Every station is being equipped with a standard set of core sensors attached to a 10-foot (3-meter) mast and includes the capability to add supplementary sensors in the future. “Off-the-shelf” commercially-available sensors are being selected based on performance, durability, and cost. The core parameters being measured are:
In general an aspirated air temperature sensor is superior to one mounted in a passive radiation shield. Errors in a passive shield can be as large as several degrees C in calm wind and strong solar radiation/sunlight conditions.
The measurement of precipitation is complicated by the fact that it can occur in liquid or solid form. There are three established methods of measuring liquid precipitation (i.e., rain): tipping-bucket; weighing-bucket; and vibrating-wire. To measure solid precipitation (i.e., snow), the instruments have to be heated.
Solar radiation (i.e., sunlight) is one of two variables needed to develop the transfer function between air temperature measured at a CRN station and temperature measured using conventional (alternate backup) instrumentation at nearby or co-located historical stations.
In addition, solar radiation is an important variable in agricultural and hydrometeorological models. The U.S. has a poor record of maintaining a high quality solar radiation data base. A new attempt is the SURFRAD Network which has established a complete set of measured state variables including solar radiation in all of the climate zones in the US. The CRN can contribute to this effort by improving the spatial resolution. The structure of the time series of solar radiation also can be used to assess the type of clouds during daytime.
Wind speed at thermometer height is the other variable involved in establishing the transfer function between temperature measured at a CRN station and temperature measured using conventional instrumentation.
Hourly observations of these variables will be transmitted in near-real time and summary-of-day statistics will be computed operationally at the National Climatic Data Center (NCDC). The GOES satellite system operated by NOAA/NESDIS is being tested as the method of communication for the hourly data.
The problem of under-catch in precipitation gauges (Groisman and Legates 1994), resulting from wind-induced turbulence at the gauge orifice and wetting losses on the internal walls of the gauge, can seriously affect the utility of precipitation data for climate change studies. This under-catch is even more significant during the winter due to the deleterious effect of the wind on snowfall. Additionally, tipping-bucket gauges under-measure precipitation during heavy-rain events and snow events (McKee et al. 1995 and 1996). This is an issue to which careful attention is being paid during the evaluation phase for selection of the type(s) of precipitation gauge(s) for the CRN. There are a number of ways to reduce the “wind effect”, including the use of one or more wind shields. According to WMO (World Meteorological Organization) practice, the gauge should be placed on level ground and surrounding objects should not be closer than a distance equal to four times the height (WMO 1996). Under-catch of rainfall due to wind speed can be reduced to less than 2% when the gauge has an aerodynamic design and when the gauge height is low, around 0.5 meter (1.64 feet).
http://www.ncdc.noaa.gov/crn/crnmeasurementtypes.html
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