I've started this thread to open a discussion about what may be an under-appreciation of thunderstorms and their effects on safety in aviation.
Recent studies have even emphasized the importance of re-reviewing FAA guidelines for thunderstorm avoidance.
It may be that if the effects of turbulence upon airspeed are better understood, accidents can be prevented.
I'll start this discussion with an animation showing, at 30 second intervals over a 21 minute period of time, the intense windshear layers which form near, around, and over the top of thunderstorms as the result of convection inducted turbulence (CIT)
Credit:
Turbulence above thunderstorms
The blue areas in this diagram are cloud water content (above some threshold) and the red areas are model-produced turbulence (actually referred to as subgrid turbulence kinetic energy).
Its the kinetic energy part I'll discuss below. First, some background on what the above image shows:
Scientists at the Research Applications Laboratory (RAL) have investigated aircraft encounters with turbulence above thunderstorms and find that the FAA guidance is a bit naive. One of these incidents occurred on 10 July 1997 over Dickinson, North Dakota. A commercial turbojet aircraft encountered severe turbulence as it was negotiating a path through a number of scattered thunderstorms. At the time of the encounter it was passing directly over a developing deep convective cloud. In this incident, 22 passengers sustained minor injuries and the aircraft sustained enough damage to cause it to make a precautionary landing at Denver, Colorado. This aircraft encountered what is referred to as convection induced turbulence (CIT)." This type of turbulence is common enough that the CIT acronym is well known within the commercial aviation community; it is used to describe turbulence in the clear air either above the thunderstorm top, under the anvil, or near the lateral visible boundaries.
The aircraft's transit of the storm:
The cauldren of turbulent air it overflew:
Source:
AMERICAN METEOROLOGICAL SOCIETY Bulletin: Recent advances in the understanding of near-cloud turbulence 2/28/11
Regardless of the source dynamics, the spatial scale of the lateral bands and downstream structures are all approaching those that strongly influence commercial aircraft and they may pose an additional hazard adjacent to the storm and be responsible for the known hazard downwind.
5. Summary and future outlook: Fundamental understanding of near-cloud turbulence (NCT).
We have summarized recent progress made in understanding the NCT aviation hazard. These fundamental advances in our basic understanding were enabled by high-resolution
numerical simulations of observed events, complemented by improved data on turbulence encounters. These studies have shown that NCT is a complex phenomenon that crucially
depends on cloud characteristics, the structure of the near-cloud environment, and perturbations to that environment by cloud circulations and gravity waves. Yet, we are
acutely aware that we are only beginning to scratch the surface and a variety of basic problems are still to be solved regarding the dynamics underlying NCT. Outstanding
questions include:
--- What are the characteristics of NCT and how does it vary spatially and temporally?
--- How is NCT related to the mode of convective organization and its intensity?
--- What is the relative importance of gravity wave breaking and Kelvin-Helmholtz instability to the turbulence hazard?
--- What are the processes leading to the enhanced hazard near thunderstorm anvils?
--- What is the structure and mechanism of turbulence in thunderstorm wakes?
--- How common is the hazard posed by turbulence associated with ducted gravity waves?
--- What is the relationship between observable cloud features, the mesoscale environment, and NCT that may be useful for pilots and aviation forecasters?
--- What is the climatology of NCT?
Answering these questions and further research on the other processes detailed in this article is necessary to advance the fundamental understanding of NCT and could also
provide the framework to develop new approaches for turbulence avoidance. With recent improvements in high-resolution modeling capabilities available to researchers, we
believe that such much-needed advancement is achievable. Unfortunately, despite the importance of this problem and the opportunities for progress, there is relatively limited
activity in this area with only a few groups around the world actively engaged in NCT research. We hope that this article has spurred additional interest in this topic and we
encourage others to study this challenging problem.
I'll admit I'm not scientist, but I'd like to touch on a few things I have some personal experience with concerning the effects upon airspeed (and instantaneous leaps in speeds) associated with aircraft repeatedly crossing shear layers / boundary layers, below.
First, some statistics:
Source:
Statistical Summary of Commercial Jet Airplane Accidents Worldwide 1959 - 2006
An FAA Advisory Circular: Pilot Windshear Guide (11/25/88)
