Thunderstorms spreading over Skagerrak on 24/11/06
Hello,
I want to take a look at somewhat surprising (?) development over Skagerrak (between Denmark and Sweden/Norway) where several thundery convective cells occured today's afternoon hours.
Satellite imagery depicts remarkable deep-moist convection in the highlighted area.
Now I try to find the reasons for this large thunderstorm activity...
The 500hPa-Isohypses and temperatures show a well-defined shortwave trough moving across the area of interest. The curvature ahead of the trough axis leads to somewhat cyclonic curvature vorticity advection and also cyclonic shear vorticity advection adding eachother to cyclonic vorticity advection (CVA - or positive vorticity advection in the northern hemisphere). This results into strong upward motion. Within the trough, the temperatures decrease to -25 up to -30°C.
Additonally, relatively mild temperatures in 850hPa prevail in the area of interest. Hence, some difference between 500hPa and 850hPa temperatures - 25 - 30K could be formed indicating a potential unstable atmospheric layer ( in fact, this does not suffice to consider instability because there could be some inversions capping deep-moist convection).
Lightningwizard charts from Oscar van der Velde show steep midlevel lapse rates highlighting a very dry layer.
Skagerrak is overspread with a field of lower equilibrium temperatures of -30°C that is enough for electrified convection. The equlibrium temperatures (ET) express the temperatures of the cloud tops . The lower the ET, the higher deep-moist convection will be and chances for electrified convection will arise. Generally, ET in exceed of -10°C are enough for showers.
In fact, GFS 12z analysis demonstrates that CAPE is present (this CAPE is MLCAPE calculated by the lowest 180hPa).
To conclude this analysis, the only available sounding nearby the area of interest (Southern Norway)
It shows little instability values of 140 J/kg but strong vertical wind shear (around 20 m/s in 850hPa, 25m/s in 700hPa). The layer between 750 and 600hPa as well as from surface to 850hPa is nearly adiabatic indicating strong updraft motions. The top of convection is nearby 500hPa - exactly where the -30°C isoline intersects the temperature line. Thus, the Lightningwizard charts did a good job.
Including the strong vertical wind shear in lower levels and the dry layer between 800 and 600hPa, I would have expected severe wind gusts within the area of interest, exceeding 70-80km/h (50 mph).
