View Full Version : Thundetstorm Severity

Capt Groper
28th Nov 2018, 06:28
Can any body provide a reference doc or explanation why CBs in Equatorial regions have a higher cloud base and are generally less severe in relation to wind gusts than those experienced in mid latitudes over land.
For example SIN vs SYD.

Le Flaneur
3rd Dec 2018, 01:27
I wouldn't fly through any thunderstorms, but here are my notes for flying around oceanic thunderstorms:

Oceanic thunderstorms (barotropic, meaning the storms formed in a front-less region of uniform temperature) tend to have less mass and significantly less reflectivity than continental thunderstorms (baroclinic, meaning storms formed from distinct air masses of colder and warmer air) of equivalent height.

The reduced mass and lower reflectivity levels found in oceanic thunderstorms occur due to the fact that they have, on average, less water content than do thunderstorms that form over land. As a result, a higher gain setting and/or a lower tilt setting may be required to adequately detect thunderstorm threats at higher cruise altitudes.

It is not unusual for oceanic thunderstorms to be very “skinny” but still have significant vertical development, especially in equatorial regions. This type of thunderstorm has very little moisture content and is extremely difficult for radar to see.

Oceanic storms do not generate hail in large quantities or large sizes because hail can only be generated by strong updrafts (convection), which is produced by large temperature differences in the air mass. Hail forms in vigorous convective clouds that have high water content.

Over land, solar heating produces a rapid change in temperature of the land, which is then imparted into the air near these hot surfaces. These large temperature changes produce convection currents that can drive large quantities of moisture aloft. In order for hail to be produced, these convection must persist for a significant period of time and over large spatial regions.

Over the ocean, solar heating only slightly heats the water and the air above it, resulting in the production of only minor convection currents which are insufficient to sustain hail formation.

Additionally, land surfaces (and the air associated with them) heat unevenly and thereby produce uneven air currents that contain large shears (variation wind speed and direction) in both horizontal and vertical directions and over both large and small scales; whereas, oceanic surfaces (and the air associated with them) are more uniformly heated and thus only have minor shears within them.

These shear conditions are required for hail production (methodology that allows for multiple layers of ice and increases in particle mass/size (and thus an increased hazard to aviation)). So typically, oceanic storms do not have the meteorological properties required for hail production. In completeness, some oceanic storms become large enough (eg, hurricanes) to exhibit strong convection and enough shear for small hail production but these conditions are severe enough that the radar reflectivities in these regions are huge and thus the hail conditions are easily avoided by pilots simply on the basis of radar reflectivity alone.

Storms that develop out over the water establish their meteorological characteristics from these conditions; once they begin to go over land they slowly begin to take on more continental characteristics but that takes many hours or even days for these storms to morph into continental storms. Most storms die out prior to changing their characteristics and then new storms form from the atmospheric instability that initiated the original storm; so for the most part, storms that develop over the ocean retain these characteristics throughout their brief life time and once they go ashore they die out and other storm cells/complexes that may show continental characteristics will take their place.

3rd Dec 2018, 20:26
Can any body provide a reference doc or explanation why CBs in Equatorial regions have a higher cloud base and are generally less severe in relation to wind gusts than those experienced in mid latitudes over land.
For example SIN vs SYD.
I’m not sure that is the case. You’re more likely to get higher relative humidity near the equator, which leads to lower cloud bases. All other things being equal, mid-latitude thunderstorms over land have higher bases (I was under one at 14,000’ a few days ago in South Africa).

As far as wind gusts go, one reason I can think of is that there is generally not much in the way of wind in low latitudes in the low to mid layers as the pressure gradients are fairly slack most of the time. Precipitation can bring winds aloft down to the surface, so that can be additional to the normal outflow, which makes mid-latitude storms potentially more gusty.

If you take SIN and SYD, in SIN you could have 30/26 with CB which is a cloud base of c.1,600’AGL. In SYD, it’s possible but with a westerly wind that dew point would be quite unusual; more likely 30/15 which is 6,000’.

3rd Dec 2018, 21:05
Can any body provide a reference doc or explanation why CBs in Equatorial regions have a higher cloud base and are generally less severe in relation to wind gusts than those experienced in mid latitudes over land.
For example SIN vs SYD.

i cannot quote you the source but a micro-burst researcher once explained it to me as being due to the stronger upper level winds in the mid latitudes adding energy to the cell and enhancing the circulation speeds of updrafts and downdrafts, This was the so called escalator effect. Thus the resultant turbulence would likely be more severe.

This seemed a credible explanation. But as I stressed, I cannot point to any academic papers which explicitly state that this is the case.

Happy to be corrected.

3rd Dec 2018, 23:25
Quite a long time ago, the RAF Publication "Elementary Meteorology for Aircrew" has the interesting assertion (in part) : "------- thunderstorms can be safely penetrated ----".

This has been comprehensively disproved, the hard way; are not the degrees of severity of the turbulence rather moot, when even the "lesser" has the capability to cause severe damage or loss of the aircraft.

I well recall an interesting bar conversation, too many moons ago.

A graduate of the RAF of the era of the above mentioned publication, by now working for a very well known international airline, an F/O on B707-320, inquired of the Fleet Manager Standards of said airline, a very experienced and highly respected pilot, in a very unctuous and ingratiating tone and manner: "Captain XXXXX, when, in your immense experience, would you consider it safe to penetrate a Cb.

The answer had the rest of us falling about laughing.

The answer: "Listen, son, if you never go in one, you'll never have a fxxxxg accident in one!!".

Those were the exact words, I can remember the scene like it was yesterday, and it wasn't.

Tootle pip!!

6th Dec 2018, 17:54
Cloudbase of CBs are directly related to Temp/ Dewpoint spread of the ricing air , anywere on the planet in a uniform airmass.
As for the rest of the stuff with regards to Equatorial vs the rest I have no experience.
As for sitting in a CB looking out, I have, by mistake!!
The Dornier 328 did not come apart , it never will, it will stall and spin! It didnt!
The Boeing 737-800 is also rugged and we got out swiftly!

I am with Leadsleads old Fleet Cpt: Stay the hell away!And thanks Lead for making me look up unctuous and ingratiating , must find a sentence to use that in!

Would like to add that most clouds I see have no clear label on them and dont care much about how they are supposed to behave.
Ie, Turbulence, strong updrafts, hail size etc.