In this clip I will explain how class calculates the entrainment flux and the boundary layer height two variables that are used in the mixed layer equation.
In the mixed layer equation the evolution of temperature in time is proportional to the surface flux. And the entrainment flux and inversely proportional to the boundary layer height.
If we take a look at the tab basic within class only two of these variables appear explicitly the surface flux and the initial boundary layer height.
Because the evolution in Boundary Layer height and of the entrainment flux cannot be specified it is important to know how class calculates them
first I will explain how class calculates the boundary layer evolution in time.
From our observations we can see that the boundary layer grows during the day
in addition to the surface flux. In class the boundary layer evolution depends on the total amount of heat that is untrained from the free troposphere.
The amount of heat in trained is a function of the surface flux and the stable stratified loued above the boundary layer that is defined by the jump at the inversion Delta theta
the link with the surface flux can be understood as follows.
Large eddies originating at the surface reach the boundary layer height. And untrain warm air from the free troposphere down into the boundary layer class models the surface contribution as a fraction of the surface flux beta.
In the default case class uses a value of zero point two for calculations.
The jump of the potential temperature prevents the growth of the boundary layer acting as a lid.
A larger jump causes parcels to penetrate less into the free atmosphere and consequently the boundary layer growth is small.
Now let's look at how class calculates the evolution in time of the in train and flux using a sketch with two temperature profiles here the solid line represents the profile that I showed you before. And the dotted line shows you the profile observed a bit later.
The amount of heat that is untrained the entrainment flux is the difference between the two profiles indicated by the red shaded area the entrainment flux depends on the boundary layer growth as you can see here if the boundary layer grows faster the untrained red shaded area increases and therefore the amount of untrained heat as well as the entertainment flux will be larger to.
The entrainment flux also depends on the jump at the inversion if a jump is larger. The red shaded area is larger. And therefore the entrainment 's flux is larger too.
In conclusion the jump of potential temperature has two opposite effects on the dynamics it prevents boundary layer growth because it acts as a lid.
But at the same time it increases the amount of heat and trained in the layer
in order to investigate the effect of the initial temperature jump on boundary layer physics please look at the hands on example for point to exercise number three. Note that the jumper varies in time as well and only the initial value can be specified in class.
The time evolution of the temperature jump depends on the characteristics of the free troposphere such as the lapse rate of temperature gamma feta these variables have to be specified in class please watch the instruction clip tab basic.
In the next knowledge clip the interaction between the boundary layer and the surface will be explained.