Ivens (Maximum convective wind gust)
It has been proven to be accurate and reliable for use in operational forecasting, particularly in the European area when other techniques originating from the USA produced excessively high velocities.
The technique is a statistical method developed using multiple regression. The following parameters are considered:
- The difference between maximum daytime temperature (T_{x}) and θ_{w} at 850 hPa: If the showers developed over the sea, sea surface temperature is used instead of T_{x}.
- \(T_x-\theta_w500\) and \(\theta_w850-\theta_w500\)
- The square roots of the above parameters.
- Wind velocity U at 850 and 250 hPa.
There was a marked difference in the maximum gust in relation to upper air data in the case of \(T_x-\theta_w850<9 ^\circ C\) or \(T_x-\theta_w850\geq 9 ^\circ C\). As a result of this multiple regression equations were derived separately.
$$if \quad T_x-\theta_w850<9 ^\circ C:\quad$$
$$FF_{max}=14.9+0.976*U850+1.27(\theta_w850- \theta_w500)$$$$if \quad T_x-\theta_w850\geq 9 ^\circ C:\quad$$
$$FF_{max}=15.9+0.174*U850\sqrt{T_x- \theta_w500} +0.057*U250\sqrt{T_x-\theta _w500}+0.92*(\theta _w850-\theta _w500)$$
where FF_{max} is the maximum gust in knots, T_{x} the maximum daytime
temperature (deg C), U850 and U250 the wind at 850 and 250 hPa (kts) respectively and
θ_{w}850, θ_{w}500 the wet bulb potential temperature at
850 and 500 hPa (deg C) respectively.
Note that should any of the expressions within the square root operators evaluate to less than zero (highly unlikely in real atmospheric conditions), then the expression will be set to zero internally to avoid arithmetical errors.