Temperature and Radiation Sensor Performance During Lightning Events

The South Alabama Mesonet (http://chiliweb.southalabama.edu) is a network of 26 research-grade fully automated weather stations located primarily in Southern Alabama. In reviewing temperature and quantum radiation data from 2013 through 2015 from the Mesonet station at Fairhope, Alabama, for quality control purposes, it was noticed that during or before/after heavy rainfall events the temperature and/or radiation values showed distinct spikes. On 8 January 2013 all instruments on the Fairhope tower were replaced with new instruments. 81 cases in the year of 2014 were identified with temperature and/or radiation spikes (including at night) and over two thirds of these cases showed downward spikes in temperature. In some cases only temperature and no radiation spikes were observed or vice versa. We suspect lightning strikes in the vicinity of the station were the cause of these spikes, given that the sensor manuals reference unstable temperature measurements can occur during electromagnetic interference. Level II WSR-88D reflectivity data was downloaded for all 81 cases. For 6 cases radar data was not available. For the remaining 75 cases, storm cells with high reflectivity values were identified in the area of the station at the time of the temperature/radiation spikes. The distance between the centers of the storm cells and the station were calculated along with the compass direction of the cell relative to the station and the cell’s maximum reflectivity value at the time closest to the spike time. The goal will be to investigate the relationship between spiking and storm cell distance/intensity.

Lightning data was requested and received from Vaisala including lightning strike locations and times within a 100 km radius from the Fairhope station over the 3 year period from 2013 to 2015. This data will be compared with the times of the 81 spike cases, and the percentage of cases where lightning occurred very close to the station will be recorded. The results from the lightning data will be compared to the results from the radar analysis.

The years 2013 and 2015 will be investigated in a similar way, i.e. using both radar reflectivity and lightning data. This will provide information regarding the effects of lightning as compared to sensor aging. The difference in spiking between the temperature and radiation sensors will be investigated as well. For example, in some cases, just the temperature spiked and not the radiation or vice versa. We would like to understand why.

To our knowledge, no prior research has been done on this specific topic. Should it be found that lightning is indeed causing these issues with the temperature and radiation sensors, it will confirm that these spike issues are transient and not pointing to sensor failure requiring Mesonet technician dispatch. Hence, saving time and financial resources. However, one spike in our time frame caused a temperature sensor to be faulty for the rest of the year, giving values wildly different from the other sensors. This may have been a direct hit by lightning or a strike in very close proximity to the station. The Vaisala data will help determine this. Knowing the source of the spikes will allow them to be removed during the quality control process without the entire record becoming suspicious and being discarded.