Dan said...
"I would think that the surface type (land or water) would have no effect on storm mode or boundary layer wind fields, assuming an identical synoptic environment to one over land. Convection (nor surface winds) doesn't 'care' what the surface type is - the only thing near the surface that it cares about the moisture content at the boundary layer. Backed winds are usually associated with the circulation of a surface low"
Oh I assure you Dan it does matter what surface type you are over. Assuming ALL the atmospheric conditions are the same, no a storm doesn't care and it will act the same. That is why I said "mesoscale environments" favorable for tornadoes are less frequent over water.
Surface features aren't going to influence storm mode directly, but it influences the boundary layer winds and the boundary layer winds effect the storm mode, so yes the surface winds do care about what surface they are over. It is a fact that surface friction will upset the geostrophic balance of the winds and back winds toward the low pressure center. Over land you have more friction than you do over water. The Coriolis force will be weaker over land due to the decreased wind speeds (Coriolis force is proportional to wind speeds), hence the pressure gradient force will be relatively stronger over land and cause backing surface winds (better directional shear for tornadoes in the low levels). Over water you basically have a geostrophic wind at the surface (balance between the pressure gradient force pulling the wind to the left and the Coriolis force pulling the wind to the right), so the winds follow the isobars (lines of equal pressure).
There are tons of other things that are influenced in the same way. Hurricanes moving over land always die rapidly. One of the reasons this happens is because the increase in friction at the surface. Over water the winds circle around the low pressure center at the surface. Once it moves over land friction increases and surface winds back, turning more towards the center of the low pressure. The low pressure center fills and the hurricane dies (pressure gradient decreases). There are other things that kill off hurricanes over land, but this is one thing and another example of the frictional affects of land on the boundary layer winds.
Another example is lake effect snow. There is less friction for the winds traveling over water so the winds are blowing faster. The winds slow down where they encounter land and friction. You get an area of convergence from this and convection in some cases (once again there are other reasons for lake effect snow).
I also stated how you typically don't get as much instability over water as you do over land and that is because of the boundary layer not heating up as much/as quickly. A good example of this differential heating is the land sea breeze.
Dan said...
"Backed winds are usually associated with the circulation of a surface low, which, granted, is more likely to form over land (or along the coast) due to baroclinicity"
If you take that same surface low Dan and put one over water and one over land, the one over land is going to have more backed surface winds for the reasons I explained above. I promise you this is true. These are not my thoughts and ideas. It is well documented meteorological fact and it has nothing to do with baroclinicity.
Like I said in my first post, there are obviously mesocyclone tornadoes over water, but they are less frequent. It's not because "the storm cares about what surface it's over". The storm most certainly doesn't know or care what surface it's over. They are less frequent over water because the mesoscale environments that favor tornadic supercells are less frequent over water. Relatively speaking, surface winds don't back as much over water and you don't get as much instability (due to lower boundary layer heating). This is physics and I promise you it's true. I didn't make this stuff up myself.
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