MMC: > Air goes up at moist adiabatic rate, but has to be forced down at the > dry adiabatic rate
Of course. Thanks. Does it follow that although the net effect of moist convection is to transport heat upward, the actual circulation of air transports heat downward whenever air is being forced to rise despite a lapse rate lower (more stable) than the dry adiabatic rate? I had actually thought that typical lapse rates were near dry adiabatic, and that dry convection was fairly common wherever there's not enough moisture to condense into clouds, and enough sunshine to exceed losses by long-wave radiation from the surface. f.m.maugis: > Concerning hurricane and energy, nobody is speaking about Coriolis forces. That's partly because these are tropical storms, and the Coriolis force is small in the tropics. Also, in a low-pressure system the Coriolis force acts outward same as centrifugal force does, so it doesn't introduce a qualitative difference. > Anyway, there is a continuum between the very hot center of our planet and > the very cold space. It's not generally monotonic: it's often cooler underground than than at the surface, because groundwater is derived from precipitation that falls from cooler altitudes. Alvia Gaskill > http://www.aoml.noaa.gov/hrd/tcfaq/C5e.html > Subject: C5e) Why don't we try to destroy tropical cyclones by cooling > the surface waters with icebergs or deep ocean water ? That seems to presume that the strategy would be to simply starve the hurricane of energy by brute-force cooling of the entire area beneath it. That approach is obviously implausible. But can a hurricane remain at full strength, or close to it, if a little of the area beneath it is much cooler than the rest? Or will that mean that part of it is being driven at one rate and part at another, so that both dissipate much of their kinetic energy in turbulence? > http://groups.google.com/group/geoengineering/attach/2ea05e9f69344c48/250px-Hurricane_profile.svg.png?part=4&view=1 That looks as though the hurricane is being driven in part by heat from the stratosphere. Of course, it takes energy to move hot air downward and have it replaced with cooler air from below. It's stratified; that's why it's the stratosphere. But even if the hurricane as a whole is losing energy by bringing air with high potential temperature down from the stratosphere, part of it could be strengthened where that heat is going back up to the cool upper troposphere. In principle, I think the net effect could be to make an inefficient heat engine less inefficient. Sort of like how a turbojet engine has to extract more energy from its exhaust than the amount of work it does compressing its intake air, since the turbine and compressor aren't a perpetual motion machine, but the effect is to make the combustion faster and more complete so the whole thing works better. On Jun 6, 8:38 pm, "Alvia Gaskill" <[email protected]> wrote: > Some more info about the effect of hurricanes or more generally, tropical > cyclones on SST (sea surface temperature) from NOAA and the Wikipedia. Most > of the temperature decrease is due to the mixing of water in the upper layer > of the ocean by winds and most of the decrease occurs after the storm has > passed. Limited data show that the decrease from evaporation of water is > much less. NOAA also throws some cold water on artificial dissipation > strategies including the one that got this discussion started, ocean pipes. > They didn't address indirect approaches like the cloud ships and the desert > cover. > > http://www.aoml.noaa.gov/hrd/tcfaq/H7.html > > Subject: H7) How does the ocean respond to a hurricane and how does > this feedback to the storm itself? > Contributed by Joe Cione > > The ocean's primary direct response to a hurricane is cooling of the > sea surface temperature (SST). How does this occur? When the strong winds of > a hurricane move over the ocean they churn-up much cooler water from below. > The net result is that the SST of the ocean after storm passage can be > lowered by several degrees Celsius (up to 10° Fahrenheit). > > Figure 1 shows SSTs ranging between 25-27°C (77-81°F) several days > after the passage of Hurricane Georges in 1998. As Figure 1 illustrates, > Georges' post storm 'cold wake' along and to the right of the superimposed > track is 3-5°C (6-9°F) cooler than the undisturbed SST to the west and south > (i.e. red/orange regions are ~30°'C [86°'F]). The magnitude and distribution > of the cooling pattern shown in this illustration is fairly typical for a > post-storm SST analysis. > > One important caveat to realize however is that most of the 3-5°C > (6-9°F) ocean cooling shown in Figure 1 occurs well after the storm has > moved away from the region (in this case several days after Georges made > landfall). The amount of ocean cooling that occurs directly beneath the > hurricane within the high wind region of the storm is a much more important > question scientists would like to have answered. Why? Hurricanes get their > energy from the warm ocean water beneath them. However, in order to get a > more accurate estimate of just how much energy is being transferred from the > sea to the storm, scientists need to know ocean temperature conditions > directly beneath the hurricane. Unfortunately, with 150kph+ (100mph+) winds, > 20m+ (60ft+) seas and heavy cloud cover being the norm in this region of the > storm, direct (or even indirect) measurement of SST conditions within the > storm's "inner core" environment are very rare. > Thankfully in this case "very rare" does not mean "once in a > lifetime". Recently, scientists at the Hurricane Research Division were able > to get a better idea of how much SST cooling occurs directly under a > hurricane by looking at many storms over a 28 year period. By combining > these rare events, HRD scientists put together a "composite average" of > ocean cooling directly under the storm. > > Figure 2 illustrates that, on average, cooling patterns are a lot > less than the post storm 3-5°C (6-9°F) cold wake estimates shown in Figure > 1. In most cases, the ocean temperature under a hurricane will range > somewhere between 0.2 and 1.2°C (0.4 and 2.2°F) cooler that the surrounding > ocean environment. Exactly how much depends on many factors including ocean > structure beneath the storm (i.e. location), storm speed, time of year and > to a lesser extent, storm intensity (Cione and Uhlhorn 2003). > While the estimates in Figure 2 represent a dramatic improvement when > it comes to more accurately representing actual SST cooling patterns > experienced under a hurricane, even small errors in inner core SST can > result in significant miscalculations when it comes to accurately assessing > how much energy is transferred from the warm ocean environment directly to > the hurricane. With all other factors being equal, being "off" by a mere > 0.5°C (1°F) can be the difference between a storm that rapidly intensifies > to one that falls apart! With that much at stake, scientists at HRD and > other government and academic institutions are working to improve our > ability to accurately estimate, observe and predict "under-the-storm" upper > ocean conditions. These efforts include statistical studies, modeling > efforts and enhanced observational capabilities designed to help scientists > better assess upper ocean thermal conditions under the storm. With such > improvements, it is believed that future forecasts of tropical cyclone > intensity change will be significantly improved. > > Reference > Cione, J. J., and E. W. Uhlhorn, 2003: Sea Surface Temperature > Variability in Hurricanes: Implications with Respect to Intensity Change. > Monthly Weather Review, 131, 1783-1796. > > Last updated August 13, 2004 > > http://en.wikipedia.org/wiki/Typhoons > > Tropical cyclones are characterized and driven by the release of large > amounts of latent heat of condensation, which occurs when moist air is > carried upwards and its water vapour condenses. This heat is distributed > vertically around the center of the storm. Thus, at any given altitude > (except close to the surface, where water temperature dictates air > temperature) the environment inside the cyclone is warmer than its outer > surroundings.[2] > > Mechanics > > Tropical cyclones form when the energy released by the condensation of > moisture in rising air causes a positive feedback loop over warm ocean > waters.[14] > A tropical cyclone's primary energy source is the release of the heat > of condensation from water vapor condensing at high altitudes, with solar > heating being the initial source for evaporation. Therefore, a tropical > cyclone can be visualized as a giant vertical heat engine supported by > mechanics driven by physical forces such as the rotation and gravity of the > Earth.[15] In another way, tropical cyclones could be viewed as a special > type of mesoscale convective complex, which continues to develop over a vast > source of relative warmth and moisture. Condensation leads to higher wind > speeds, as a tiny fraction of the released energy is converted into > mechanical energy;[16] the faster winds and lower pressure associated with > them in turn cause increased surface evaporation and thus even more > condensation. Much of the released energy drives updrafts that increase the > height of the storm clouds, speeding up condensation.[17] This positive > feedback loop continues for as long as conditions are favorable for tropical > cyclone development. Factors such as a continued lack of equilibrium in air > mass distribution would also give supporting energy to the cyclone. The > rotation of the Earth causes the system to spin, an effect known as the > Coriolis effect, giving it a cyclonic characteristic and affecting the > trajectory of the storm.[18][19] > > What primarily distinguishes tropical cyclones from other > meteorological phenomena is deep convection as a driving force.[20] Because > convection is strongest in a tropical climate, it defines the initial domain > of the tropical cyclone. By contrast, mid-latitude cyclones draw their > energy mostly from pre-existing horizontal temperature gradients in the > atmosphere.[20] To continue to drive its heat engine, a tropical cyclone > must remain over warm water, which provides the needed atmospheric moisture > to keep the positive feedback loop running. When a tropical cyclone passes > over land, it is cut off from its heat source and its strength diminishes > rapidly.[21] > > Chart displaying the drop in surface temperature in the Gulf of Mexico > as Hurricanes Katrina and Rita passed over > The passage of a tropical cyclone over the ocean can cause the upper > layers of the ocean to cool substantially, which can influence subsequent > cyclone development. Cooling is primarily caused by upwelling of cold water > from deeper in the ocean because of the wind. The cooler water causes the > storm to weaken. This is a negative feedback process that causes the storms > to weaken over sea because of their own effects. Additional cooling may come > in the form of cold water from falling raindrops (this is because the > atmosphere is cooler at higher altitudes). Cloud cover may also play a role > in cooling the ocean, by shielding the ocean surface from direct sunlight > before and slightly after the storm passage. All these effects can combine > to produce a dramatic drop in sea surface temperature over a large area in > just a few days.[22] > > Scientists at the US National Center for Atmospheric Research estimate > that a tropical cyclone releases heat energy at the rate of 50 to 200 > exajoules (1018 J) per day,[17] equivalent to about 1 PW (1015 watt). This > rate of energy release is equivalent to 70 times the world energy > consumption of humans and 200 times the worldwide electrical generating > capacity, or to exploding a 10-megaton nuclear bomb every 20 > minutes.[17][23] > > While the most obvious motion of clouds is toward the center, tropical > cyclones also develop an upper-level (high-altitude) outward flow of clouds. > These originate from air that has released its moisture and is expelled at > high altitude through the "chimney" of the storm engine.[15] This outflow > produces high, thin cirrus clouds that spiral away from the center. The > clouds are thin enough for the sun to be visible through them. These high > cirrus clouds may be the first signs of an approaching tropical cyclone.[24] > > http://www.aoml.noaa.gov/hrd/tcfaq/C5e.html > > Subject: C5e) Why don't we try to destroy tropical cyclones by cooling > the surface waters with icebergs or deep ocean water ? > > Contributed by Neal Dorst > > Since hurricanes draw their energy from warm ocean water, some > proposals have been put forward to tow icebergs from the arctic zones to the > tropics to cool the sea surface temperatures. Others have suggested pumping > cold bottom... > > read more » > > GeorgesSST.jpg > 198KViewDownload > > SSTprofile.jpg > 57KViewDownload > > 250px-Hurricane_profile.svg.png > 18KViewDownload > > magnify-clip.png > < 1KViewDownload > > GulfMexTemps_2005Hurricanes.gif > 104KViewDownload --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
