19. Glossary

19.1. Terms

ANN - Artifical Neural Network.

Non-Interactive Download Link

This is a web-link that can be used to download data from the WEPROG website without logging in.

The Non-Interactive Link uses a unique access key instead of a login procedure for authentification. This means that you do not have to visit the WEPROG website, but instead can make changes to the download link to select what data you would like to download.

The Non-Interactive Link consists of a list of settings which can be changed to define what data should be downloaded. The general format is &attribute-name=value

Forecast Interface

The forecast Interface is the web page which is used to access forecast data using graphs and tables.

Analysis

Analysis in a meteorological sense is the production of an accurate image of the true state of the atmosphere at a given time, represented by a collection of numbers, usually on regular model grids. Objective analysis is an automated procedure for performing such analysis – versus subjective, hand analysis. See also Data Assimilation.

General Term: The separation of an intellectual or material whole into its constituent elements to determine either their nature (qualitative analysis) or their proportions (quantitative analysis).

ANN - Artificial Neural network When using the term ANN in this user guide, we really refer to ANN models according to the definition found e.g. in Wikipedia: Neural network models in artificial intelligence are usually referred to as artificial neural networks (ANNs); these are essentially simple mathematical models defining a function f : X -> Y or a distribution over X or both X and Y, but sometimes models are also intimately associated with a particular learning algorithm or learning rule. A common use of the phrase ANN model really means the definition of a class of such functions (where members of the class are obtained by varying parameters, connection weights, or specifics of the architecture such as the number of neurons or their connectivity). (source: Wikipedia, http://en.wikipedia.org/wiki/Artificial_neural_network).

CFL criterion

The three mathematicians named Courant, Friedrichs, and Lewy created a criterion which imposes a restriction on the size of the integration time step based on the reciprocal of the smallest spatial step. Because of the CFL criterion, a modeller cannot arbitrarily choose a horizontal grid spacing without also taking into account the time step of the model.

Cyclonic circulation

Atmospheric circulation associated with a cyclone (depression, low pressure area). It is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Data Assimilation

Data assimilation is an analysis technique in which the observed information is accumulated into the model state by taking advantage of consistency constraints with laws of time evolution and physical properties. It is the process of combining observations and short- range forecasts to obtain an initial condition for NWP. The purpose of data assimilation is to determine as accurately as possible the state of the atmospheric flow by using all the available information.

First Guess

The use of short-range forecasts as a first guess has been universally adopted in operational systems into what is called an “analysis cycle”. Initially climatology, or a combination of climatology and a short forecast were used as a first guess. The first guess or background field is our best estimate of the state of the atmosphere prior to the use of the observations.

Forecast

Forecast is a scientific predictions about future states of the atmpsphere made with a numerical model or method. A forecast incorporates meteorological, oceanographic, and/or river flow rate forecasts; makes predictions for locations where observational data will not be available; and is usually initialized by the results of a nowcast. see also textit{Numerical Forecasting}.

Front

In meteorology, generally, the interface or transition zone between two air masses of different density.

Frontal System

The orientation and nature of the fronts within the circulation of a frontal cyclone (cyclonic circulation).

Hindcast

Hindcast is a scientific predictions about past states of the atmpsphere made with a numerical model or method. These predictions rely on either observed or forecast data, not on hypothetical data. A hindcast incorporates past or historical observational data.

Initial Conditions

Initial conditions in a global model are prepared by making a synthesis of observed values of atmospheric fields taken over a for example 24 hour period and short-range forecasts provided by the global model itself. This synthesis is a process of assimilating observed values into a model. The use of both observations and model forecasts in the construction of initial values is required. High quality data are sparsely and irregularly distributed over the globe. Short-range model forecasts carry knowledge forward in time of earlier observations and also provide a crucial background for extracting useful information from expensive satellite observations.

Isopleth

An isopleth is a line of equal value (a Greek word iso - equal; pleth - value). A weather map contains isopleths of different weather parameters.

Isobar

Isopleth of Pressure

Isotherm

Isopleth of Temperature

Isotach

Isopleth of Wind Speed

Jet Stream

A jet stream is a narrow stream of relatively strong winds. The existence of the polar front jet streams is tied to the presence of horizontal temperature gradients. If temperature gradients exist through a deep layer of the troposphere, a pressure gradient force increases with height throughout the layer, and so does the wind.

Mesosphere

The mesosphere starts just above the stratosphere and extends to 85 kilometers (53 miles) high. In this region, the temperatures again fall as low as -93 degrees Celsius as you increase in altitude. The chemicals are in an excited state, as they absorb energy from the Sun. The mesopause separates the mesophere from the thermosphere.

Nowcast

Nowcast is a scientific predictions about the present states of the atmpsphere made with a numerical model or method. A nowcast incorporates recent (and often near real-time) observed meteorological, oceanographic, and/or river flow rate data; covers the period of time from the recent past (up to a few days) to the present; and makes predictions for locations where observational data are not available. The present is the time at which the nowcast is made, and at which the most recent observations are from a few minutes to an hour old.

Numerical Weather Prediction

NWP is an initial- boundary value problem: given an estimate of the present state of the atmosphere (initial conditions), and appropriate surface and lateral boundary conditions, the model simulates (forecasts) the atmospheric evolution.

Numerical Integration

A solution of the governing equations of hydrodynamics by numerical methods. The numerical solutions are carried out with the aid of computers ranging from desktop workstations to the most powerful computers available.

Numerical Forecasting

(Also called mathematical forecasting, dynamical forecasting, physical forecasting, numerical weather prediction.) The integration of the governing equations of hydrodynamics by numerical methods subject to specified initial conditions. Numerical approximations are fundamental to almost all dynamical weather prediction schemes since the complexity and nonlinearity of the hydrodynamic equations do not allow exact solutions of the continuous equations.

Mesoscale

Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers. From a dynamical perspective, this term pertains to processes encompassing deep moist convection and the full spectrum of inertio-gravity waves but stopping short of synoptic-scale phenomena, which have Rossby numbers less than 1.

MSEPS - Multi-Scheme Ensemble Prediction System

Parameterisation

The representation of physical effects in a dynamic model in terms of simplified parameters, which represent a series of simplifications of the full turbulence model to remove complex terms and form a closed set of equations that lead to a hierarchy of so-called closure models of decreasing complexity.

Percentile

In statistics a percentile (or a centile) is a measure indicating the value below which a given percentage of the data set fall. Percentiles are usually used, when the data set is divided into 10 equally sized sets. In the context of the MSEPS application the data set is comprised of 75 weather ensemble forecasts of the specific variable or e.g. 300 power forecasts (4 x 75 forecasts at different heights). For example, the 10th percentile (P10) is the value below which 10 percent of the forecasts may be found. The 25th percentile is in statisitcs also known as the first quantile (Q1), the 50th percentile as the median or second quartile (Q2) and the 75th percentile as the thrid quantile (Q3). Note that percentiles and quartiles are specific types of quantiles.

Primitive Equations

The Eulerian equations of motion of a fluid in which the primary dependent variables are the fluid’s velocity components. These equations govern a wide variety of fluid motions and form the basis of most hydrodynamical analysis. In meteorology, these equations are frequently specialized to apply directly to the cyclonic-scale motions.

Quantile

(Note: Definition is a context adjusted copy from Wikipedia): Quantiles are points taken at regular intervals from the cumulative distribution function (CDF) of a random variable, in our case a set of forecasts. Dividing ordered (forecast) data into q essentially equal-sized data subsets is the motivation for q-quantiles. The quantiles are the data values marking the boundaries between consecutive subsets of forecasts. Put another way, the k^mathrm{th} q-quantile for a forecast is the value x such that the probability that the forecast will be less than x is at most k/q and the probability that the forecast will be more than x is at most (q-k)/q=1-(k/q). There are q-1 of the q-quantiles, one for each integer k satisfying 0 < k < q.

Some q-quantiles have special names:

• The 2-quantile is called the median
• The 3-quantiles are called tertiles or terciles ? T
• The 4-quantiles are called quartiles ? Q
• The 5-quantiles are called quintiles ? QU
• The 6-quantiles are called sextiles ? S
• The 10-quantiles are called deciles ? D
• The 12-quantiles are called duo-deciles ? Dd
• The 20-quantiles are called vigintiles ? V
• The 100-quantiles are called percentiles ? P
• The 1000-quantiles are called permilles ? Pr

More generally, one can consider the quantile function for any distribution. This is defined for real variables between zero and one and is mathematically the inverse of the cumulative distribution function.

Quartile

In statistics, the quartiles of a ranked set of data values (in our case forecasts) are the three points that divide the data set into four equal groups, each group comprising a quarter of the data. A quartile is a type of quantile. The definition most known of the lower quartile is being the middle number between the smallest number and the median of the data set. The second quartile is the middle of the data set, also called the median. The third quartile is defined as the middle value between the median and highest values of the data set. In other words, the 3 quartiles can be expressed as being 25%, 50% and 75% of the data values (forecasts).

Stratosphere

The stratosphere starts just above the troposphere and extends to 50 kilometers (31 miles) high. Compared to the troposphere, this part of the atmosphere is dry and less dense. The stratopause separates the stratosphere from the next layer.

Synoptic Scale

Used with respect to weather systems ranging in size from several hundred kilometers to several thousand kilometers, the scale of migratory high and low pressure systems (frontal cyclones) of the lower troposphere.

Thermosphere

The thermosphere starts just above the mesosphere and extends to 600 kilometers (372 miles) height. The temperatures go up with increasing altitude due to the Sun’s energy. This layer is known as the upper atmosphere.

Troposphere

The troposphere starts at the Earth’s surface and extends 8 to 14.5 kilometers high (5 to 9 miles). This part of the atmosphere is the most dense.

Almost all weather is in this region. The tropopause separates the troposphere from the next layer. The tropopause and the troposphere are known as the lower atmosphere.