Hurricane Dynamics
All of the systems described thus far contrast sharply with a hurricane, which is almost by definition a storm with somewhat circular symmetry. Tropical hurricanes form over ocean waters with a surface temperature in excess of 26 degrees. From a satellite, a cloudless eye of some 10 to 30 kilometers in diameter is usually evident, and precipitation in the eye wall, fueled by moist inflow at low altitude, is intense (see Figure 10). Cloud-base temperatures in the eye wall are high--often greater than 22 degrees--and surface winds rotate counterclockwise (in the Northern Hemisphere) at speeds in excess of 50 meters per second. At the top of the storm and well outward from the eye, the cirrus layer outflow is clockwise.
Aircraft flights through hurricanes do show the presence of electrical fields, but the graupel-liquid water-ice turns out to be at the wrong place at the wrong temperature and in insufficient volume to give the spatial charge distribution necessary to produce a lightning discharge. The vertical shear of horizontal wind is present in the region of updraft and outflow all the way around in the eye wall--but the supercooled water is not, so there is not enough separation of charge.
Aircraft observations show that hurricanes exhibit a dearth both of updrafts greater than 8 meters per second and of supercooled water at temperatures colder than -5 degrees. Warm cloud-base temperatures provide a great depth of cloud for coalescence processes to form rain without the involvement of ice processes. Except in the strongest updrafts near the melting level, there is usually less than half a gram of supercooled cloud water per cubic meter. Even in the eye wall, the typical hurricane has maximum updrafts that are less than 8 meters per second and maximum supercooled cloud liquid-water content less than 2 grams per cubic meter.
In addition, eye-wall updrafts are far from upright; the wind structure in the eye wall forces the updraft outward and upshear relative to the surface (see Figure 10). This, coupled with the radial outflow, allows precipitation formed by coalescence at lower levels to fall out of the updraft before it reaches the melting level. It also allows ice particles formed at higher levels to “seed” the upstream edge of the updraft, thereby ensuring that little supercooled water survives to reach the -10 degree level. This in turn prevents the proper liquid-ice particle mixture from forming at colder temperatures, which is where most of the charge separation takes place in other systems.
Finally, the orbital period of ice in the anvils resulting from convection in the eye wall is only 30 to 40 minutes, so ice is rapidly distributed around the upper eye and thence outward to the whole outer hurricane. Thus hurricanes lack lightning activity both because the vertical velocity is too low to carry supercooled water up to higher altitude and because of an excess of ice, such that even if water were carried up it would be nucleated prior to forming the graupel necessary for charge separation.
The occasional hurricane that is electrically active has strikingly different dynamics from the typical storm. Such systems are often characterized by highly asymmetrical precipitation--sometimes confined to a “supercell” in just one quadrant--and updrafts greater than 15 meters per second associated with the precipitation. Flights through such a system are far less benign than routine hurricane fly-throughs, with much more turbulence. The supercell usually rotates around the eye, and its electrical activity delineates the eye wall. The weaker vertical updrafts result from a more general symmetrical heating around the hurricane and an overall reduction in the available potential energy for convection.
Vapor and Global Radiation Balance
Although the prediction of lightning is the primary motivation for understanding cloud electrification, convection and the other mechanisms that produce thunderstorm clouds also may profoundly influence the radiation balance of the earth’s atmosphere. Like the better-known greenhouse gas carbon dioxide, water vapor has less impact on incoming radiation from the sun than on outgoing infrared radiation. In the presence of water vapor, the atmosphere is heated at lower levels and cooled aloft. Unlike carbon dioxide, however, water vapor is not evenly distributed in the atmosphere.
Clouds obviously also have a significant influence on the radiation balance, but because of the variable nature of both cloud cover and temperature at various altitudes, the exact influence is very difficult to assess. In particular, the effect of cirrus clouds produced at the tops of the convective systems described in this article depends on the optical depth at various wavelengths, which is in turn influenced by particle size, shape and distribution, cloud spatial distribution, and the amount of moisture and aerosol transported to upper levels. The efficiency of removal of water at lower levels determines the mass flux aloft to form cirrus cloud, and the details of the ice evolution in the critical shear zones determine the size and shape of the particles and their optical properties--processes we cannot yet quantitatively assess on a significant scale. Nonetheless, because it is associated with powerful convective activity, lightning frequency may be related to the input of water vapor into the upper troposphere.
Hurricanes, too, move large amounts of water and water vapor. Much of it may be left as rain along its path, but a hurricane also lofts much water as ice to high levels, detectable as dense cirrus clouds in satellite views of hurricane outflow (see Figure 10). The tops of such clouds reach low temperatures--often well below -50 degrees and occasionally below -80 degrees. This cirrus returns solar radiation to space and influences thermal radiation from lower in the atmosphere. In the overall scheme of things, the area of the earth influenced by immediate hurricane-produced cirrus is rather small; the water vapor resulting from cirrus evaporation, however, may have a somewhat greater effect.
The magnitude of the uncertainties water vapor poses to the earth’s radiation budget is probably as great or greater than the contribution of increased carbon dioxide to computations of so-called global warming and calls into question the reliability of numerical models that ignore these effects. The depth and spatial distribution of cirrus clouds near the top of the troposphere influence both the albedo for solar radiation and the radiative flux to spaceÑtogether with the overall equivalent temperature of the earth’s atmosphere at different levels. A complex feedback process may be at work here.
Thus looking for an answer to the simple question of why hurricanes aren’t so hot electrically raises questions of a fundamental nature that can lead to techniques and knowledge with much broader application.
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© American Scientist 1998