Convection - Differences Between Summer And Winter

[desperately seeking]

Hi All,

Convection confuses me. In the next few days we're forecast to have an LP sat bang over us. In the summer (especially if it was a warm LP which I know this one's not) I'm pretty certain we would be more or less guaranteed slow moving heavy - possibly thundery - showers more or less anywhere in the country. At this time of year, because the sun is less strong, this seems to be less likely - I keep looking at the LP that will be over us and having to remind myself that convection is less likely to happen - but is that true?

This is where my question lies; how does the difference between a cold LP (like the PV that will be over us soon) and a warm LP (for example a Spanish Plume in August) affect convection and do we know where convection is more likely to occur in a cold and warm LP? What other factors (leaving out the effect of the sun) can contribute to convective showers being generated?

TIA

DS

[Nick F]

Hi All,

Convection confuses me. In the next few days we're forecast to have an LP sat bang over us. In the summer (especially if it was a warm LP which I know this one's not) I'm pretty certain we would be more or less guaranteed slow moving heavy - possibly thundery - showers more or less anywhere in the country. At this time of year, because the sun is less strong, this seems to be less likely - I keep looking at the LP that will be over us and having to remind myself that convection is less likely to happen - but is that true?

Convection is indeed less likely over inland areas during winter away from windward coasts, because the sun is too weak to generate the strong updrafts from heating the ground needed for convection. Convection is more likely over sea and along coastal areas, if cold upper air is present, as this creates the steep lapse rates over the relatively warmer seas - which allows air warmed over the sea to rise quickly (convection). There are instances in winter when convection can occur almost anywhere inland, particularly along potent cold front boundaries (think 28th Jan 2004 Thundersnow) where very cold undercuts milder air ahead of front and forces it to rise quickly along a steep angle. Troughs too can bring convection inland – provided there is sufficient forcing, moist air and steep lapse rates aloft.

This is where my question lies; how does the difference between a cold LP (like the PV that will be over us soon) and a warm LP (for example a Spanish Plume in August) affect convection and do we know where convection is more likely to occur in a cold and warm LP? What other factors (leaving out the effect of the sun) can contribute to convective showers being generated?

TIA

DS

Convection is most likely to occur, any time of year, when there are steep lapse rates aloft (i.e. steep drop in temperature) helped by a forcing mechanism to force warm moist surface air upwards quickly – such as a cold front, trough, mountains, wind convergence. Steep lapse rates occur in summer when relatively colder air aloft over-runs warm and moist air below, usually when an upper trough approaches – this creates instability both over land and sea, but is enhanced by surface heating over land if the surface is moist (leading to surface-based rather than high-based storms). Spanish Plumes are basically when a conveyor of warm moist air is destabilised when differential thermal advection takes place – or in layman’s terms when colder air from the west with an approaching upper trough over-rides layers of warm moist air advecting N or NE.

Steep lapse rates in winter are likewise created by cold air aloft moving over relatively warmer/moister airmasses below – lapse rates enhanced over the sea which tend to lend to warmer surface air than overland during cold weather.

I could go on all afternoon - but I hope this may answer some of your queries.