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Questions for Tom Knutson
My main issue has to do with the higher intensity hurricanes and whether you have any idea what causes your model, even at the highest resolution, to underestimate those Cat 4 and 5 hurricanes which seem like those that could increase in a warmer world based on limited observations (e.g., Webster et al and also ELsner et al.). It looks like your more recent results improved a little with the cat 4 hurricanes. What gave this improvement (resolution?) and what direction are you looking into to improve your results further?
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Why is it that the threshold temperature rises with an overall change in SST? Is it the *contrast* in SST of the tropics vis-a-vis mid-latitudes which is important to hurricane formation? In other words, as the mid-latitude SST increases, the tropic SST has to get that much warmer? Or is it not quite that simple?
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through, but I was too busy in the early part with my talk to notice the online questions...Anyway, hopefully I can address everyone's questions here.
Hurricane intensities as simulated by models will depend on both model resolution and some details of the physics in the models. Our 18-km grid regional climate model tops out at around 47 m/s, but we can improve on this a lot by doubling the resolution and tinkering with the surface flux parameterization (e.g., evaporation and momentum fluxes).
The GFDL hurricane prediction model (9 km grid resolution) simulates cat 3's and 4's fairly well in terms of climatological number but is a bit deficient on cat 5's. It's also used operationally for hurricane prediction, so we get to learn more about its biases based on operational experience. This work with the GFDL hurricane model that I showed has not yet been published. We were keen to move to a model that could simulate cat 4-5 storms as we saw a tendency for a different response to climate warming at the high end vs the run-of-the-mill tropical storm or hurricane.
Further improvements will involve illustrating how dependent some of our downscaling results are on the particular climate model we choose for the downscaling. That is a measure of uncertainty about the whole problem, and one that the whole climate change community will have to help in solving.
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My guess is that the threshold temperature is determined based on the sea surface temperature in the tropical locations relative to the rest of the tropics. Whether the tropical vs extratropical contrast could also affect things is an interesting question that we haven't really looked into. We think that the tropics is special in that you cannot have large contrasts of temperature in the upper troposphere, even though you can have such contrasts at the surface. So the place where tropical storms will be most active will be where the surface is warmest, because that will have the greatest contrast exists (in the vertical) between the surface and the (relatively uniform) upper tropospheric temperature.
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