Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 85

Fig. 4.5 Sample plot of a time-height cross section of potential temperature from RASS measure-
ments over Augsburg, Germany, for a height interval between 40 and 540 m and over 72 hours
from May 26 to 28, 2008. Green : between 15 and 20 ◦ C, red : between 25 and 30 ◦ C. Temporal
resolution is 10 min, vertical resolution is 20 m
ABL. The only direct and key variable for the analysis of the presence of a mix-
ing layer is the vertical profile of virtual temperature. Temperature profiles can
directly be measured with a radio-acoustic sounding system (RASS). Figure 4.5
shows an example. There is also the option to derive vertical temperature profiles
from Raman-LIDAR soundings (Cooney 1972 ) and passive radiometer measure-
ments, but especially from passive remote sensing the vertical resolution is usually
not sufficient for boundary-layer research. See Section 4.3.2 below for further
details.
MLH can be determined from the lowest height where the vertical profile of
potential temperature increases with height, indicating stable thermal stratification
of the air. The great advantage of RASS measurements is that the magnitude of sta-
bility (inversion strength) can be assessed quantitatively, which is not possible from
the acoustic and optical sounding devices described before. Figure 4.5 displays a
time-height cross section of potential temperature over three days starting at mid-
night for a height range of 540 m. On the afternoons of the second and the third
day well-mixed boundary layers formed, which can be deduced from the vertically
constant potential temperature. New surface layers form on the evenings of all three
days at about 6 p.m. The depths of these new surface layers increase during the
night to about 200 to 300 m. Above these nocturnal surface layers low-level jets
(see below) form, indirectly visible from the white areas indicating missing data in
Fig. 4.5 . Stronger winds like those in low-level jets blow the sound pulses from the
RASS out of the focus of the electro-magnetic antenna and hence lead to a failure
of the temperature detection.