Lapse rates over water

[chrisbray]

Hello,

I was looking at a mesoscale discussion today at the SPC and they were showing a chart of low level lapse rates over Wisconsin, Lake Michigan, and lower Michigan. The charts showed 8s and 8.5s across the states, but over the lake it was merely 6 c/km. I've noticed that the same effect is in place for CAPE values. I know that it is at least partly because the surface temperature over the lake is much cooler than over land, but do the models also tend to just fill in lower values over the Great Lakes because there are no surface observations?

If the lapse rates are always significantly lower over the great lakes, and there is hardly any SBCAPE over them as well, how does convection, for example crossing from Wisconsin to Michigan across Lake Michigan, survive without weakening immensely? Or does the convection have to be elevated in order to make it across the lake and maintain severe intensity? Just curious,

Chris

 

[rdale]

There are surface obs over the lake from bouys, ships, etc. so models are aware of low level conditions.

Sometimes surface temps are quite warm over the water, and sometimes a very low inversion sets up. Predicting what storms will do when crossing the lake is much more art than science

 

[MClarkson]

"do the models also tend to just fill in lower values over the Great Lakes because there are no surface observations?"

no. Most grid squares do not have observations. The interpolation algorithms for putting the right values (from the observations you do have) into the right grid squares are massively complex. They usually do a pretty good job.

In the summer, most bodies of water are cooler than the nearby land. This does have the effect of reducing convective strength. Sometimes the forcing is strong enough or the air unstable enough to continue propagating good thunderstorms over cooler waters. But often you will see convective systems lose strength as the reach the coast.... for someone who spent a lot of time living in the US northeast near the ocean... that is really annoying.

 

[Michael Thomas]

In the summer, most bodies of water are cooler than the nearby land. This does have the effect of reducing convective strength. Sometimes the forcing is strong enough or the air unstable enough to continue propagating good thunderstorms over cooler waters. But often you will see convective systems lose strength as the reach the coast.... for someone who spent a lot of time living in the US northeast near the ocean... that is really annoying.

I know the feeling, it is very much the same for the east coast of Australia. Along the east coast of Australia, dew points are usually higher on the coast/offshore but surface-temps are typically cooler. With regards to CAPE, the increased low-level moisture can help offset the lower surface temps but all thing being equal, CIN will be greater along the coast. I suspect this increased CIN, and not necessarily a lack of CAPE, is often responsible for killing off storms approaching the coast (at least for the east coast of Australia). Of course, there are the odd exceptions where surface-based storms will actually strengthen when/after crossing the coast. I think this is a nice example (30 Dec, 2008) - http://www.theweatherchaser.com/radar-loop/IDR662-brisbane/2008-12-30-03/2008-12-30-13

In the cooler months though, surface temps are often warmer over the ocean. Under the right conditions, the steeper lapse rates offshore, and resulting low-level buoyancy, will often aid in the formation of waterspouts.

BTW, just realised I got fairly off topic but it is still vaguely relevent

 

[Jeff Duda]

In the summer, most bodies of water are cooler than the nearby land. This does have the effect of reducing convective strength. Sometimes the forcing is strong enough or the air unstable enough to continue propagating good thunderstorms over cooler waters. But often you will see convective systems lose strength as the reach the coast.... for someone who spent a lot of time living in the US northeast near the ocean... that is really annoying.

Just expanding on your response.

This happens because 1) the thermal heat capacity (specific heat) of water is much higher than that of "land" (I think somewhere in the neighborhood of 1000-2000 J/kgK for typical soil/Earth and 4196 J/kgK for pure water), so the same solar input simply doesn't cause as large a temperature change; and 2) the albedo of a water surface (even a choppy one with lots of waves) is much higher than that of "land" (the albedo of a water surface is typically in the range of 0.9-0.95, whereas it ranges from maybe as low as 0.1 to 0.3 for many common surface land cover types, to as large as 0.9 for fresh snowpack).

Reason (1) provides much of the explanation for why the peak of the Atlantic tropical season occurs in September rather than June/July. There's a significant lag of SSTs due to the higher specific heat capacity of the water. It also explains why lake-effect snow is more common in late autumn/early winter, as the water surface "holds onto" the heat for longer. The land surface type (soil vs. water) makes a huge difference on the lower atmosphere temperature profile. With less sensible heat flux over a water surface due to cooler water surface temperatures (and perhaps due to reduced turbulence also from lower roughness length of water compared to grass/trees etc.), the boundary layer is generally cooler, thus parcels will have less CAPE compared to their bretheren over nearby land masses.