Temperatures of Air Parcels Lifted from the Surface: Background, Application and Nomograms

Abstract

When the temperature of a parcel lifted adiabatically from the surface to a given pressure level (SLT) is subtracted from the ambient temperature at that level, the result is the surface index (SLI) for that level. This quantity, for the 500-mb pressure level, has been found useful in the very-short-range prediction of deep and often severe convection. We examine the thermodynamics of the lifting process, finding that the SLTs are sensitive mainly to surface dewpoint for very warm humid air and to surface temperature for cold air. Nomograms are presented for determination of SLT's for 850, 700 and 500 mb, given surface temperature and dewpoint and a nominal surface pressure of 950 mb. An approximate correction is given for departures from this pressure. These are intended for use with the routine aviation surface observations. An additional nomogram is provided for use over the sea. An example of application is shown, based on hourly surface observations and on ambient temperatures interpolated between standard rawinsonde analysis times. Severe convection is observed where large parcel buoyancy is indicated by SLI at 500 mb, with no negative buoyancy at 700 or 850 mb. Apparently unreliable measurements of dewpoint in warm humid air constitute a significant analysis problem, and reliable large-scale predictions of ambient temperature are a requirement for optimum real-time use.