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Cities Influence Distribution of Precipitation

At the mesoscale, cities can influence the distribution of precipitation. This phenomenon is well documented in the five-year study of precipitation called METROMEX (Metropolitan Meteorological Experiment), which encompassed a 56 mile diameter relatively flat area around St. Louis, Missouri. As much as 300% more precipitation was observed downwind of the St. Louis area as up wind of the area during the July and August growing season as shown in the accumulated precipitation figure.

A precipitation measurement at the St Louis International Airport, which is located 10 miles northwest of the city, would record only half the precipitation that would fall in Eastern St. Louis. Most of this difference is ascribed to urban heat radiation and aerosol pools emanating from the city and must be considered in siting precipitation measuring instruments.

This post is an excerpt from the Belfort Instrument Engineering Guide to Siting Precipitation Gauges.

Mountains and Hills Influence Distribution of Precipitation

Mesoscale orographic phenomena is most often described as mountains blocking rain producing weather systems and casting a shadow of dryness, or a “Rain Shadow,” behind the mountain. This phenomenon occurs when warm moist air is pulled by the prevailing winds towards the top of mountains where it condenses and precipitates before crossing over the top, leaving a rain shadow or dry air on the leeward side of the mountain as shown in the example of a rain shadow.

Mountains and rough terrain can impact mesoscale precipitation in many other ways as described in the writings of Robert A. Houze. In his paper Orographic Effects on Precipitating Clouds, (#2011RG000365, Reviews of Geophysics, 50, RG1001/2012) he provides a more comprehensive perspective of orographic effects on precipitation and how complex terrain modifies precipitation.

He shows how the effect of upslope and downslope winds can increase precipitation on either the windward or leeward side of mountains and how rainfall may be maximized on mountain ridges and minimized in valleys. Siting rain gauges to measure average precipitation over large geographic areas must carefully consider orographic factors on the measurement. We highly recommend a careful study of the referenced Houze paper before siting gauges near hilly or mountainous terrain.

Lakes Influence Distribution of Precipitation

NOAA describes lake effect snow as heavy bands of snowfall that may be 20 to 30 miles wide and 100 miles inland on the leeward side of a large lake or other body of water. It can “hover over one location for several hours, dropping several feet of snow; however, 10 to 15 miles on either side of that narrow band skies may be sunny with no snow at all.”

This phenomenon is well documented on the leeward side of the Great Lakes, but similar though less dramatic effects have been observed to the lee of smaller warm bodies of water and should be considered when siting precipitation gauges.

Valleys Influence Distributions of Precipitation

In the absence of mountains, there is significant evidence that river valleys can significantly affect the spatial distribution of precipitation. One example is the Canadian River valley that runs generally west to east across the northern Texas panhandle and lies 800 to 1000 feet below the surrounding plateau. The snowfall on the windward side of the valley has been recorded at half the level of the leeward side of the valley and twice the level of the valley floor (Chris Kimble, NOAA Amarillo, Texas).

In the complex terrain of southern Germany, the interaction of valleys and circulation patterns on small scale precipitation distribution has been analyzed (M. Liu et. al, Hydrol. Earth Syst. Sci Discuss 9, 14163-14204, 2012). “The results show an interaction of valley orientation and the moisture flow direction of the CPs (circulation patterns) at the intermediate-scale, i.e. when the valley is shielded from the moisture flow, the precipitation amount within the valley is comparable to that on the mountain crest; when the valley is open to the moisture flow, the precipitation within the valley is much less than on the mountain.”

More recently, in an excellent AMS paper presented in 2013, we found the following quote:

“The ageostrophic frontogenesis, acting as a mesoscale ascent-focusing mechanism, helps air parcels to rise above the temperature inversion into a conditionally unstable atmosphere, which results in enhanced precipitation focused along the SLRV (St. Lawrence River Valley)” (Milrad, Precipitation Modulation by the St. Lawrence River Valley in Association with Transitioning Tropical Cyclones, AMS 2013). In other words, more rain falls in the river valley than in the surrounding areas.

Locating precipitation gauges in river valleys, especially those surrounded by high plateaus or mountains could result in measurements not representative of the surrounding area.

When siting precipitation gauges, carefully consider the above mesoscale influences on the spatial distribution of precipitation and the potential for misrepresenting precipitation falling in the wider geographic area to be represented by the measurement.