Radio acoustic sounding system (RASS) applications and limitations

Radio acoustic sounding systems (RASS) measure (virtual) atmospheric temperature, usually looking vertically from the ground. Briefly, RASS use a radar to measure the speed of propagation of an acoustic disturbance and derive there from the virtual temperature. Virtual temperature is the temperature of dry air at the same density as the moist ambient air, and is the appropriate measure for air density. It is up to a few degrees larger than (kinetic) temperature in very humid (tropical) conditions. Historically, temperature in the atmosphere has been measured by radiosondes, consisting of in-situ sensors carried on by a small balloon. Remote sensing instruments such as RASS have substantial advantages over radiosondes. RASS measurements can be made continuously at high time resolution (minutes) if desired, whereas launching radiosondes even hourly is very expensive and labor-intensive. Furthermore, RASS measurements are inherently averaged over a spatial domain of order 10⁶ m³, giving a more reliable estimate of the temperature than the essentially 1-dimensional radiosonde measurement. In addition to their obvious uses, RASS, like any instrument, have limitations. This presentation concentrates on RASS applications in the lower troposphere for basic planetary boundary layer research and air quality applications. It also includes a discussion of accuracy and precision and the ongoing quest for understanding of and corrections to the bias problem