The 5-day animated view of Total Precipitable Water (PW), published daily at the U of Wisconsin, provides us with critical information that is needed for gaining a full understanding of the primary messages obtained from analysis of the Weather Maps. I go back to it often and search for more insights into the process of PW formation and distribution. Please open it now, at http://tropic.ssec.wisc.edu/real-time/mtpw2/product.php, and I will offer a few comments about the picture that unfolds. Most of what we see here stays true on practically every day of the year.
The most obvious observation tells us that a very high percentage of all the PW found in the atmosphere after adding it up in all parts of the globe, perhaps 90% of it or more, originates strictly from the evaporation of warm waters within the tropical zone. As one might expect, a relatively narrow strip of waters close to the equator is the most productive of all. It’s also worth noting that many rain forest regions are about as productive as ocean waters as sources of evaporation, by virtue of recycling processes. The remaining PW production in the tropics, as seen within margins which have a way of slowly making small seasonal adjustments between north and south, is basically well-contained within latitudes 30N and 30 S. Land surfaces are, of course, quite unproductive except for rain forests, but they are not alone. It is apparent that evaporation rates drop off fairly sharply with every incremental decline in surface water temperatures in all parts of the tropics, leading to results that seem drastic in some cases. I’ll now open the weather map that clearly demonstrates the closeness of the relationship between PW concentrations and tropical surface water temperatures when detailed comparisons are made:
Where tropical waters are warmest, and well within the tropical borders, we can be sure that any limit to how much vapor is lofted into the high troposphere will be quite high. We can also deduce that most of this vapor does not travel laterally for more than a short distance but just stays pretty much in place before becoming saturated and raining out. There are almost no jetstream winds blowing high up over the tropics that could otherwise pick up vapors of any density and carry them off. Everything is different upon approaches to the tropical margins, where jet winds do exist, in a state quite ready for interaction. We can see exactly how and where this begins and continues on the animated website, when we view how concentrations of PW in stream-like patterns are seen moving quickly away from certain locations along the tropical margins in the same eastward direction commonly taken by jetstream winds. This map will give us a good look at relevant wind positioning on this day, which is quite normal, starting in both hemispheres at latitudes in the 15-20 area.
Any surface waters with temperatures above 25C found along the line of these lowest jet wind latitudes will be able to loft vapors to an altitude where interactions with jet winds are unavoidable. We have a truly amazing tool for watching this happen! Remember that the PW values we see on the website include some that are at lower altitudes and may be moving in contrary directions or not at all. They will typically constitute a lesser part of the full value of PW in the outward moving streams.
Carl