Observations and numerical study of the morning transition: A case study from SOS99

The morning transition from stable to convective conditions is important to air quality studies because of the different concentrations of pollutants that can occur in the nocturnal boundary layer and the overlying residual layer. Yet, this transition is one of the more difficult features to simulate properly in the meteorological portion of air quality models because of insufficient vertical resolution and boundary-layer parameterization physics. In some cases, the models simply do not maintain and strengthen the nocturnal inversion that forms on calm, clear nights as a result of radiative flux divergence. Consequently, vertical mixing occurs too rapidly as the surface heat flux increases in response to solar heating.

During the 1999 Southern Oxidants Study (SOS99), scientists from the NOAA Environmental Technology Laboratory (ETL), NOAA Aeronomy Laboratory, and NOAA Air Resources Laboratory deployed a network of integrated boundary-layer observing systems in and around Nashville, Tennessee. The instruments deployed near Dickson, Tennessee included a boundary-layer wind profiler with RASS, a monostatic sodar, a laser ceilometer, and a 10-m meteorological tower. In addition, near-surface chemical sampling was conducted by the Tennessee Valley Authority and the National Center for Atmospheric Research. The meteorological tower and ground surrounding the tower were instrumented with both fast and slow sensors to acquire measurements necessary to do complete surface energy budget calculations. During two three-day periods, members of NOAA/ETL also flew a tethersonde system equipped with five sondes to measure wind, temperature, and humidity profiles in the lowest 300 m.

In this paper we use the tethersonde and supporting ground observations to study the behavior of the Blackadar boundary-layer parameterization scheme embedded in the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) during the morning transition. The individual terms in the ground temperature tendency equation are compared with observations to determine whether one or more terms is causing the ground temperature in the model to warm too rapidly. The low level stability profile in the model is compared with the stability profile measured with the tethersonde to see if the time required to break the nocturnal inversion is accurate. The growth of the convective boundary layer in the model is compared to observations of boundary-layer growth from the sodar and wind profiler. Finally, the effect of the morning transition on surface chemistry measurements during the case study is discussed.