Mixing Height and Smoke

Mixing height (also called mixing depth) is the height above ground level through which relatively vigorous vertical mixing occurs. Low mixing heights mean that the air is generally stagnant with very little vertical motion; pollutants usually are trapped near the ground surface. High mixing heights allow vertical mixing within a deep layer of the atmosphere and good dispersion of pollutants. As such, mixing heights sometimes are used to estimate how far smoke will rise. The actual rise of a smoke plume, however, considers complex interactions between atmospheric stability, wind shear, heat release rate of the fire, initial plume size, density differences between the plume and ambient air, and radiant heat loss. Therefore, an estimate of mixing height provides only an initial estimate of plume height.

Mixing heights usually are lowest late at night or early morning and highest during mid to late afternoon. This daily pattern often causes smoke to be concentrated in basins and valleys during the morning and dispersed aloft in the afternoon. Average morning mixing heights range from 300 m (~980 ft) to over 900 m (~2,900 ft) above ground level (Holzworth 1972). The highest morning mixing heights occur in coastal areas that are influenced by moist marine air and cloudiness that inhibit radiation cooling at night. Average afternoon mixing heights are typically higher than morning heights and vary from less than 600 m (~2,000 ft) to over 1400 m (~4,600 ft) above ground level. The lowest afternoon mixing heights occur during winter and along the coasts. Mixing heights vary considerably between locations and from day to day. Ferguson and others (2001) generated detailed maps and statistics of mixing heights in the United States.

Smoke plumes during the flaming stage of fires often can penetrate through weak stable layers or the top of mixed layers. Once the plume dynamics are lost, however, the atmosphere retains control of how much mixing occurs. Low-level smoke impacts increase once a convective column collapses.

The depth of the mixed layer depends on complex interactions between the ground surface and the atmosphere in a region called the planetary boundary layer (PBL). As such, it is difficult to measure exactly and there are many ways in which it is calculated. At times, it is possible to estimate the mixing height by noting the tops of cumulus clouds or the presence of an upper-level inversion, which may appear as a deck of strata-form clouds.

Typically, National Weather Service (NWS) smoke management forecast products will estimate the mixing height by the so-called parcel method. This method considers turbulence related only to buoyancy. When a parcel is lifted adiabatically from the surface, the point at which it intersects the ambient temperature profile, or where it becomes cooler than its surroundings, is the mixing height. Usually the maximum daily temperature is used as the parcel’s starting temperature and its adiabatic lapse rate is compared with the afternoon (0000 UTC) sounding profile. Conversely, the minimum daily temperature is used to compare with the morning (1200 UTC) raob for calculating morning mixing heights. If an elevated inversion (see next section) occurs before this height is reached, the height of the inversion base would determine the mixing height. If a surface inversion exists, then its top marks the mixing height. For example, the mixing height in figure 7.3 is at the top of the surface-based inversion at about 750 mb (approximately 2,400 meters or 7,800 feet above ground level).

Instead of approximating a mixing depth, physical calculations of the PBL are possible through numerical meteorological models. These calculations are more precise than the parcel method because they consider turbulence generated by wind shear as well as buoyancy. Each prognostic model, however, may calculate the PBL slightly differently as some functions are approximated while others are explicitly derived to enhance computational efficiency and the vertical resolution, which varies between models, affect PBL calculations.