Joint modulation of coastal rainfall in Northeast Australia by local and large‐scale forcings

This study investigates the interaction of the Madden Julian Oscillation (MJO) with local scale forcings in regulating precipitation and its diurnal variation over coastal areas in Northeast (NE) Australia. Radar results show that the variation of rainfall with MJO phases exhibits both large-scale and local-scale influences. During the enhanced convection phases of the MJO, widespread increased rainfall signals are generated by large-scale forcings associated with the MJO convection, but the environmental factors controlling the type and amount of precipitation during each phase is different. By contrast, the locally enhanced rainfall probability during suppressed convection phases of the MJO possibly results from mesoscale convective systems such as sea breezes and the interaction of easterly trade-winds and topography. The amplitude of the rainfall diurnal cycle in suppressed convection phases is generally larger than in enhanced convection phases of the MJO. However, the impact of the MJO on diurnal rainfall characteristics (e.g., diurnal timing and amplitude) varies from phase to phase suggesting that each MJO phase needs to be considered separately. Simulations from the UK Met-Office Unified Model with grid-spacing of 2.2 km have been used to understand the processes driving this observed interaction of large-scale and mesoscale variability. The simulations show that coastal rainfall during suppressed convection phases of the MJO is sensitive to the trade-wind inversion height as well as moisture distribution. The findings are important for assessing numerical model skills at small scales and highlight the importance of process-based understanding at these scales.

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.

Every year, millions of migratory shorebirds fly through the East Asian-Australasian Flyway between their arctic breeding grounds and Australasia. This flyway includes numerous coastal wetlands in Asia and the Pacific that are used as stopover sites where birds rest and feed. Loss of a few important stopover sites through sea-level rise (SLR) could cause sudden population declines. We formulated and solved mathematically the problem of how to identify the most important stopover sites to minimize losses of bird populations across flyways by conserving land that facilitates upshore shifts of tidal flats in response to SLR. To guide conservation investment that minimizes losses of migratory bird populations during migration, we developed a spatially explicit flyway model coupled with a maximum flow algorithm. Migratory routes of 10 shorebird taxa were modeled in a graph theoretic framework by representing clusters of important wetlands as nodes and the number of birds flying between 2 nodes as edges. We also evaluated several resource allocation algorithms that required only partial information on flyway connectivity (node strategy, based on the impacts of SLR at nodes; habitat strategy, based on habitat change at sites; population strategy, based on population change at sites; and random investment). The resource allocation algorithms based on flyway information performed on average 15% better than simpler allocations based on patterns of habitat loss or local bird counts. The Yellow Sea region stood out as the most important priority for effective conservation of migratory shorebirds, but investment in this area alone will not ensure the persistence of species across the flyway. The spatial distribution of conservation investments differed enormously according to the severity of SLR and whether information about flyway connectivity was used to guide the prioritizations. With the rapid ongoing loss of coastal wetlands globally, our method provides insight into efficient conservation planning for migratory species.

Factors influencing atmospheric visibility (VIS) in coastal areas are more complex than those for inland and far oceans owing to the complex circulation and aerosol sources. This study analyzed the factors influencing VIS under sea-land breeze circulation (SLBC) for different external aerosol sources based on field survey data in southern Chinese coastal areas. First, SLBC characteristics observed during the experiment period showed that on SLBC days, sea breeze occurs more frequently (∼50%) than land breeze (∼27%), and the wind speed (WS) is generally small, with a mean sea and land breeze WSs of ∼2.18 m/s and ∼2.38 m/s, respectively. Then, analysis of factors influencing VIS was conducted for different land/sea breeze conditions and external aerosol source conditions indicated by the HYSPLIT4 model simulations. Results showed that the aerosol particle number concentration (PNC) and relative humidity (RH) both had negative correlations with VIS, while only very weak relationships between WS and VIS were found, possibly due to small WSs on SLBC days or because local aerosols were not pure marine aerosols. Further two-factor analysis of VIS showed that the power-law function relating VIS with PNC in each RH bin ranges from ∼-0.3 to ∼-1.5, and VIS exhibited sharper exponential decline with increasing PNC under high RH. A new method of retrieving aerosol-extinction hygroscopic growth factor (fext) with the measured VIS, RH, and PNC was developed to investigate the optical hygroscopic growth property of aerosols. Results show that aerosols in the study area have similar fext under different land/sea breeze and external aerosol source conditions; the deliquescence RH of aerosols is ∼60%, suggesting that mainly polluted marine aerosol was observed during experiments in this area.

The effect of synoptic flow on the coastal boundary layer and land sea breeze circulation on the west and east coasts has been investigated by a numerical simulation using PSU/NCAR-MM5 mesoscale model. A study on 24th May 2003 during ARMEX observation period shows occurrence of land-sea breeze circulation and formation of thermal internal boundary layer (TIBL) on both the coastal regions in the daytime but of some differences. An early development and accelerated propagation of sea breeze circulation is noticed on the west coast compared to the east coast under the influence of the prevailing southwesterly flow. The strength of the sea breeze is more on the west coast. The mixing heights during daytime in the coastal regions are reduced by the formation of TIBL. The horizontal extent of TIBL is very less (24 km) on the west coast under the complex topographic effect. The sea breeze frontal activity is more intensive on the east coast with vertical velocity of the order of 2-4 m/sec by the retarding influence of southwesterly synoptic flow.

One of the most prominent mesoscale phenomena in the coastal zone is the sea-breeze/land-breeze circulation. The pattern and its implications for the weather in coastal areas are well described, and with mesoscale-resolving operational NWP models the circulation can be captured. In this study, a straightforward method to identify sea and land breezes based on the change in wind direction in the column above a grid point on the coastline is presented. The method was tested for southern Sweden using archived output from the HARMONIE-AROME model with promising results, describing both the seasonal and diurnal cycles well. In areas with a complex coastline, such as narrow straits, the concept of the land–sea breeze becomes less clear, and several ways to address this problem for the suggested method are discussed. With an operational index of the sea and land breezes, the forecaster can better understand and express the weather situation and add value for people in the coastal zone. Further, the indices can be used to study systematic biases in the model and to create climatologies of the sea and land breezes. Significance Statement A wind pattern that is frequently occurring in the coastal zone is the sea-breeze/land-breeze circulation, and the purpose of this study is to test a new method to automatically identify sea breezes and land breezes in weather forecasts. Knowing if a sea breeze or a land breeze is occurring is helpful for the operational weather forecaster in understanding the weather situation. It can also be used to study systematic model behavior, for example, errors in the forecast temperature during sea-breeze conditions. The method has been tested for seven coastal sites in Sweden and shows promising results both in case studies and multiyear statistics.

The characteristics of the land and sea breeze near the west coast of Sumatra are studied using hourly 10-m wind observations from the Bengkulu Airport for the year 2018, with an emphasis on the properties of the land breeze. Spectral analysis shows that the land–sea breeze cycle is a dominant part of the overall circulation in the region, with disturbances at the diurnal frequency accounting for roughly half the overall disturbance kinetic energy. A method is presented for isolating the near-diurnal parts of the flow through a combination of high- and low-pass filtering, with land and sea breezes defined in terms of the shore-perpendicular component of the filtered winds. By this definition, a land breeze occurs each day, with a median onset time of 1900 LT, a median duration of 15 h, and a median maximum speed of 1.8 m s−1 occurring near 0200 LT. The characteristics of the land breeze are found to depend strongly on the phase of the Madden–Julian oscillation. A dependence was also found during the Asian and Australian monsoons, particularly for the onset time and maximum speed. Sea breezes occur almost every day but are much shorter (about 8.5 h) and stronger (>3 m s−1) than land breezes. Comparisons between airport observations and ERA5 surface winds show that while ERA5 accurately captures the onset time, duration, and timing of the maximum speed for sea breezes, it only captures the onset time and duration for land breezes. For both, the maximum speed is significantly underestimated. Significance Statement Detailed characteristics of tropical land and sea breezes are lacking in the literature. Utilizing 1 year of hourly wind observations from the west coast of Sumatra and a newly developed detection algorithm, we found that land and sea breezes occur essentially every day. Land breezes start just after sunset but last many hours after sunrise and reach their maximum speed almost halfway through their median 15-h duration. These properties vary intraseasonally and seasonally. Sea breezes begin around 1000 LT and are much shorter and stronger than land breezes. ERA5 does a reasonable job capturing many characteristics of the land–sea breeze circulation but underestimates the maximum wind speed in both cases.

The daytime boundary-layer heating process and the air-land heat budget were investigated over the coastal sea-breeze region by means of observations over the Sendai plain in Japan during the summer. In this area, the onset of the sea breeze begins at the coast around 0900 LST, intruding about 35 km inland by late afternoon. The cold sea breeze creates a temperature difference of over 10°C between the coastal and inland areas in the afternoon. On the other hand, warm air advection due to the combination of the counter-sea breeze and land-to-sea synoptic wind occurs in the layer above the cold sea breeze in the coastal region. Owing to this local warm air advection, there is no significant difference in the daytime heating rate over the entire atmospheric boundary layer between the coastal and inland areas. The sensible heat flux from the land surface gradually decreases as distance from the coastline increases, being mainly attributed to the cold sea breeze. The daytime mean cold air advection due to the sea breeze is estimated asQ adv local =−29 W m−2 averaged over the sea breeze region (0∼35 km from the coastline). This value is 17% of the surface sensible heat fluxH over the same region. The results of a two-dimensional numerical model show that the value ofQ adv local /H is strongly affected by the upper-level synoptic wind direction. The absolute value ofQ adv local /H becomes smaller when the synoptic wind has the opposite direction of the sea breeze. This condition occurred during the observations used in the present study.

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.

This study investigates the role of the interaction between the Madden–Julian oscillation (MJO) and local‐scale forcings in regulating precipitation and its diurnal variation during the austral summer over coastal areas in northeast (NE) Australia using radar data. The variation of rainfall is influenced by both large‐scale and local‐scale forcings. During the enhanced convection phases of the MJO, widespread increased rainfall signals are generated by large‐scale forcings associated with the MJO, but the environmental factors controlling the type and amount of precipitation during each phase are different. By contrast, the locally enhanced rainfall during suppressed convection phases of the MJO likely results from the interaction of mesoscale land–sea breezes, strong large‐scale background winds and topography. Different responses of mean and heavy precipitation to the MJO occur in some MJO phases. The impact of the MJO on diurnal rainfall characteristics is spatially inhomogeneous and likely regulated by local forcings. Although stratiform rainfall is more common, convective rainfall predominantly contributes to the total precipitation over coastal regions of NE Australia. The MJO's influence on convective rainfall is generally stronger and more statistically significant than its impact on stratiform rainfall. The widespread increased rainfall probability during the enhanced convection phases is likely due to increases in both stratiform and convective rainfall. In contrast, the locally enhanced precipitation signal during some suppressed convection phases mainly results from an increase in convective rainfall.

A combination of observations and a numerical model revealed the meso‐scale structure of the near‐surface atmospheric conditions over the Persian Gulf. Low‐level winds were dominated by a single, coherent, perennial land–sea breeze circulation (LSBC) that varied seasonally and diurnally. In summer the sea breeze was deeper and wider than in winter. At night the core of the LSBC over the Gulf was confluent with uplift, whereas in daytime it was difluent with subsidence. Sensitivity tests with the model revealed the influence on the LSBC of the land–sea distribution, orography and the ambient wind. The latter resulted in different conditions over the north, east, south and west coasts. Over the north coast, where the opposing ambient wind created a sea breeze front, landward penetration was very limited; over the south coast it was over 250 km. The thermal effect of the Iranian mountains accentuated the depth and penetration, landward and seaward, of the LSBC, thus influencing the duration of the land and sea breezes over the east coast. From spring to autumn the marine boundary layer over the Gulf was shallow, cool, moist and stable, with strong vertical gradients of temperature and humidity at its top. Its depth increased in the ambient flow from northwest to southeast and also, in daytime, from west to east, because of different magnitudes of the subsidence in the sea breeze circulation over the west and east coasts. Copyright © 2004 Royal Meteorological Society

Estimation of the effective zone of sea/land breeze in a coastal area

Sea breezes are the most common winds experienced by people living in coastal regions. In general, the sea breeze is a flow of winds that particularly takes place in coastal areas. Sea breezes cause irregular climatic conditions. The salty air coming from the sea breeze has got many adverse effects like deterioration. The collision of two powerful sea breezes fronts can cause severe thunderstorms across the coastal regions. Therefore, it is important to know the location of the sea breeze front to define the regions affected by sea breezes. In order to detect the sea breeze front from satellite images, it is important to segment the satellite images. Image segmentation which helps in extracting the objects of interest and make the image more meaningful for further processing. Then using contour detection, the outline of sea breeze, which is sea breeze front can easily be extracted. A proper methodology with user interface is proposed in this paper for detecting the sea breeze front from the satellite images.

The land-sea breeze circulation is important for the Metropolitan Region of São Paulo (MRSP), influencing predominant wind direction during the night and day, and so, the transport of pollution in the local scale. In the summer of 2014, there were 43 exceedances of the state air quality by ozone, when the South Atlantic Subtropical High strengthened over southeast Brazil. We aimed to study how the land-sea breeze circulation influenced the transport of the pollutants CO, NO, NO2 and O3 in the study area using the WRF/Chem model in the period 28/01-01/02/2014. Two scenarios were considered: CTRL – vehicular emissions based on current emission inventories and SENS – removing 75% of emissions in the MRSP. Results were analysed through maps with the spatial distribution of pollutants in the domain and showed the importance of the land-sea breeze circulation for the transport of pollution. Analysis of the divergence field proved useful for identifying the sea breeze front. Higher O3 concentrations were simulated in the prefrontal convergence line, due to stagnation and accumulation of pollutants brought by the passage of the sea breeze over polluted areas, resulting in the transport of ozone and other pollutants to distant areas northwest during the afternoon and evening. There was also transport of pollutants to the south in the early morning caused by the land breeze. Upward air motion due to the convergence in the prefrontal region caused vertical transport of ozone during the afternoon.

A detailed study of the hourly variations of refractivity has been made from the meteorological data for a number of years at stations along the coasts of the Arabian Sea and the Bay of Bengal, during the clear months October to May. The prevailing winds and sea surface temperatures have also been taken into consideration when drawing inferences on the occurrence of anomalous propagation associated with the land and sea breezes.Ducting conditions over the coastal land areas will be absent in the sea breeze during the day when convection is active, but will be experienced from about the evening when the land cools and the temperature inversion begins to form and prevents convection. The ducting increases in intensity till the land breeze begins to flow. With the remaining sea-breeze moisture and the strong inversion, while there may be a fall in refractivity near the surface, ducting conditions will prevail. After sunrise, during the land breeze, there will be an increase in the intensity of the ducting due to the evaporation of moisture in the ground. However, with the increased convectional mixing with the upper layers, the ducting conditions will disappear. Later the warm land breeze which flows over the adjacent sea will cause conditions suitable for anomalous propagation over the sea. Thus, a shifting of the ducting conditions from the land areas to the sea takes place during the warm forenoon till the sea breeze sets in. it is expected that normal propagation conditions will prevail from the time of onset of the sea breeze till about sunset.Along the Arabian sea coast, from October to February, the pattern of ducting conditions changes wi h latitude. The times of onset and duration of the sea breeze undergo changes. However, from March to May, there is an appreciable change due to the altered conditions over the Arabian sea. An anticyclone develops near about the middle regions of the sea and air from the warmer areas flows over colder areas to the west of the anticyclone and later strike the west coast as a westerly over Bombay and to its northern latitudes. As a result anomalous propagation and ducting extends over a large area of the Arabian sea with little diurnal variation, particularly over the latitudes to the north of Bombay. During these months, ducting will be absent only over the land when convection is largest. Normal propagation will prevail all along the coasts of the Arabian sea during the monsoon months.Along the coasts of the Bay of Bengal, the conditions during October to February are different from those on the Arabian Sea. There is no common feature among Madras, Visakhapatnam and Calcutta. The sea breeze is practically absent at Calcutta, it is observed to occur at Visakhapatnam, and the northeast monsoon influences the land and sea breezes at Madras during October and November. However, from March to May, there is a large change in the prevailing conditions due to the variation in the distribution of the sea surface temperature and wind circulation over the Bay of Bengal, and are similar to those on the Arabian Sea. Ducting conditions prevail most of the time along the coasts in March-April and extend over the whole Bay in May when warmer air from the land flows over the cooler sea. During this minth, the large-scale ducting condition over the sea is accentuated near the coasts by the prevailing land breeze which is warmer than the sea most of the day and even night.An interesting feature over the Bay of Bengal coast is observed during the monsoon. While normal propagation prevails over most of the country, the land and sea breezes are strongest and prominent at Madras in this season and ducting conditions are experienced to the south of Madras.