Joint Centre for Mesoscale Meteorology (JCMM)
JCMM INTERNAL REPORT NO.110
The relationship between water vapour imagery and thunderstorms
by Nigel M Roberts
Mid-latitude thunderstorms are often difficult to predict, even just a few hours ahead. Usually, the area that is generally favourable for storm development is known, but to be more precise about where and when thunderstorms will break out can be a big problem. Many factors play a role and add to the complexity. A list of some of these factors includes; variations in orography, the urban heat island effect, sea breeze fronts, humidity variations, low-level convergence, gust fronts and mesoscale or synoptic scale dynamical forcing. This report concentrates primarily on one aspect, the dynamical forcing and temperature and humidity structure associated with upper/middle level vorticity anomalies that can be identified in water vapour (WV) imagery. It has been noted from a variety of case studies that thunderstorms can be found within and on the edges of dry regions in WV imagery. Most of these case studies focus specifically on cyclogenesis or split front events. The aim here is to show greater generality in the relationship between WV dry regions and the triggering of thunderstorms; firstly, by showing several good examples and then by presenting the results of a study of over three hundred events to show just how common this relationship is. JCMM report 109 provides a useful background to this report, particularly in the interpretation of WV imagery.