Thunderstorm Cumulonimbus Clouds
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How do we get
thunderstorm cumulonimbus clouds?
Meteorologists look at the daily soundings, plot them on a graph like this one and assign Convection Parameters. See them listed below.
T - The surface temperature, usually near 1000 mb pressure. Also identifies the right hand side curve on a skew T log P chart or tephigram.
Td - The surface mixing ratio, absolute humidity. Also identifies the left side curve on the chart.
LCL - Lifting condensation level, the lower left corner of the negative area, where the dry adiabat and the mixing ratio line passing through the surface temperature and dewpoint intersect.
The bottom of the thunderstorm cumulonimbus clouds.
- Level of free convection, the lower corner of the positive area. This is where the surface parcel must be lifted to keep moving upward on its own.
- Convective condensation level, where descending LFC meets the rising LCL given that enough heat is accumulated below for them to actually meet.
- Convective temperature, the surface temperature on the dry adiabat at which LCL and LFC meet to form CCL and free convection.
- unstable region where a lifted parcel would become warmer than the ambient
- stable region where the lifted parcel would be colder than the surrounding air.
- Equilibrium level, the top of the positive area, where the parcel ascent curve intersects with the environmental curve above the LFC. Typical cloud top.
- Energy balance level, not shown but somewhere in the upper negative area, just high enough so that the area below the EBL within the upper negative area is equal to the total positive area, below. This is normally the very top of the storm clouds.
Tc is often unrealistic and can be difficult to reach. So much for surface-based convection.
Time Lapse of thunder storm
Because of layers slipping past each other, convection starting at some upper level is more likely to occur. One of two things happen.
Warm air slips in under much colder air or the colder air flows over top of warm air.
Warm air moving in - differential thermal advection they call it. The warm air has to be moist as well.
Once it contacts the colder air above, up it goes. Then we can use the same graphical techniques as before to determine this air's LCL and cloud base.
It may be within a different type of cloud, and observers may see things they call alto cumulus castellanus (ACC) or something more severe, thunderstorm cumulonimbus clouds (CB).
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