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DID YOU KNOW?

Welcome to Did You Know, a daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks.


Precipitation rate from TC Laura in the USA - windy.com

Tonight's question is topical with a strong Tropical Cyclone whacking the USA coastline, but it also quite complex so you'll need to use that noggin.

"Nitso what is the maximum intensity of rainfall a Tropical Cyclone can produce?

The answer - There is no set number but for a conditionally unstable atmosphere with some (500J/kg+) Convective Available Potential Energy inside the vortex of a TC, the maximum precipitation rate per hour of the cyclonic circulation and associated convective processes within it is roughly equal to the Precipitable Water of the atmosphere + a small percentage. (NOAA WPC 2017) If that went whoosh straight over your head, then read on...


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Precipitable water is the depth of water in a column of air from the surface to the top of the atmosphere. Whenever a weather balloon is launched it measures how much water it encountered on its trip up through the atmosphere. We can also use microwave imagery to assess how much moisture air contains. Basically a column of air can go from holding no water (0mm) in a desert or the polar region all the way up to about 80mm in a fully saturated deeply tropical airmass. Tropical Cyclones can only exist in Precipitable Water values that are above about 40mm, once the value drops below that, there is not enough moisture in the air to sustain their constant need to maintain their deep convection and they need to rely on moisture coming in (advecting) from other areas.


Here is an image of the precipitable water this afternoon as Laura made landfall in the USA. You can see about 60-75mm of precipitable water in the atmosphere and using the established NOAA guidelines the theoretical maximum precip rate from the cyclone over Louisiana is right up around 70-90mm/hr and with no added source of orographic (explained later on) or frontal type lift, then we should not be witnessing any locations exceeding that threshold.

MIMIC-TPW - Nth America August 27 2020

During a Tropical Cyclone, I have seen precipitable water values in Darwin peak at 72mm, Townsville 66mm, and Mackay 63mm. So theoretically if the atmosphere can lift an air parcel in that fully saturated UNSTABLE environment it can dump the weight of that water on us. In the real world, Tropical Cyclone rain rates over the course of its time over a location are much much lower than the theoretical maximums. Here is a graph of the average rainfall rates inside Tropical Cyclones with both intensity and distance from the centre plotted. You will notice that stronger cyclones produce heavier rain rates and the closer you are to the centre the heavier the rain rates are as well. In a different "Did You Know" we will explore the nature/type of rainfall that comes out of a Tropical Cyclone.




It is important to understand that the theoretical maximums would apply to places like Karratha, Port Hedland, Darwin, Gove, Normanton, Weipa etc. These are locations without any added sources of orographic lifting. For places in North-East Queensland like Innisfail, Tully, Mackay, Cairns etc these maximums can be exceeded by a great deal because the mountain ranges provide a secondary source of violent uplift as extreme winds race up the mountain slopes and create an incredible amount of extra forcing. The theoretical maximums therefore only apply to air being lifted by the cyclonic vortex itself.

One thing is for sure, a Tropical Cyclone itself can not provide you with the intensity of rainfall that a mid afternoon outback thunderstorm on a hot, humid 40 degree day can. Strong storms can double or even triple the maximum rain rates that Tropical Cyclones can provide you with. However those rain rates can only last a few minutes, a cyclone's rainfall can last days and its theoretical maximum rainfall can theoretically last many hours.



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