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Weather Queries

On May 9, 2013, an early afternoon, garden-variety thunderstorm passed overhead, moving toward the north-northeast. The temperature was in the mid-60s, and the dewpoint was near 60°F before the storm. Fog came in off the ocean about 10 minutes after the storm departed. I took a photo of the receding storm (see the Figure), but I neglected to photograph the fog rolling in or dissipating. How does a thunderstorm produce or induce fog, or can it do both? Did the ocean contribute to the development of the cumulonimbus cloud?

Tom Adams

Marblehead, Massachusetts

I looked at the sea-surface temperatures (SST) provided by the Taunton, Massachusetts, National Weather Service office for the afternoon of May 9, 2013. They ranged from 50 to 54°F off Marblehead. Thus, the sea surface was at least 10 degrees cooler than the air temperature at the coastline at about the time the thundershowers arrived. It is unlikely that the ocean contributed to further development of the cumulonimbus cloud. Because it was early afternoon, it is reasonable to assume that the boundary layer air feeding the thunderstorm from below was well mixed over land, that is, convective currents could occur freely below the cloud and feed its updraft. Once over the ocean, however, the air beneath the thundercloud would be stable, the water being colder than the air above. This would have the effect of weakening any updraft feeding the storm. It is possible that the thundershower developed inland along a sea-breeze front before winds aloft carried it out to sea. With light surface winds prevalent over the region on May 9, a sea breeze could have progressed well inland. Convergence of low-level air along this front could have led to shower development.

Fog was probably present at sea before the thundershowers arrived. With the dew point near 60°F and a sea-surface temperature 6-10°F lower, vapor in the air in contact with the water surface could easily have been chilled to the dew point. Condensation would then occur, and fog would form. The low-level outflow of air from a weak thundershower might have been just enough to push the fog bank to the shoreline.

What would happen if somehow a large patch of ocean could be strongly heated, almost to the boiling point, and an incipient tropical storm passed over this patch. Could it happen?

Steve Kwiatkowski

Kerry Emanuel, a professor at the Massachusetts Institute of Technology, and several co-authors tackled this question in a 1995 article published in the Journal of Geophysical Research entitled “Hypercanes: A Possible Link in Global Extinction Scenarios.” The highest sea-surface temperature ever recorded is probably in the neighborhood of 35°C (95°F), but not in an area frequented by tropical storms. Thus, there are no instances of tropical storm development over hot ocean water in the natural world. The authors resorted to a computer model of a hurricane to investigate what might happen if a storm formed over ocean water at 50°C (122°F). This is far from the boiling temperature, but the results are still spectacular.

The model runs in a geographical area 1,000 km wide with a bell-shaped sea-surface temperature anomaly: the sea surface is 50°C at the center of the anomaly, decreasing to 27°C (81°F) at a large distance from the center. The atmosphere is considered to be initially at rest (no wind at any altitude). The temperature and moisture stratification (how temperature and dew point vary with altitude) are typical of the tropical atmosphere. A weak vortex is placed over the ocean “hot spot.” The stipulation of no ambient wind ensures that the vortex will remain over the hot spot long enough to intensify into a hurricane.

A hurricane, unlike any ever observed, quickly develops. The central pressure drops below 300 millibars (mb) within 30 hours. Very few real hurricanes have central pressures below 900 mb. Winds wrapping around the hurricane eye wall exceed 500 mph, which is a substantial fraction of the speed of sound. Even the strongest real hurricanes (Category 5 on the Saffir-Simpson scale) fail to reach 200 mph. Such an apocalyptic storm is so beyond earthly experience that it has a special name: a hypercane.

Could it ever have happened? Emanuel and his coauthors suggest that it could. About 65 million years ago, a bolide (a large, extraterrestrial body that hits the earth at very high speed, explodes upon impact, and creates a large crater), perhaps 10 km in diameter, struck the ocean near the northern tip of the Yucatan Peninsula at an incoming speed of about 22 km per second. The energy associated with this impact created the Chicxulub Crater, ejected a tremendous amount of dust and debris into the atmosphere, and possibly beyond to orbital altitudes. Extinction of sunlight by the debris cloud, cessation of photosynthesis by plants, widespread fires, and other calamities very likely led to the extinction of dinosaurs and many other life forms on earth. Geological evidence strongly supports this.

Emanuel and co-authors hypothesize that water flowing back into the crater immediately after impact would have become very hot, and that a succession of hypercanes would have formed over the hot spot. In the main hypercane updraft, possibly exceeding 30 meters per second (67 mph), much of the cloud condensate, starting as tiny droplets, would not have time to grow enough to fall out as precipitation. Consequently, a large fraction of it, freezing as it reached the high troposphere, would be lofted to the stratosphere.

The hypercane model develops a ring of air between five and 35 kilometers from the eye that is still rising at five meters per second (11 mph) at an altitude of 20 km (12.4 miles), which is in the middle stratosphere. If ten grams of water substance (ice and vapor) per kilogram of air enters the middle stratosphere within the ring of rising air just described, then the transport of water into the middle stratosphere is about 1.7 × 107 kg per second. This flux of ice and vapor is enough to saturate the layer between 100 and 50 mb globally in about 40 days. If the ocean hotspot persisted this long and if a succession of hypercanes continued to deliver this water flux to the stratosphere, it is highly likely that an extensive stratospheric cloud layer would develop.

A layer of stratospheric clouds would contribute further to the extinction of solar radiation already caused by stratospheric dust previously deposited following the bolide impact. These clouds might persist from many days to weeks but would eventually evaporate. The high concentration of stratospheric water vapor left behind would persist far longer and pose another hazard to life on the surface: potentially damaging doses of ultraviolet radiation caused by ozone (O3) depletion in the stratosphere.

In the stratosphere, water vapor (H2O) is a source of OH and HO2. These are called free radicals, specifically, the hydroxyl radical and the hydroperoxy radical, respectively. A free radical is a molecule with an unpaired (free) electron. Most free radicals, including OH and HO2, are highly chemically reactive. The following catalytic cycle occurs in the stratosphere, in which OH is the catalyst (it participates in the initial reaction and is reproduced in the final reaction):

OH + O3 → HO2 + O2

HO2 + O3 → OH + 2O2

The net result is the production of three oxygen molecules from two ozone molecules. If the middle stratosphere were nearly saturated with water vapor, the production of OH and HO2 radicals would be high, and very substantial destruction of ozone could occur, primarily through the reactions above. Ozone shields the earth's surface from harmful ultraviolet radiation, which can be detrimental to life in high doses.

There is no conclusive proof that hypercanes formed following the Chicxulub impact, nor that serious ozone depletion resulted in life-threatening doses of ultraviolet radiation. On the other hand, the extinction of many life forms at this time is generally accepted, and hypercanes cannot be ruled out as a contributing cause.

You can watch an excellent and authoritative 45-minute video on hypercanes at http://www.youtube.com/watch?v=7-dK5kogPUE.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut St., Suite 850, Philadelphia, PA 19106.

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