PRECIPITATION TRENDS AND THE HYDROLOGIC CYCLE

AOS 101 Weather and Climate

Lisha M. RoubertUniversity of Wisconsin-Madison

Department of Atmospheric & Oceanic Sciences

Precipitation•Precipitation: Any or all of the forms of

water particles (liquid or solid) that fall from clouds and reach the ground. It includes:▫liquid precipitation (drizzle, rain)▫freezing precipitation (freezing drizzle,

freezing rain)▫frozen precipitation (snow pellets, snow

grains, ice crystals, ice pellets, hail)

Precipitation Formation-Summary• Precipitation is product of the

condensation (gas to liquid) of atmospheric water vapor.

• For condensation to take place, the air must be saturated and there must be a surface on which the vapor can condense. In the air above the ground, tiny particles known as cloud condensation nuclei serve as the surfaces on which water vapor condenses.

• For precipitation to form, millions of cloud droplets must coalesce into drops large enough to avoid breaking apart or evaporating during their descent.

Cloud Seeding• Supposed to boost

precipitation by introducing artificial condensation nuclei, such as salt and dry ice into clouds that have the right conditions.

• The artificial nuclei then attract water vapor within the clouds system to become larger droplets.

• Once the raindrops get big and heavy enough, the droplets fall as rain

• Research about the effectiveness is inconclusive to date.

Precipitation and Evaporation Trends

• For precipitation to occur we need water vapor. To have water vapor in the atmosphere we need evaporation.

• Precipitation exceeds evaporation in the Tropics and in the very high latitudes.

• But why is evaporation greater in the subtropics than in the Equator?

A few reasons:•The eastern equatorial Pacific is a region

of cold currents and upwelling.•Deep convective clouds reduce the

amount of solar radiation.•Low wind speeds in the equator reduce

evaporation.•Ocean transport.

Pole-ward Transport by the Hadley Cell

Comparing Vertical Velocity with E-P

•Regions of ascent (negative values=blue) agree with regions where P>E (brown).

•Regions of descent (positive values=red) agree with regions where E>P (green).

Vertical velocity at 500 hPa, July average in units of pascals per second. Ascent (negative values) is concentrated close to the solar equator; descent (positive values) is more diffuse.

Vertical Velocity

Evaporation-Precip

Annual Precipitation for the US

Precipitation for Wisconsin

• From 1950 to 2006, Wisconsin as a whole has become wetter, with an increase in annual precipitation of 3.1 inches.

• This observed increase in annual precipitation has primarily occurred in southern and western Wisconsin, while northern Wisconsin has experienced some drying.Change in Annual Average

Precipitation (inches) from 1950 to 2006 

Extreme Precipitation Events Projections for Wisconsin

• Typically, heavy precipitation events of at least two inches occur roughly 12 times per decade (once every 10 months) in southern Wisconsin and 7 times per decade (once every 17 months) in northern Wisconsin.

• By the mid-21st century, Wisconsin may receive 2-3 more of these extreme events per decade, or roughly a 25% increase in their frequency.Projected Change in the

Frequency of 2" Precipitation Events (days/decade) from 1980 to 2055 

Precipitation in Madison, WI

Today:•You will be making maps of precipitation

for Wisconsin and for other regions to study precipitation trends.

•You will be using the NOAA Earth Sciences Research Laboratory (ESRL) Online mapping tool.

•You will also be studying the Annual Variation of the water balance for different regions.