Tropical stormsWith the cyclone low pressure systems, we have already covered one possible storm system that occurs on Earth. However, there is a different kind of system. While cyclones are driven by the jet stream dynamics in the upper atmosphere, tropical (hot core) storms are driven by the massive accumulation of convective energy on the ground that drives a significant updraft in the lower atmosphere. Such an accumulation requires sea water to warm up - the thin layer of solid ground that gets diurnally heated has nowhere near the thermal energy storage capacity to drive a storm.So far the simulation has tracked convective energy, but assumed that all of it ends up in the development of small-scale convective clouds during a day/nigh cycle, so it never accumulates in sufficient quantities to power a storm. When the storm simulation is switched on, part of the convective energy is allowed to accumulate and a storm forms as soon as a particular threshold is crossed. As long as convective energy is available, the storm then spins up and grows both in size and windspeed, drifting to a different location partially governed by Coriolis forces and partially by the energy gradients. If there is no further energy available (such as after landfall), the storm spins down and gradually dissolves.
Activating storm simulationEvery storm adds clouds and rainfall into the simulation, however generally the cloud layer is a bit reduced with respect to using all convective energy for small-scale clouds, so if storms are switched on in the weather simulation one needs to increase mid and high level clouds slightly to get the same average cloud coverage and temperature as before.The config file example29.cfg uses the standard Earth model to create storms. The relevant section is shown here:
The keyword storms switches on the simulation, the keyword grad_motion_factor determines how much the motion of storms is determined by energy gradients (aka whether storms actively move towards regions of high convective energy or merely drift across them by accident - with a high gradient motion factor storms will generally avoid land and hence live longer). When the simulation terminates, a breakdown of the storms that have been generated is written. This presents the number of storms as well as the average lifetime, followed by the percentage of storms that fall into a particular category. The category is the Saffir-Simpson Hurricane scale up to number 5, then extended to include super-storms possible on other planets. Category 0 means that the storm can be classified as 'Tropical Storm' but has not yet reached Hurricane force.
Plotting storm tracksIf we want to understand how and where the storms move, we need a new kind of plot, a scatter plot which shows the groundtracks of the storms across the surface of Earth.
As usual, the plot requests to declare a file to write the results into, and it accepts a delay to avoid writing tracks for the phase in which the simulation still finds its equilibrium. The interval keywords specifies the number of seconds after which a new dot is written for the position of all currently active storms and the type keyword calls for a particular scatter plot (here the storm tracks). Using the keywords filter_low and filter_high, it is also possible to write only track segments for a particular category only and so study where a storm is strongest. The result might look like this, more or less resembling how actual tropical storms move across Earth:
While storms do contribute to the cloudiness in a surface element, their actual effect is difficult to see as they typically only occupy a fraction of an element and other weather dynamics also creates clouds.
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