The water vapor content of the atmosphere is of critical importance to nearly every facet of atmospheric science. Observing the processes that determine the water vapor content of the atmosphere is, therefore, of fundamental importance. As the earth’s surface is the source of most atmospheric water vapor, observations of vertical turbulent transport of water vapor in the planetary boundary layer have been the subject of many research projects. Evapotranspiration can vary significantly across the earth’s surface due, for example, to variability in insolation, vegetation cover, soil properties, and precipitation. Area-averaged water vapor flux measurements are necessary to get a large-scale view of surface–atmosphere interactions.
The Extreme Forecasting Index (EFI) is an integral measure of the difference between the ensemble forecast (ENS) distribution and the model climate (M-climate) distribution. This allows the abnormality of the forecast weather situation to be assessed without defining specific (space- and time-dependant) thresholds. The EFI takes values from -1 to +1. If all the ensemble members forecast values above the M-climate maximum, EFI = +1; if they all forecast values below the M-climate minimum, EFI = -1. Experience suggests that EFI magnitudes of 0.5 - 0.8 (irrespective of sign) can be generally regarded as signifying that "unusual" weather is likely whilst magnitudes above 0.8 usually signify that "very unusual" weather is likely. Although larger EFI values indicate that an extreme event is more likely, the values do not represent probabilities as such.
What does this mean? The prediction for excess water vapour and the presence of an “atmospheric river” will grace eastern Australia this weekend, and is classified as an unusual and extreme event, in areas shaded red (image courtesy ECMWF).
