Chapter 7 part A Flashcards
The following six features of the large-scale tropics were identified as “necessary, but not sufficient” conditions for tropical cyclogenesis:
- sufficient ocean thermal energy (SST > 26°C to a depth of 60 m)
- enhanced mid-troposphere (700 hPa) relative humidity
- conditional instability
- enhanced lower troposphere relative vorticity
- weak vertical shear of the horizontal winds at the genesis site, and
- displacement by at least 5° latitude away from the equator.
(i) sufficient ocean thermal energy (SST > 26°C to a depth of 60 m)
(ii) enhanced mid-troposphere (700 hPa) relative humidity
(iii) conditional instability
(iv) enhanced lower troposphere relative vorticity
(v) weak vertical shear of the horizontal winds at the genesis site, and (vi) displacement by at least 5° latitude away from the equator.
- (i), (ii) and (iii)
- seasonal indicators of genesis potential
- thermodynamic parameters measure the ability to support deep convection
- seasonal indicators of genesis potential
- (iv), (v) and (vi)
- daily likelihood of genesis
- dynamical parameters, such as vertical wind shear
- daily likelihood of genesis
“Necessary but not sufficient” means that
all of these conditions must be present simultaneously before tropical cyclogenesis can occur, but even if all of these conditions are met, tropical cyclogenesis may not occur.
Thus, the necessary, but not sufficient, criteria for tropical cyclogenesis may be summarized as
- the ability to support deep convection in the presence of a low- level absolute vorticity maximum.
- The low-level vorticity maximum reduces the local Rossby radius of deformation focusing the convective heating locally.
The ability of the initial convection to survive for many days depends on its
vorticity, stability, and depth—defined by the Rossby radius of deformation, LR.
The Rossby radius, LR,
is the critical scale at which rotation becomes as important as buoyancy.
compare Lr values
- When the disturbance size is wider than LR,
- it persists;
- systems that are smaller than LR will
- disperse.
- LR is inversely proportional to
- __latitude
- so it is very large in the tropics.
- __latitude
- However, the high vorticity in tropical cyclones reduces the Rossby radius and enables tropical cyclones to last for many days and even weeks.
Theories of Tropical Cyclone Formation
- Conditional Instability of Second Kind (CISK)
- TC formation from MCV
- Top-down merger for development
- Top-down showerhead theory for development
- Botton-up development theory
TC formation is associated with:
- mesoscale convective systems (MCSs) and
- their accompanying mesoscale convective vortices (MCVs).
The transition from MCS to a TC-like vortex require:
the generation of low- level cyclonic vorticity below the MCS.
A mesoscale convective vortex (MCV) is a
- low-pressure center (mesolow) within an mesoscale convective system (MCS).
- An MCV core is only 48 to 97 km wide and 1.6 to 4.8 km deep.
mesoscale convective vortices (MCVs)
- mid-level vortices found in many tropical cloud clusters.
- They occur both in:
- mid-latitude and
- tropical mesoscale convective systems (MCSs).
The figure below shows an idealized vertical cross-section through an MCV.
- An MCV has the maximum vorticity in the middle, with decreasing intensity above and below.
- There is a cold anomaly below and a warm anomaly above the vorticity maximum.
An idealized MCS contains the regions:
-
A convective region where new convective cells are formed and progress to maturity.
- low to mid-level convergence with
- divergence aloft.
-
A stratiform region formed from the remnants of the old cells from convective region.
- there is mid-level convergence,
- with low-level and upper-level divergence.
There are three theories that attempt to explain how an MCV associated with an MCS can result in the ……………………………. :
a) Top-down Merger; b) Top-down Showerhead and c) Bottom-up Development.
formation and/or amplification of a vortex at the surface
a) Top-down Merger for Development
two or more smaller mid-level vortices merge into a larger mid- level vortex. The influence of this larger vortex is then felt through a deeper depth of the atmosphere, influencing development at the surface.
The depth or vertical thickness to which a vortex penetrates is given by
the Rossby penetration depth:
𝐷 = 𝜔𝐿⁄𝑁
where w is the inertial frequency, L is the horizontal scale of the vortex, and N is the Brunt-Vaisala frequency.
the Rossby penetration depth explination
- The larger in horizontal extent, or the more intense a vortex is (more intense vortices have higher inertial frequencies), the deeper the Rossby penetration depth.
- If two or more smaller, mid-level vortices merge then they will form a
- larger vortex that has a deeper Rossby penetration depth, and
- can build down to lower levels.
- larger vortex that has a deeper Rossby penetration depth, and
Top-down Showerhead theory for development
- an existing mid-level vortex in the stratiform region of an MCS.
- Rain falling from mid-level stratiform clouds
- causes evaporative cooling in the low levels,
- cools the lower levels
- cold anomaly that bows the isentropes upwards
- pressure surfaces bulge downward,
- extending the mid-level circulation downward.
- pressure surfaces bulge downward,
- cold anomaly that bows the isentropes upwards
- subsidence
- advects positive vorticity downward.
- cools the lower levels
- causes evaporative cooling in the low levels,
- evaporation of rain
- lower levels more humid and cool
- Once the downdrafts abate, convection can now fire in the low levels
- low level convergence
- spin up of low level cyclonic vorticity
- Once the downdrafts abate, convection can now fire in the low levels
- lower levels more humid and cool
Bottom-up Development Theory
- low-level potential vorticity (PV) anomaly produced from a separate convective updraft moves underneath the MCV.
- the interaction shifts the profile of divergence and convergence such that there is:
- is enhanced low-level convergence and
- spin-up of a low-level vortex, at the expense of the original MCV.
- the interaction shifts the profile of divergence and convergence such that there is:
Another general consideration for development is the need to
- eliminate convective downdrafts.
- those areas are not favorable for development
- convective downdrafts lead to:
- low level divergence
- reduce cyclonic spin up
- low level divergence
- convective downdrafts lead to:
- those areas are not favorable for development
…………………….. will reduce convective downdrafts by ……………..
Having moisture in the mid and low levels
by reducing the evaporative cooling and entrainment which lead to downdrafts.
Conditional Instability of Second Kind (CISK)
This theory explains how tropical convection develops into a tropical cyclone.
a positive feedback
This theory explains how tropical convection develops into a tropical cyclone. In its simplest form, CISK can be explained as follows:
- Latent heating of the atmosphere
- leads (through the hypsometric relationship) to a lowering of surface pressure
- The lowering of the surface pressure
- leads to enhanced radial inflow and convergence,
- which enhances the convection and latent heat release,
- which further decreases the surface pressures.
- which enhances the convection and latent heat release,
- leads to enhanced radial inflow and convergence,
(CISK cannot explain how cloud clusters form into a tropical cyclone, but it can be used to explain how a tropical cyclone, once formed )
CISK cannot explain how cloud clusters form into a tropical cyclone, but it can be used to explain how a tropical cyclone, once formed, intensifies:
since as the vortex becomes stronger the Rossby radius of deformation in the core can become small enough to allow a mass-field disturbance to force a velocity field adjustment.
