Very Large Potential Temperature Lapse Rates and Low Turbulence Levels during Spring Thaw on the Prairies

Abstract

Observations show that two phenomena that are generally considered mutually exclusive occur both night and day during spring thaw over flat cultivated prairies: extremely large potential temperature lapse rates and low levels of atmospheric turbulence. Magnitudes of the large lapse rates routinely exceeded 20°C (100 m)?1, whereas levels of turbulence during night and day were at values usually associated with moderately stable and near neutral conditions, respectively. Coexistence of these phenomena occurs during the time of year when a significantly large portion of the prairie is characterized by water and/or ice while the remainder comprises bare soil surfaces. Whereas freezing water is a sensible heat source during the night and melting ice a heat sink during the day, the bare soil acts in a reverse manner being a heat sink during the night and a source during the day. Air moving over the prairie surface is, therefore, subject to, both day and night, an alternating succession of warm and cool surfaces. Observed boundary layer depths characterized by the very large lapse rates were about 10 m in depth. Calculations based on the first law of thermodynamics show that these depths are consistent with sensible heat fluxes from the prairie surface of about 2 W m?2. Assessments of these small heat fluxes through applications of the energy balance equation show that they are in agreement with the known behavior of melting ice and freezing water during spring thaw. Extremely large potential temperature lapse rates and low levels of turbulence seem, therefore, to occur because small heat fluxes are being introduced into the air on an interruptible basis. Very large lapse rates persist because the lack of a sustained heat flux does not allow for development of the vigorous turbulence needed for their eradication.