Mixing Condensation Level

Mixing Condensation Level (MCL) is the lowest height at which saturation may occur if the near surface layer is or will be mixed completely by wind action. You may relate this to a situation in which you have a radiation inversion keeping the boundary layer wind away from the surface. Mixing is occurring at the top of the inversion. When the radiation inversion breaks, the boundary winds will reach the surface, and mixing will take place through the laye  formerly protected from the turbulent winds by the inversion. Before we proceed, lets look at what occurs in the atmosphere when mixing occurs, and how this is different from convection. Mixing occurs in a layer when vertical wind speed shear or vertical direction wind shear occurs. As was discussed in the previous lesson, shear causes turbulence due to updrafts and downdrafts. These updrafts and downdrafts produce the mixing action.

When mixing occurs, the air in the downdrafts warms dry adiabatically; in an updraft it cools dry adiabatically until saturation is reached, then it cools saturation (moist) adiabatically. The air in the layer tends to become more unstable due to warming in the lower levels if saturation is reached within the layer. The moisture present in a layer tends to become evenly distributed through the mixed layer. The mean mixing ratio line through the layer before mixing closely approximates the dew point curve through a layer after mixing. In the horizontal, we can expect to see relatively equal relative humidity percentages. In the convective-cloud formation process, we find columns of rising air that become saturated and form clouds. Relative humidity in a horizontal

layer in this case would show large changes between cloud columns and the ambient air. Since the moisture in a mixed layer is evenly distributed throughout the layer, we do not expect to see scattered clouds forming. Instead, we expect saturation to be reached at the same level throughout the mixed layer. The turbulent process will cause small, relatively evenly spaced areas where downdrafts are occurring where the humidity is slightly lower. Because of this, clouds formed in mixing layers are strato-cumuloform. You will find that mixing layers, when approach-ing saturation, progress from clear skies to thin broken to overcast layers to dense overcast layers. Generally, no clouds will be evident in a mixed layer until the dew point or frost point depression decreases to less than 2.5C. Typically, the depression is less than 1.5C in observed cloud layers caused by mixing. This contradicts the general thumb rules for cloud coverage presented later in this lesson on cloud layer analysis.

Now that we understand what happens in a layer when mixing occurs, we can see that computation of a MCL on an analysis is useless. Mixing will not begin to occur in your air mass unless certain changes occur. The changes that need to occur for mixing to begin must be forecast. If the mixing process is expected because of increasing wind speeds with a frontal passage, the forecaster must first adjust the lower dew point and temperature curves to reflect the changes expected with frontal passage. If mixing is expected to occur after a radiation inversion breaks, the forecaster must first adjust the temperature curve to approximate the low level temperature at that time. In both cases, the forecaster will need to forecast changes in the vertical wind profile to determine the top of the

mixing level. After the forecaster has made these changes on the Skew T, you can compute MCL as follows:

1. Draw a horizontal line at the level of the top of the mixed layer.

2. Bisect the dew point curve with a mixing ratio line by the equal area method.

3. Bisect the temperature curve with a dry adiabat by the equal area method.

4. MCL is the level at which the mean mixing ratio and the mean dry adiabat intersect. See figure 6-2-9 for an example of a computed MCL.

We have just looked at several methods used to determine where cloud bases will form. In reality, more than one factor can combine to start cloud formation. You may have a frontal surface moving through your area that will provide mechanical lift, but daily heating may occur to add convective lift to the process. You may have increasing winds to add mixing to the process. The forecaster must take these factors into consideration when applying the guides you have calculated to his forecast.

Just how high will these clouds develop after they form? Well, lets find out.

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