By
Gp Capt (Dr) B Nandi
1. Encyclopedia Britannica provides a typical thunderstorm image. Worthy to note the Dumbbell like structure or human waist structure. Large latent heat liberation takes place at the cloud base due to the first condensation of convective air. These latent heat liberation makes the base broad. There after latent heat liberation amount reduces and cloud width also reduces. After crossing freezing level latent heat liberation starts increasing due freezing and sublimation. These continues to increase till auto freezing level 0f -400C, where anvil forms. Thus, the waist like formation of a single cell thunderstorm happens.
2. There is a similarity with pushing a human and thunderstorm cloud. If you push at the bottom lower part will move forward so, the human body will lean backward and fall. If you push at the top the upper part will move forward so, the human body will lean forward and fall. But if you push by the waist the human body will move forward. Similarly, it is believed that thunderstorm is moved by the mid-tropospheric wind close to freezing level. Some study claims that movement of thunderstorm can be predicted by vectoral mean of the entire cloud layer, which also turns close to mid tropospheric wind. If we think of a moving thunderstorm, the relative wind for the thunderstorm at mid troposphere will be zero, while at top the stronger wind (usually) will penetrate into cloud and blow the upper part as anvil down the stream, while in the lower level relatively air will enter the cloud in upstream direction and move up helping cloud formation. Therefore, the mid tropospheric air gets a good chance, just to mix with the cloud because of relative wind to thunder storm is very weak at mid-troposphere. Mid tropospheric wind outside the cloud is colder than the cloud air (a warm rising current). On entraining into cloud cold heavier air starts accelerating downward. On mixing cloud evaporates in the entraining dry air. Cooling due to evaporation causes further downward acceleration. This acceleration will be higher if cooling is more. Cooling will be more for drier entraining air. Mid troposphere is often dry in pre-monsoon season that’s how Pre-monsoon thunderstorm produces stronger down draft than monsoon thunderstorm, when mid troposphere is highly moist.
We may recall the formula for Buoyant acceleration when a parcel of air is heated up
a = (Tp -Te)g/Te
Where, a = acceleration of the parcel of air
Tp = Temperature of the parcel of air
Te = Temperature of surrounding air
g = acceleration due to gravity = 9.8 ms-2
Let us assume that temperature difference between entraining air and cloud is 0.3K, which seems pretty reasonable, and that the temperature at the entrainment level is 2730 K.
Then, a = – (0.3)*9.8/273
or, a = – 0.011 ms-2
So, if the initial velocity of the air parcel is nil it will reach Z=5000m (mid troposphere to ground) with velocity given by the formula
Then Since V2= 2aZ
by V= √(2aZ) = 10m s-1
In the above calculation evaporation cooling not considered. Further downward acceleration due to evaporation cooling will produce stronger down draft. Colder and drier the entraining air stronger will be the down draft. This means that in a strong thunderstorm cloud is much warmer than environment and likely to produce stronger down draft. Other method of down draft by drag by the falling precipitation not discussed here.
3. So for strong down draft forecast we look for
(a) Mean RH between 600hPa and 500hpa for identifying the dryness of mid-tropospheric entraining air. drier the air stronger the down draft as evaporative cooling will be higher. This is most importantant parameter for strong down draft.
(b) High CAPE indicates high temperature of the rising air at mid troposphere, making the entraining air comparatively colder.
(c) Strong mid tropospheric wind will mean stronger momentum transfer to down draft.
(d) Moderate divergence in upper troposphere and strong convergence at lower troposphere is likely to produce strong thunderstorm and thus strong down draft. What rises faster also falls faster.
(e) The heavy precipitation can generate high drag of air and strong down draft.
(f) Melting of hail or graupel can cause large cooling of the air and thus strong down draft.