From: Subject: Tuesday Weather Topic in Greater Depth Date: Thu, 22 Oct 2009 10:29:10 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w06t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 13 = October 2009=20

MASS BUDGETS AND THE HYDROLOGIC CYCLE

Water resides as a liquid, solid or vapor within one of several = reservoirs=20 constituting our planetary system. Incoming solar radiation provides the = energy=20 for changes in the physical phase of the water and an exchange of water = between=20 these reservoirs. This exchange is often called the hydrologic cycle. = The=20 earth's hydrologic cycle is an extremely important factor, not only = directly=20 sustaining life, but permitting the planet to remain a habitable place, = by=20 moderating the flow of solar energy into the planetary system, the flow = of long=20 wave radiation out of the system, and the distribution of energy within = the=20 planetary system.

Various disciplines within the earth sciences focus upon certain = aspects of=20 the water cycle. Meteorologists usually focus primarily upon water in = the=20 atmosphere, precipitation and, to a certain extent, evaporation - the = primary=20 mechanisms involving the exchange of water between the atmospheric = reservoir and=20 those reservoirs that reside on the earth's surface. Besides considering = the=20 extent of clouds and the amount of water vapor in the atmosphere, = meteorologists=20 routinely measure precipitation - the depth of water accumulated on a = unit area=20 from rainfall or snowfall in 24 hours or a month or a year.

On the other hand, hydrologists, concerned with water on or under the = earth's=20 surface, typically focus upon exchange processes on land. Often, they = are=20 concerned with flow rates in rivers and the amount of soil moisture that = affects=20 the water table.

THE BUDGET PHILOSOPHY

One method used to quantify the flow of water between various = reservoirs on=20 the planet is the mass budget. Like the financial budget that many of us = try to=20 maintain in our personal lives, the mass budget can simply be stated as = "the=20 gain (input) is equal to the loss (output) plus storage". On the earth's = surface=20 we can rephrase this statement to say that any water gained by = precipitation=20 would be balanced by the loss due to evaporation and surface runoff, = plus any=20 storage term which would be associated with either a change in the = soil's water=20 table or a change in lake or ocean levels.

We could apply this type of mass budget approach to any size system - = ranging=20 from the size of a cornfield through the Mississippi River watershed = that covers=20 a large portion of the nation , to the entire planet.

PLANETARY AVERAGES

Let us consider the entire planetary system on a long-term basis as=20 represented by annual averages. Over this time scale, we can assume that = the=20 gain to the surface, as represented by the annual precipitation averaged = over=20 the globe should equal the loss through evapotranspiration, since we = cannot=20 detect large changes in stored water over time.

The annual average precipitation for the entire planet is = approximately 33=20 inches (83 centimeters), with the same amount of evaporation taking = place=20 annually. Obviously, any locale can depart greatly from these annual = global=20 averages, with some desert locales such as Death Valley, CA receiving = minuscule=20 amounts of precipitation annually, while some stations on Hawaii receive = over=20 200 inches (5 meters) per year. Usually, more evaporation usually takes = place=20 over the oceans, while more precipitation typically takes place over the = land.=20 The differences between rates over land and ocean are reconciled by a = net runoff=20 of the excess rainwater from the continent by rivers, and a net onshore = flow of=20 moisture in the atmosphere.

AVERAGE RESIDENCE TIME

So how fast does water cycle through a particular reservoir? Let us = trace an=20 "average water molecule" that is free to move between any of the = hydrologic=20 reservoirs. The average residence time that this water molecule would = spend in=20 any particular reservoir depends upon the combined effect of reservoir = size (as=20 indicated in Table 6.1 in the Weather Studies text) and the rates = at=20 which water either replenishes or depletes the reservoir.

Applying mass budgeting techniques to each hydrologic reservoir, = water=20 molecules would cycle through the clouds most rapidly, spending an = average of=20 only 1.3 hours in a cloud as a cloud droplet or ice crystal and between = 9 to 10=20 days as a water vapor molecule in the atmosphere. On the earth's = surface, our=20 water molecule would only remain for approximately 2 weeks, but as = ground water,=20 it would remain in the top soil for approximately 3 months and as much = as 10,000=20 years in deep aquifers. Because of the immense size of the oceans, the = molecule=20 could spend an average 3300 years in the world oceans. The longest = recycling=20 time for a water molecule would be in the polar ice caps and glaciers of = the=20 world, where the average residence time would be on the order of 11,500 = years.=20

These average residence times verify our observations that clouds are = indeed=20 short lived, with new water molecules from the atmosphere rapidly = replenishing=20 the water droplets and ice crystals that are continually removed from = the cloud=20 base by precipitation (as rain or snow) and by evaporation (or = sublimation) into=20 the atmosphere. The water vapor in the atmosphere recycles on the same = time=20 scales as those average time scales that we associate with weather = systems that=20 appear on weather maps. The times needed for the water to pass through = the ocean=20 and the ice caps (some times called the "cryosphere") are on the same = time=20 scales as those usually associated with long term climatic change, such = as=20 experienced since the last large scale glaciation of the Northern = Hemisphere=20 some 11,000 years ago.

Return to the Tuesday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =