##### Surface wind from gradient wind

This utility allows you to calculate the expected surface wind speed and direction for a
given geostrophic wind. Initially gradient wind speed is calculated for cyclonic (sub-geostrophic) and
anticyclonic (super-geostrophic) flow as follows; if

$$V_{sub-geostr} = \frac{1}{2} \left(-rf + \sqrt{r^2f^2 + 4rfv_g} \right)$$ $$V_{super-geostr} = \frac{1}{2} \left(rf - \sqrt{r^2f^2 - 4rfv_g} \right)$$ $$f = 2\Omega sin\phi$$ Note: If there's an error in calculating the super geostrophic wind, remember there is, dynamically a limit to the strength of winds around an anticyclone. Pressure gradient force acts outwards so closer isobars fight against circular motion, in effect the expression within the square root section of the super geostrophic equation must be positive or zero. The resultant speeds are then reduced and the direction backed for the selected atmospheric stability profile and surface type as follows:

`r`is the radius of flow curvature (nautical miles),`Φ`the latitude (degrees),`f`the coriolis parameter (s^{-1}) and`V`the geostrophic wind velocity (ms_{g}^{-1}):$$V_{sub-geostr} = \frac{1}{2} \left(-rf + \sqrt{r^2f^2 + 4rfv_g} \right)$$ $$V_{super-geostr} = \frac{1}{2} \left(rf - \sqrt{r^2f^2 - 4rfv_g} \right)$$ $$f = 2\Omega sin\phi$$ Note: If there's an error in calculating the super geostrophic wind, remember there is, dynamically a limit to the strength of winds around an anticyclone. Pressure gradient force acts outwards so closer isobars fight against circular motion, in effect the expression within the square root section of the super geostrophic equation must be positive or zero. The resultant speeds are then reduced and the direction backed for the selected atmospheric stability profile and surface type as follows:

Stability and surface | Speed reduction factor | Direction backed (degs) |
---|---|---|

Land - clear night | 0.2 | -40 |

Land - average | 0.4 | -30 |

Land - unstable | 0.5 | -20 |

Sea - stable | 0.8 | -10 |

Sea - unstable | 0.9 | -5 |