Comparison of Thermodynamic Variables Measured from a Mobile Platform with U-Tube and Gill Shields

Measurements have been made with Rotronics Hygroclip sensors concurrently mounted on a moving vehicle within the NSSL U-Tube and within a 10-plate Gill shield under a wide variety of environmental conditions. The U-Tube was originally designed primarily for use in severe storm environments under which anomalously cool and humid measurements are likely to occur if the sensor is not well protected from wetting by raindrops, and in which rugged protection from large hail and debris may be required. Sensors commonly used within the U-Tube by NSSL and collaborating organizations have typically included the HMP45C and fast-response thermistors. In non-severe and rain-free conditions both U-Tube and Gill shields have been used by various investigators, although with no systematic comparison. During 2016-2017, data have been collected from a large number of cases geographically distributed across central and western regions of the United States. The performance of the Hygroclip sensor in the different shields is compared under conditions of clear sky, partly cloudy, overcast, light rain, moderate/heavy rain, and light snow. While expected wetbulbing effects are noted during most heavy rain events, many light rain cases show no discernible cool bias with the Gill shield. The most noteworthy difference under rain-free conditions is a lag in temperature measurements with the U-Tube, which is particularly recognizable under conditions conducive to small-scale variability such as a stable nocturnal boundary layer in complex terrain. This lag is presumed to be a consequence of thermal inertia from the much larger U-Tube in spite of its fan aspiration. Much less lag is noted in humidity measurements. It is suggested that the ideal configuration for an “all-weather” mobile observing system should include both the U-Tube and Gill shields, and that there may be little additional value to use of fast-response temperature sensors within the U-Tube or similar structures when mobile at highway speeds.