The method of Ivens (1987) is used by the forecasters at KNMI to predict the
maximum wind velocity associated with heavy showers or thunderstorms. The
method of Ivens is based on two multiple regression equations that were
derived using about 120 summertime cases (April to September) between 1980 and 1983.
The upper-air data were derived from the soundings at De Bilt, and
observations of
thunder by synop stations were used as an indicator of the presence of
convection.
The regression equations for the maximum wind velocity (w
max ) in m/s
according
to Ivens (1987) are:
- if Tx - θw850 < 9°C
- wmax = 7.66 + 0.653⋅(θw850 - θw500 ) + 0.976⋅U850
- if Tx - θw850 ≥ 9° C
- wmax = 8.17 + 0.473⋅(θw850 - θw500 ) + (0.174⋅U850 + 0.057⋅U250)⋅√(Tx - θw850)
where
- Tx is the maximum day-time temperature at 2 m in K
- θwxxx the potential wet-bulb temperature at xxx hPa in K
- Uxxx the wind velocity at xxx hPa in m/s.
The amount of negative buoyancy, which is estimated in these
equations
by the difference of the potential wet-bulb temperature at 850 and at 500 hPa,
and horizontal wind velocities at one or two fixed altitudes are used to estimate
the maximum wind velocity. The effect of precipitation loading is not taken into
account by the method of Ivens.
(Source:
KNMI)