The entrainment zone

E. Linacre and B. Geerts


Experiments on very flat terrain in Illinois have revealed some of the features of the atmosphere on top of the planetary boundary layer (PBL). The PBL is the well-mixed boundary layer near the Earth surface, and depending on wind speed and stability, the mixing can be the result of either surface friction (forced, mechanical turbulence) or thermals (convective turbulence). Therefore the PBL is also called the mixing layer or convective boundary layer (Section 1.8). The top of the PBL, or mixing depth, is sometimes ill-defined, especially in a stable, wind-forced PBL, and at other times very visible, for instance when flying through it in an ascending or descending aircraft. The PBL is well-defined when the overlying free atmosphere experienced subsidence, producing warm and dry conditions just above the PBL top. The structure of this PBL top is complicated in complex topography, hence the flat, fairly homogenous farmland of Illinois is ideal to study the PBL top, known as the entrainment zone.

The entrainment zone is where air aloft becomes incorporated into the PBL, mixing with the fluxes of heat, moisture and chemicals from the ground on the lower face of the boundary layer. Air becomes entrained either by an increase of the PBL depth as thermals become increasingly warm and rise higher, or by air moving downwards into the PBL. Entrainment controls the chemistry of producing ozone from air pollutants.

The experiments on the entrainment zone involved Doppler radar to measure the wind at various elevations above the ground. Three wind profilers were used, about 7 km apart in a triangle, to measure reflectivity and wind profiles at high spatial and temporal resolution. Evident changes of reflectivity (i.e. echo intensity) at the top of a boundary layer were due to a steep gradient of the humidity profile there. Doppler lidar equipment was also used to estimate the wind profile profile. Temperature profiles were obtained from acoustic sounders, as well as balloon radiosondes. There was a ceilometer (to measure cloud base height), and a network of automated surface weather stations which also measured evaporation and sensible-heat transfer at the ground.

On 16th August 1995 it was hot and humid in Illinois, and there was a weak inversion aloft (around 2 km). The convective PBL grew from 500 m at 8am to 1,500 m at 2 pm (the cumulus cloud base was at 1.5 km), and then decline to about 1,000 m by 5 pm. The large variation of radar reflectivity in the entrainment zone indicated alternating thermals and downdrafts of the order of 0.2 m/s every few minutes within the PBL.

On 10th September, when it was dry, there was a subsidence inversion at 1,500 m throughout a cloudless day. The PBL grew from 400 m at 8 am to 1,400 m by 12:30 pm, thereafter rising slightly to 1,600 m by 5 pm.

The 25th July 1996 was quite different, with surface winds of about 7 m/s and only a weak inversion at 2,500 m early in the day. The PBL grew rapidly to cloud base at about 1,500 m by 11.30 am, with evidence of considerable vertical motion within the layer and at its top. The evidence consisted of fluctuations of reflectivity values. It indicated overshoot of the convective thermals into the free air, which triggered the formation of clouds above 2 km high by 1 pm. The clouds in turn affected the net radiation to the ground, and hence the rate of evaporation.

A tentative analysis of data gathered in several daysí experiments indicates that a low height of the boundary layer is associated with a low Bowen ratio at the surface, i.e. relatively high rate of evaporation from wet ground, and little transfer of sensible heat. Boundary layers were deeper in dry weather, as might be expected.



(1) Angevine, W.M., A.W. Grimsdell, L.M. Hartten and A.C. Delany 1998. The Flatland boundary layer experiments. Bull. Amer. Meteor. Soc., 79, 419-31.