRF winds remains as a challenge in forecasting. The analysis also shows the importance of accurate wave forecast during a short-duration sudden wind (~12 m/s) occurrence in southern Tamil Nadu near Rameswaram during the pre-monsoon period. Low pressure forms over Tamil Nadu due to the land surface heating, resulting in a sudden increase of winds. High winds and steep waves which cause damage to the property of the coastal community near Rameswaram also were well simulated in the high-resolution forecast system with WRF winds.

Physical mechanisms of surface wind variability at Incheon Airport

AbstractThe mechanisms of surface wind variability observed by Aerodrome Meteorological Observation System at Incheon International Airport (IIA), located near the western coast of the Korean Peninsula, are investigated using cyclostationary empirical orthogonal function (CSEOF) analysis. Despite the complex nature of surface wind, as various factors contribute to its variance, distinct features of surface wind variability are separated reasonably into CSEOF modes. The physical interpretation of each mode using the ERA‐Interim reanalysis data is accomplished through regression analysis in CSEOF space. In essence, three atmospheric features dominate the surface wind variability: East Asian monsoon, land–sea breeze, and baroclinic waves. East Asian monsoon, the first mode, explains ~12% of the total variance of the surface wind at IIA. The annual reversal of the monsoon circulation due to the thermal contrast between the Asian continent and the western Pacific is captured as the single most dominant source of surface wind variability at IIA. The second CSEOF mode reveals that zonal circulation induced by land–sea breeze dominates the diurnal variation of the surface wind at IIA, and explains ~8% of the total variance. The largest fraction of variability is associated with baroclinic waves, which is separated into many CSEOF modes due to their irregular phase and wavelength, and the locations of vortex centre. Modes 3–10 are all associated with baroclinic waves and they together explain ~42% of the total variance. Baroclinic waves are amplified as they move over the ocean due to strong turbulent flux from the ocean surface induced by the associated upper tropospheric vortices.

Theories for Past and Future Monsoon Rainfall Changes

Long-standing biases in simulations of past and present climate states and climate model disagreement even in sign of future monsoon rainfall changes evince limitations in our theoretical understanding. The dominant theoretical paradigms for understanding monsoon rainfall—convective-quasi equilibrium (CQE), the moist static energy (MSE) budget, and monsoons as local Intertropical Convergence Zone (ITCZ) shifts—all jettison the traditional “land-sea breeze” paradigm. Summer monsoon precipitation falls when the assumptions of CQE are most satisfied but those of the ITCZ shift framework are least satisfied. Zonal asymmetries, changes in ITCZ width and strength, hydrology-vegetation-CO2 coupling, and timescale-dependent responses complicate inferences of monsoon rainfall from paleoclimate proxy records. The MSE budget framework applied to deliberately designed simulations can illuminate key mechanisms underlying monsoon responses to external forcings, presenting a path toward falsifying model projections. Sustained, rapid progress in monsoon rainfall theory is urgently needed by society and is plausible based on recent advances.

GPS-based precipitable water vapour retrieval and variability using measured and global reanalysis data in the coastal regions of China

ABSTRACT The atmospheric water vapour is an important indicator of the climate state and evolution, and also the key parameter affecting the hydrological cycle, atmospheric convection, and the weather. This study validated the accuracy of surface pressure and temperature data interpolated from European Centre for Medium-Range Weather Forecasts (ECMWF) Interim ReAnalysis (ERA-Interim) and ECMWF 5th ReAnalysis (ERA5) datasets, and evaluated the performance of the interpolation meteorological data for precipitable water vapour (PWV) retrieval using the measured meteorological data and Global Positioning System (GPS) data of the year 2014 obtained from 25 stations of the Chinese Coastal GPS Observation Network. We then analysed the temporal and spatial distribution of water vapour in the coastal regions of China using a 3-year GPS PWV time series calculated with the interpolated reanalysis meteorological data at 25 stations from 2014 to 2016. Evaluation of the reanalysis meteorological data results showed that the accuracies of the interpolated pressure and temperature data from ERA5 were slightly superior than those from ERA-Interim. The root-mean-square errors (RMSEs) of the surface temperature and pressure interpolated from two reanalysis datasets were less than 2.4 K and 1.6 hPa, respectively. Data based on GPS PWV products that used interpolated parameters were very close to those of meteorological observations, with biases within ±0.4 mm and RMSEs below 0.5 mm in most areas; these data also strongly agreed with radiosonde observations. However, the interpolated meteorological data from reanalysis datasets could not reflect the true change during typhoon events, which could not be used in GPS PWV retrieval. The analysis of the temporal and spatial distribution showed that the distribution of water vapour was mainly affected by latitude, land-sea distribution, and water vapour advection. The variations in water vapour were mainly seasonal with the highest PWV occurring in the summer and the lowest always occurring in the winter. Significant diurnal variations of water vapour varied with season and latitude, with an amplitude of about 0.9–3.5 mm; the peak value occurred in the afternoon or early morning, and was affected by surface evaporation, large-scale land–sea breeze circulation, and valley-wind circulation.

Diurnal Cycle of Surface Winds in the Maritime Continent Observed through Satellite Scatterometry

AbstractThe diurnal cycle of surface winds throughout the Maritime Continent plays a significant role in the formation of precipitation over the islands of the region and over the surrounding seas. This study investigates the connection between the diurnal cycles of surface wind and offshore precipitation using data from four satellite scatterometer instruments and two satellite precipitation radar instruments. For the first time, data from three scatterometer instruments are combined to yield a more temporally complete picture of the surface wind diurnal cycles over the Maritime Continent’s surrounding seas. The results indicate that land–sea breezes typically propagate over 400 km offshore, produce mean wind perturbations of between 1 and 5 m s−1, and propagate as gravity waves at 25–30 m s−1. Diurnal precipitation cycles are affected through gravity wave propagation processes associated with the land–sea breezes, and through the convergence of land breezes from nearby islands. These observational results are then compared with previous mesoscale modeling results. It is shown that land–sea breezes occur too early, and are too intense in these modeling results, and this may partly explain why these modeling results also exhibit an early, overly intense diurnal precipitation cycle. This study also investigates variations in the diurnal cycle of surface winds at seasonal and intraseasonal time scales. Previous work has suggested that seasonal and intraseasonal variations in surface heating affect the land–sea breeze circulation and diurnal precipitation cycles; we argue that variations in background winds also play a defining role in modulating coastally influenced local winds.

Observed decrease of summer sea-land breeze in Shanghai from 1994 to 2014 and its association with urbanization

Sea-land breeze (SLB) is widely known as a common wind circulation in coastal cities, which plays an important role to heat transport and pollution diffusion. Using 21-year observation data, a continuous decrease of SLB is discovered in a Chinese metropolis city - Shanghai. In general, the thermodynamic difference between sea and land accounts for the magnitude of SLB. However, other factors associated with urbanization could also affect the SLB significantly, such as anthropogenic aerosol loading, surface roughness, heat release, and so on. This study statistically analyzes the influence of these factors on SLB under the background of global warming. As expected, the decrease of SLB is a combined effect of global warming and great urbanization in Shanghai. The latter, which affects the circulation in both thermodynamic and dynamic ways, is consisted of three parts: temperature increase with urbanization, change of atmospheric radiative forcing, and variation of the underlying surface. At night, the urbanization effect and global warming effect play the similar role by decreasing the temperature gap between sea and land, which would cause the decrease of land wind. At day, decreasing downwelling solar radiation partially offsets the temperature increase caused by both global warming and urbanization although the increasing trend of temperature remains. Different from the expected increasing trend of sea breeze based on the increasing temperature in Shanghai, sea breeze demonstrates a decreasing trend, which could be caused by the dynamic effect from increased surface roughness in Shanghai along with the cooling contribution from radiation-influential factors such as anthropogenic aerosols.

Toward Understanding the Differences of PM2.5 Characteristics Among Five China Urban Cities

Characteristics of PM2.5 are investigated and compared based on observations from 2008 to 2017 in Beijing, Shenyang, Shanghai, Chengdu, and Guangzhou, which cover five different climate regions of China. Significant seasonal variations of PM2.5 are found for all five cities, with the largest values in winter and the smallest in summer. The differences of PM2.5 among the five cities are most likely associated with the emission sources, meteorology and topography. The strong emissions with surrounded mountains in Beijing make PM2.5 high, particularly in winter. The basin topography surrounding by mountains in Chengdu makes the pollution hard to disperse and then causes high PM2.5. By contrast, the sea-land breeze helps disperse the air pollution in coastal cities such as Guangzhou and Shanghai by mixing and transport and makes PM2.5 in those cites relatively low. Moreover, the population shows high values in Beijing and Shanghai, and the vehicle amount shows high values in Beijing and Chengdu, contributing to their high PM2.5. Similar monthly variations of PM2.5 are found for the five cities, while the magnitude of variations are different. The five cities demonstrate more different diurnal variations of PM2.5. Associated with the sea-land breeze, there are very weak diurnal variations of PM2.5 in Shanghai and Guangzhou. Differently, significant diurnal variations exist for Beijing, Shengyang and Chengu, with the minimum values in the afternoon and high values at night.