Coastal meteorology

Coastal meteorology deals with atmospheric processes that are influenced by the transition between land and sea. Differences in surface temperature, roughness and topography generate local atmospheric circulations and modify larger-scale weather systems. These effects may extend tens to hundreds of kilometres inland and offshore. Coastal meteorology is important for understanding coastal winds, waves, storm surges, precipitation, fog and the exchange of heat, moisture and momentum between the atmosphere and the coastal ocean. The application of this knowledge is vital for the prediction of sea state and pollutant dispersal, and it is also important for public safety, ship routing and naval operations.

This article provides a brief introduction to some basic atmospheric processes that often play a major role in coastal meteorology. The aim is not to give a complete treatment of coastal meteorology, but to introduce key concepts that help explain why weather and climate near coasts often differ strongly from both inland and open-ocean conditions.

For a more detailed and comprehensive treatment, readers are referred to the textbook Rohli, R.V. and Li 2021. Meteorology for Coastal Scientists. Springer, which is specifically intended for scientists, engineers, planners, and other coastal-zone professionals who need a fuller understanding of coastal atmospheric processes. A more classic reference is Hsu, S.A., 1988: Coastal meteorology. Academic Press Inc., San Diego, 260pp.

Atmospheric boundary layer (ABL)

The ABL is the lowest part of the atmosphere, where wind velocities, temperature and humidity are directly influenced by exchanges of momentum, heat and moisture with the Earth's surface. Turbulent mixing is an important mechanism for these exchanges. The depth and structure of the ABL depend on wind conditions, surface roughness and atmospheric stability.

In coastal regions, the transition between land and sea produces strong contrasts in surface roughness and heat exchange. Air crossing the coastline adjusts to these new surface conditions, forming an internal boundary layer that develops with increasing distance from the coast. The resulting changes in wind velocity, turbulence and atmospheric stability influence nearshore wind fields and the transfer of momentum and heat to the sea surface (Garratt, 1992 ).

Thermally driven effects, the land-sea breeze

During daytime, the land surface generally heats more rapidly than the sea surface causing the air column to expand relative to the cooler marine air column. The resulting temperature difference produces horizontal pressure gradients, driving cooler marine air towards the land near the surface. The advancing marine air forces warmer air to rise, while a compensating return flow develops at higher levels (Fig. 1). This circulation is known as the sea breeze. During clear nights, the land surface generally cools more rapidly than the sea surface. The resulting reversal of the thermal contrast can produce an offshore flow near the surface, known as the land breeze (Fig. 2).

Land–sea breeze circulations are most pronounced when the large-scale atmospheric pressure gradient is weak. Their strength and inland penetration depend on atmospheric stability, background winds, coastline orientation and topography.

The sea-breeze is associated with many processes that contribute to the recirculation and trapping of pollution, the evolution of precipitating convective storms, the creation of strong nearshore thermal, moisture and aerosol gradients, and the formation and transport of fog and low cloud in the coastal zone.

Fig. 1. Simplified sea-breeze-circulation during day.

Fig. 2. Simplified land-breeze-circulation during night.

Field observations of Pattiaratchi et al. (1997) on the Perth Metropolitan Coast (western Australia) during a typical summer sea-breeze cycle (wind speed up to 12 m/s) showed the development of a wind-wave field with significant wave heights up to 0.9 m, net offshore near-bed currents and a rapid increase in the mean longshore current from < 0.05 m/s to 1.0 m/s. A 10-fold increase in suspended sediment concentration and a 100-fold increase in the longshore sand transport resulted from the effects of the sea-breeze system. Sea breeze wave-driven water circulation also completely eroded beach cusps (wavelength 20–30 m), overwhelmed the rip current system associated with the beach cusps and suppressed the infragravity waves in the incident wave and swash record.

Fig. 3. Sea fog entering the US west coast (Oregon). Photo: M. Quante

Coastal fog

Coastal marine fog (or sea fog, see Fig. 3) frequently develops when relatively warm, moist air moves over a colder sea surface (for example due to upwelling of cold water). Cooling of the air reduces the saturation vapor pressure, allowing water vapor to condense into small droplets when saturation is reached. This process is known as advection fog. Coastal fog may also develop through radiative cooling and the mixing of air masses with different temperature and humidity.

Orographic Influences

Fig. 4. Steep mountainous coastline.

Coastal mountains (Fig. 4) can strongly modify atmospheric circulation by obstructing airflow across the coastline. Depending on wind velocity, atmospheric stability and mountain height, air may flow over the barrier or become partly blocked and deflected along the coast.

The tendency for stable airflow to pass over a mountain barrier can be characterized by the Froude number [math]\displaystyle{ F_r=U/(Nh_m) }[/math], where [math]\displaystyle{ U }[/math] is the incident wind velocity, [math]\displaystyle{ h_m }[/math] is the mountain height and [math]\displaystyle{ N }[/math] is the Brunt–Väisälä frequency, which characterizes atmospheric stability. It is given by [math]\displaystyle{ N= \sqrt{(g/\theta) (d\theta/dz)} }[/math], where [math]\displaystyle{ g }[/math] is gravity acceleration, and [math]\displaystyle{ \theta }[/math] is the potential temperature. This is the temperature that an air parcel at height [math]\displaystyle{ z }[/math] would attain if it was moved dry adiabatically to a standard pressure [math]\displaystyle{ p_0= }[/math]1000 mb; in formula: [math]\displaystyle{ \theta=T \, (p_0/p)^{0.286} \, }[/math], where [math]\displaystyle{ p }[/math] and [math]\displaystyle{ T }[/math] are the pressure and absolute temperature at height [math]\displaystyle{ z }[/math].

Small Froude numbers [math]\displaystyle{ F_r < 1 }[/math] indicate that buoyancy forces strongly inhibit vertical displacement of air, favoring flow blocking and deflection along the mountain barrier. For example, with an incident wind velocity of 10 m/s and a Brunt–Väisälä frequency of [math]\displaystyle{ N \sim 10^{-2} s^{-1} }[/math], flow blocking is favored for mountain heights [math]\displaystyle{ h_m }[/math] exceeding approximately 1000 m. The steady-state response is a pressure ridge, a phenomenon referred to as damming. The topographically induced pressure fields produce along-ridge pressure gradients that can result in alongshore barrier jets. The best examples of the phenomenon are found associated with cold air damming between a coastal front and mountain ridges. A similar structure can occur when the incident flow is not uniform, such as in the vicinity of a storm.

Earth's rotation also influences the response, particularly for extensive coastal mountain ranges. The influence of Earth's rotation can be characterized by the Rossby number [math]\displaystyle{ R_o=U/(f l_m) }[/math], where [math]\displaystyle{ f }[/math] is the Coriolis parameter and [math]\displaystyle{ l_m }[/math] is the half-width of the mountain range. For [math]\displaystyle{ R_o < 1 }[/math], rotational effects are important and the flow can be approximately geostrophic (i.e. perpendicular to the pressure gradient), with the Coriolis force balancing the horizontal pressure-gradient force. For [math]\displaystyle{ R_o > 1 }[/math], advective acceleration becomes relatively more important (i.e., the flow will not remain perpendicular to the pressure gradient). The degree of flow blocking and deflection also depends on atmospheric stability, incident wind direction and barrier geometry.

In idealized models of stratified, rotating flow past coastal mountains, the offshore extent of the topographic influence is characterized by a deformation radius [math]\displaystyle{ l_r = Nh_m/f }[/math]. Depending on atmospheric stability, mountain height and latitude, this length scale can be of the order of tens to hundreds of kilometers. Steep topography is defined by the dynamical parameter [math]\displaystyle{ R_o/F_r \equiv l_r/l_m \equiv ( h_m/l_m)(N/f) > 1 }[/math]; the air flow in the coastal region then 'feels' the mountains as representing a wall.

Coastally-trapped disturbances

Along mountainous coastlines, atmospheric disturbances can become trapped near the coast through the combined influence of topographic barriers, stable atmospheric stratification and Earth's rotation. Coastally trapped disturbances with Kelvin-wave-like dynamics propagate along the coast, with the coastline on their right-hand side in the Northern Hemisphere and on their left-hand side in the Southern Hemisphere. Their propagation is governed by the combined effects of stratification, rotation and the coastal boundary. Synoptic-scale atmospheric forcing can also contribute to the development and propagation of these disturbances. These disturbances may propagate over long distances along the coastline while remaining confined to a relatively narrow coastal zone. They can produce substantial changes in coastal winds, including wind reversals and the development of low-level jets, as well as changes in cloud cover and fog occurrence.

Coastally-trapped disturbances can persist for several days, with characteristic scales of hundreds to more than a thousand kilometers alongshore and tens to hundreds of kilometers across-shore. These features generally cause significant changes in the local weather in the coastal zone, for example, replacing clear skies with clouds and fog (Fig. 3), and causing intensification and reversals of the wind field as the system moves along the coast.

Interactions with large-scale meteorological systems

Many of the meteorological phenomena that are associated with the coast are a consequence of synoptic-scale forcing. Traveling disturbances as they pass over the coastline can be strongly modified and lead to new phenomena that are peculiar to the coastal region. Land-falling storms are directly modified in the coastal region where changes in the bottom boundary conditions are dramatic.

Severe storms affect many coastal regions during the winter and, to a lesser extent, in summer. Regional differences exist, with oceanic extratropical cyclones affecting the coast and continental extratropical cyclones moving from the interior of the continents to the coast. Tropical cyclones, including hurricanes, may affect many coasts in the tropics during summer and fall. Particularly important effects are caused e.g. by cold air outbreaks along the east coast of the United States and by rapidly intensifying storms over the Gulf Stream. See also Extreme storms.

Tropical cyclones generally weaken after landfall because the supply of heat and moisture from the warm ocean is reduced and increased surface friction modifies the storm circulation. Orographic barriers can affect the dynamics of the storm through blocking and actually enhance precipitation and winds in coastal areas.

Atmospheric disturbances can strongly affect coastal ocean circulation through variations in wind stress and atmospheric pressure. Alongshore wind variations can induce coastal upwelling or downwelling, while intense storms may generate substantial changes in coastal sea level and currents. The response depends on storm duration and propagation, coastal geometry, shelf bathymetry and the dynamical properties of the coastal ocean.

Intense storms along coastlines or ice edges at high latitudes appear to form over water just beyond the ice edge, where the large vertical temperature gradient between the water and the air leads to strong low level baroclinicity. It has been shown that some polar lows can attain the intensity and structure commonly associated with hurricanes.

Fig. 5. Off-shore turbulent flux measurements.

Meteorological measurements in the coastal environment

Understanding the coastal atmosphere requires a multidisciplinary approach to combine research on air motions, cloud physics, aerosol dynamics, convection, air-sea gas exchange, surface waves and fluxes, boundary layer dynamics, and large scale storm systems to investigate interactions and feedbacks between these processes. Therefore a suite of meteorological instruments and well-organized measurement strategies are needed to capture the relevant parameters of the coastal atmosphere and air-water interface.

The small space and time scales associated with the coastal zone place severe demands on measurement systems. Space-borne remote sensing systems have the potential to measure phenomena both over the coastal ocean and over the land. The required spatial resolution is also provided by satellite data, in addition to routine photogrammetry of clouds and infrared imagery. See Table 1.

Table 1. Principal measurement techniques and numerical tools for investigating atmospheric processes in coastal regions.

Instrument or technique	Main applications in coastal meteorology
Coastal weather stations and flux towers	Wind speed and direction, atmospheric pressure, temperature, humidity, precipitation; turbulent momentum, heat and moisture fluxes.
Meteorological buoys and offshore platforms	Marine wind and atmospheric conditions, air–sea temperature differences and turbulent fluxes when suitably instrumented (Fig. 5).
Radiosondes and atmospheric profilers	Vertical profiles of temperature, humidity, wind velocity and atmospheric stability.
Doppler lidar	Wind profiles, wind shear, boundary-layer structure and turbulence.
Satellite scatterometers	Spatial distribution of marine surface winds, particularly over offshore waters.
Synthetic aperture radar (WiSAR)	High-resolution marine surface wind fields, including coastal wind jets and small-scale wind variability.
X-band marine radar (WiRAR)	High-resolution retrieval of nearshore wind speed and direction from sea-surface radar backscatter.
Weather Doppler radar	Precipitation, storm movement and radial wind velocities within precipitation systems.
Satellite radiometers and imagers	Cloud cover, atmospheric water vapour, sea-surface temperature and atmospheric temperature and humidity profiles.
Kilometre-scale atmospheric models	Simulation and forecasting of coastal winds, sea breezes, orographic effects and local atmospheric circulation.
Meteorological reanalyses	Reconstruction of historical atmospheric conditions, variability and trends.
Coastal X-band radar	Nearshore wave fields and surface current patterns; indirect information on wind-related sea-surface conditions.
Coastal HF radar	Spatial distribution of near-surface ocean currents and their response to atmospheric forcing.

Related articles

WiSAR - Wind retrieval from synthetic aperture radar

WiRAR - A marine radar wind sensor

X-band radar in marine hydrography

HF radar in marine hydrography

Extreme storms

Coriolis

Geostrophic flow

External link

• World Meteorological Organization (WMO)

Other relevant literature

Geernaert, G.L. (ed.), 1999: Air-Sea Exchange: Physics, Chemistry and Dynamics. Kluwer Academic Publishers, Dordrecht, 578pp.

Kraus, E.B. and J. A. Businger, 1994: Atmosphere-Ocean Interaction. Oxford University Press, 362 pp.

The main author of this article is Markus Quante Please note that others may also have edited the contents of this article. Citation: Markus Quante (2026): Coastal meteorology. Available from http://www.coastalwiki.org/wiki/Coastal_meteorology [accessed on 13-11-2026] For other articles by this author see Category:Articles by Markus Quante For an overview of contributions by this author see Special:Contributions/Markus Quante

Article reviewed by

Job DronkersSee the discussion page