Mobile Mesonet

The mobile mesonet is a set of vehicle-borne weather sensors. The system was developed for the VORTEX field experiment, primarily by Erik Rasmussen at the National Severe Storms Laboratory and Jerry Straka at the University of Oklahoma School of Meteorology. Significant development and testing work was carried out by Sherman Fredrickson (NSSL and Oklahoma Climate Survey) with the collaboration of the Climate Survey, and Paul Griffin and Dennis Nealson of NSSL.

Briefly, the mobile mesonet records the following data:

• Time. The correct time is obtained from the GPS satellites.
• Position. The position of the vehicle is determined from GPS. This is accurate to roughly 100 m, which is about 1/1000th of a degree. Thus, we record the position to the nearest 1/1000th degree. Sampled every two seconds; archive data gives the position every six seconds.
• " Fast" Temperature. This is measured by a very fast-response thermistor in an aspirated radiation/water shield. This is the appropriate temperature to use to represent the air temperature. Sampled and recorded every six seconds.
• " Slow" Temperature. This is measured by a thermistor located inside the aspirated radiation/water shield, that is coupled with the capacitive sorption rh sensor inside a special shield. It is a fast-response sensor, but the extra shielding slows its response. This is the appropriate temperature to combine with relative humidity for computation of derived quantities, such as vapor mixing ratio or dewpoint. Sampled and recorded every six seconds.
• Relative Humidity. This is measured by a capacitive sorption rh sensor within a special shield, all contained inside the aspirated radiation/water shield. Sampled and recorded every six seconds.
• Pressure. This is measured by a fast-response barometer attached to a pressure port using a rubber hose. The pressure port is approximately 2 m above the ground, out of the vehicle slipstream. The port removes the dynamic pressure effects of vehicle-relative wind gusts. Note that on the vans (vehicles N1, N2, N4, and FC) the pressure port was poorly positioned and pressure data were sometimes set to missing in the archived data. Obviously, this is station pressure and is the appropriate number to use in calculations of potential temperature and other quantities that rely on the actual pressure.
• Wind. Mobile mesonet winds are measured at 3 m above the ground on all vehicles. Vehicle motion is known via GPS satellite, and heading is known when the vehicle is stationary via a calibrated flux gate compass. Thus, we are able to vectorially compute the actual ground-relative wind direction and speed regardless of vehicle motion.

At the end of the first phase of VORTEX in June 1994, the project leaders discussed the successes and failures of the experiment and whether they were collecting the kinds of data required for understanding important events in severe storms. It was obvious that the mobile mesonet system, a group of cars instrumented with high-quality weather observing equipment, could give new insights into the pressure, temperature, and humidity fields near the ground. These fields give rise to the forces that cause air to accelerate and rotate, and knowledge of them would add an important new element to the understanding of severe storms.