A comparison of University of South Alabama Mesonet precipitation and solar radiation sensors

The University of South Alabama (USA) built its first mesonet station in January 2005, with the primary focus on hurricane and tropical storm landfall monitoring. Hence, most stations were installed in coastal counties and the line of counties north of the coastal counties. As of May 2009, 14 stations were operational with 12 more to be installed by October 2009. The mesonet stretches about 325 km in an east-west direction, across three states. The north-south dimension ranges from about 100 km at the western end to about 30 km in southeast Alabama north of the Florida Panhandle. The spacing between stations ranges from 5.4 to 55.6 km with an average of about 30 km. World Meteorological Organization (WMO) instrument siting standards are followed as much as possible. In some cases, minor compromises were necessary regarding nearby obstructions versus finding a safe site with a no-charge lease of land and free access to the Internet. Distances and heights of nearby obstructions are documented in station meta-data, and are available online at http://chiliweb.southalabama.edu/. There is a significant amount of sensor redundancy. This redundancy is intentional given the frequency of severe weather in the area including thunderstorms all year round and tropical storms and hurricanes during the Atlantic basin hurricane season. Data are collected at 1-minute intervals using a Campbell Scientific CR 3000 datalogger. A Campbell Scientific AM16/32 multiplexer is used to accommodate the large suite of sensors. All stations are powered by solar energy using a BP Solar SX30U solar panel which charges a 12 V battery. A fully charged battery will operate the station for approximately six days before discharging to the critical dysfunctional level of 10 volts.

Rainfall is measured by two tipping bucket rain gauges, a Hydrological Services TB3 tipping bucket rain gauge (TB3) and a Texas Electronics TE525 rain gauge (TE), placed in the northwest and northeast corners of the enclosed fence. Both gauges measure 0.254 mm of rainfall per bucket tip. The USA Mesonet dataloggers report the number of tips, as well as accumulated rainfall since 00:00 CST every minute for both buckets. The rainfall accumulation is reset to zero at 23:59 CST. The TB3 functions as the primary rain gauge as it includes a siphon tube (to deliver a preset volume of collected water to each bucket reducing under-catchment during heavy rainfall, e.g. Humphrey et al. 1997), a built-in level for more precise positioning, dual reed switches, and a sturdy tipping bucket made of synthetic ceramic coated brass. Dual rain gauges are used at USA Mesonet stations in case of damage due to severe weather or outage as a result of accumulated debris or nesting insects. The redundancy also allows for internal consistency checks at each station for quality control (QC) purposes. A Li-Cor LI-200S pyranometer measures incoming solar radiation of wavelengths between 400 and 1100 nm, also known as total solar radiation. A Li-Cor LI-190S quantum sensor measures the potosynthetically active portion of the solar radiation spectrum (400 to 700 nm). Both radiation sensors are mounted on the south side of the north-south oriented 10 m cross arm in order to avoid fence, tower, and guy wire shadows.

The four mesonet stations that operated during 2007 (Agricola, Bay Minette, Mt. Vernon, and Pascagoula) are used in this analysis. The large degree of instrument redundancy at USA Mesonet stations provides a unique opportunity to perform internal consistency checks. Solar radiation is the primary energy source for many meteorological, agricultural, and hydrological processes. Total solar radiation varies considerably with atmospheric conditions, time of day, and season. The statistical analyses show good agreement between total solar radiation (wavelengths of 400 to 1100 nm) and the photosynthetically active portion (400 to 700 nm) of the total solar radiation spectrum. Monthly variations is total radiation are explored and related to monthly variations in rainfall. It would be expected that the maximum (minimum) solar radiation values occur around the summer (winter) solstice. However, the maximum mean is observed in May, which could be explained by more cloudy days occurring in June than May. Afternoon thunderstorms on the north-central Gulf of Mexico coast, occur more frequently in June than in May. By investigating rainfall totals per month this assumption was confirmed. However, a January minimum in mean solar radiation instead of December at one site was contradicted by less rainfall (and this cloudy days) in January than December at this site. It was concluded that a larger number of non-rain producing but overcast days occurred in January at this site. This will be investigated further by looking at daily total radiation values for those months, as well as satellite and radar data. Finally, daily and seasonal variations are explored and show excellent agreement with physical principles and theoretically derived values.