Measuring
Precipitation in the Newark Area
Introduction:
In association with field exercise #4, we have been measuring precipitation in the Newark area for nearly a month. Clearly, the measurement of precipitation is one of the most important environmental observations that we make. The amount of precipitation that occurs over a given region effects the health and distribution of vegetation and animal life, controls the availability of soil moisture, is the major determinate of stream discharge values, and greatly influences ground water supplies. Certainly, we could go on with many pages of text concerning the importance of precipitation to the earth’s environment. Therefore, an accurate measurement of precipitation is crucial to a thorough understanding of the environment that surrounds us.
Measuring Precipitation:
Although it may seem that the measurement of precipitation is very simple, this is certainly not the case. First, there are many types of precipitation that may fall including rain, freezing rain, sleet, graupel, hail, and snow. In a perfect world we would have a different instrument available to measure each of these precipitation types. Unfortunately, this is not the case. At most observation sites, only a single instrument is available for the measurement of precipitation. We will take a very brief look at a few of the instruments available to us.
There are many, many different rain gauge designs found in use across the world today. However, they are all based on a similar principal, measuring the amount of rain that falls. Simple “wedge” rain gauges (like we are using to measure precipitation), tipping bucket rain gauges, heating tipping bucket rain gauges and a variety of other designs are commonly in use. Large networks of rain gauges in place across the United States give a generally accurate estimate of the spatial pattern and amount of precipitation that falls. Unfortunately, precipitation amounts can be very localized, and therefore large precipitation amounts may (and often do) go unmeasured. Moreover, there are large areas of the country that have very few rain gauges in place, limiting the spatial resolution of precipitation observations. It is often said that if you totaled the area of all the rain gauges in the United States, that area would cover only the size of a single baseball field.
Conventional
weather radar works in a very simple manner.
Short pulses of electromagnetic radiation are transmitted from the radar
site. A small portion of the energy
that leaves the transmitter is scattered by precipitation and returned to a
receiving device. The strength of the
returning signal gives information on the intensity of the precipitation, while
the time it takes for the signal to return to the radar gives the distance to
the precipitation. Doppler Radar, which
we will learn more about during our time at the NWS office, not only can
identify the intensity and location of precipitation, it can also give
information about the circulation present in a storm. Doppler radar is therefore very important in the identification
of severe thunderstorms and tornadoes.
The Field Exercise:
This week we will be visiting the National Weather Service Forecast Office in Mount Holly, New Jersey. While there, we will be shown the methods used by the NWS to measure precipitation across our area including their network of rain gauges and their WSR-88D Doppler radar.
You also need to prepare your precipitation data for completion of field exercise #4 (which is field exercise #7). On Friday (10/13/2000) please bring with you to class your precipitation data, a description of your precipitation monitoring site, and its exact location. You will later be responsible for submitting an exercise report that includes maps of each precipitation event and a map of the total precipitation during the time that we have taken measurements. Your report will be discussed further in class.