Applied atmospheric research
Nowcasting of severe weather
Introduction
KNMI issues warnings to the general public for several types of severe weather events
which can be disruptive to Dutch society. Important types of severe weather
phenomena are e.g. strong winds, heavy gusts and squalls, severe turbulence
in thunderstorms, hail and fog. For the accurate assessment and prediction of
such (usually small-scale and physically complex) phenomena at short time scales
(0-12 hours ahead), sophisticated automated guidance methods are required.
For this reason, one of the primary research issues is the development of improved algorithms
and methods to generate accurate automatic, very short term (0-12 hours ahead)
forecasts for various types of severe weather in the Netherlands.
Relevant projects in this field are:
- Nowcasting severe weather
- Development of radar severe weather products
- Development of a mesoscale atmospheric model (XHIRLAM),
and validation thereof for (small-scale) severe weather events
Project Nowcasting severe weather
In the project "Nowcasting severe weather", an inventory has been made of
the strengths and weaknesses of nowcasting techniques presently used at KNMI
and elsewhere. Methods for the prediction of gusts, clear air turbulence (CAT),
thunderstorms and strong convection, and fog have been considered. On the basis of
this inventory, several follow-up projects have been defined, in which more
advanced automated prediction techniques will be developed and tested for:
- gusts (project GUSTO (GUST Theory Research))
- thunder storms (project IOC (Indices for Thunderstorms and Convection))
- and fog.
Development of radar severe weather products
In nowcasting, the weather radar is an indispensible instrument for the
detection and monitoring of severe weather phenomena such as heavy showers,
thunderstorms, strong winds and gusts, and summer hail. The two Doppler radars
that have been installed in De Bilt and Den Helder a few years ago, offer an
increased functionality in this respect as compared to their predecessors.
Several non-Doppler products which are determined routinely, such as vertically
integrated liquid (VIL) and echo tops, contain information no the probability of
occurence of summer hail. Based on these products, a new method for the detection
of summer hail has been development in the course of the project
"Development of radar non-Doppler
severe weather products". At present, an assessment is being carried out on
the usefullness and quality of both Doppler and non-Doppler radar products for
the detection of wind and gusts.
XHIRLAM
Severe weather phenomena such as thunderstorms and fog can be highly localized in space.
Therefore, it is important that the numerical atmospheric models which are used for the
prediction of said phenomena, possess a high level of spatial detail. In order to be
able to predict small-scale weather phenomena with greater accuracy, the XHIRLAM
model has been developed. XHIRLAM is a mesoscale version (using a grid point
distance of 11km) of the atmospheric model HIRLAM,
which is used operationally at KNMI. In the
ASWAN project, the behaviour
of XHIRLAM during various severe weather events has been studied extensively.
It has been shown that in particular for the prediction of small-scale extreme
events XHIRLAM yields a clearly more accurate assessment of the weather in the
various regions of the Netherlands than the coarser HIRLAM model.
Contact informatie
More information on the project "Nowcasting gevaarlijk weer", the development
of radar severe weather products and XHIRLAM can be obtained from, respectively,
dr.
G. T. Geertsema. , dr.
I. Holleman. en dr.
A.B.C. Tijm.