Maritime meteorology

Those aspects of meteorology that occur over, or are influenced by, ocean areas. Maritime meteorology serves the practical needs of surface and air navigation over the oceans. Phenomena such as heavy weather, high seas, tropical storms, fog, ice accretion, sea ice, and icebergs are especially important because they seriously threaten the safety of ships and personnel. The weather and ocean conditions near the air-ocean interface are also influenced by the atmospheric planetary boundary layer, the ocean mixed layer, and ocean fronts and eddies.

To support the analysis and forecasting of many meteorological and oceanographic elements over the globe, observations are needed from a depth of roughly 1 km (0.6 mi) in the ocean to a height of 30 km (18 mi) in the atmosphere. In addition, the observations must be plentiful enough in space and time to keep track of the major features of interest, that is, tropical and extratropical weather systems in the atmosphere and fronts and eddies in the ocean. Over populated land areas, there is a fairly dense meteorological network; however, over oceans and uninhabited lands, meteorological observations are scarce and expensive to make, except over the major sea lanes and air routes. Direct observations in the ocean, especially below the sea surface, are insufficient to make a synoptic analysis of the ocean except in very limited regions. Fortunately, remotely sensed data from meteorological and oceanographic satellites are helping to fill in some of these gaps in data. Satellite data can provide useful information on the type and height of clouds, the temperature and humidity structure in the atmosphere, wind velocity at cloud level and at the sea surface, the ocean surface temperature, the height of the sea, and the location of sea ice. Although satellite-borne sensors cannot penetrate below the sea surface, the height of the sea can be used to infer useful information about the density structure of the ocean interior. See also Remote sensing.

The motion of the atmosphere and the ocean is governed by the laws of fluid dynamics and thermodynamics. These laws can be expressed in terms of mathematical equations that can be put on a computer in the form of a numerical model and used to help analyze the present state of the fluid system and to forecast its future state. This is the science of numerical prediction, and it plays a very central role in marine meteorology and physical oceanography.

The first step in numerical prediction is known as data assimilation. This is the procedure by which observations are combined with the most recent numerical prediction valid at the time that the observations are taken. This combination produces an analysis of the present state of the atmosphere and ocean that is better than can be obtained from the observations alone. Data assimilation with a numerical model increases the value of a piece of data, because it spreads the influence of the data in space and time in a dynamically consistent way.

The second step is the numerical forecast itself, in which the model is integrated forward in time to predict the state of the atmosphere and ocean at a future time. Models of the global atmosphere and world ocean, as well as regional models with higher spatial resolution covering limited geographical areas, are used for this purpose. In meteorology and oceanography the success of numerical prediction depends on collecting sufficient data to keep track of meteorological and oceanographic features of interest (including those in the earliest stages of development), having access to physically complete and accurate numerical models of the atmosphere and ocean, and having computer systems powerful enough to run the models and make timely forecasts. See also Weather forecasting and prediction.