Aspects of Convective Initiation Roger K. Smith University of Munich

Collaborators Gerald Thomsen (Scientist) MM5 simulations of low-level convergence lines Robert Goler (Scientist) Gravity currents, sea breezes, gulf cloud lines Ulrike Wi  meier (Ph. D. student) Application of cloud resolving models to provide understanding and empirical rules for forecasting severe convection in the tropics Severe Weather Branch, Bureau of Meteorology NTRO Case selection, briefing on forecasting problems

Topics  Conditional Instability, CAPE & CIN  Theory of low-level convergence lines  Australian examples: The Morning Glory The North Australian Cloud Line (Gulf line) Southerly Morning Glories and inland trough/dry line  Simulations of flow over the Top-End of Australia Sea breeze convergence lines as convective triggers  Gravity currents, severe storm criteria in the tropics

Conditional Instability  Conditional instability refers to the situation when an air- parcel displacement is stable as long as the parcel remains unsaturated, but which “ultimately” becomes unstable if saturation occurs.  To check for conditional instability, we examine the buoyancy of an initially-unsaturated parcel as a function of height as the parcel is lifted through the troposphere, assuming some thermodynamic process (e.g. reversible moist adiabatic ascent, or pseudo-adiabatic ascent).  If there is some height at which the buoyancy is positive, we say that the displacement is conditionally-unstable.

 If some parcels in an unsaturated atmosphere are conditionally-unstable, we say that the atmosphere is conditionally-unstable.  Conditional instability is the mechanism responsible for the formation of deep cumulus clouds.  Whether or not the instability is released depends on whether or not the parcel is lifted high enough.  Put another way, the release of conditional instability requires a finite-amplitude trigger (the heart of convective initiation!).  The conventional way to investigate the presence of conditional instability is through the use of an aerological diagram.

pressure (mb) g/kg LFC LCL 20 o C 30 o C LNB dry adiabat pseudo- adiabat

The convective inhibition (CIN) Convective Available Potential Energy - CAPE Convective Inhibition (CIN) The convective available potential energy or CAPE is the net amount of energy that can be released by lifting the parcel from its original level to its LNB.

Severe convective storms  A prerequisite for strong convection is the accumulation of large reservoirs of CAPE.  To accomplish this it is necessary to have some negative area (or CIN) in the sounding to prevent the instability from being released spontaneously.

Thermodynamic sounding, 00 GMT, May 7, 1986 at Oklahoma City

Conditions favourable for the development of severe thunderstorms in central United States

J. Atmos. Sci

Symmetric convergence line  Convergence line capped by an inversion of strength  

Convergence line capped by a layer of continuous stratification

Low-level convergence lines over Australia

gulf cloud line NACL southerly morning glory northeasterly morning glory 03 Oct EST

The Morning Glory

Bores on rivers

A gulf cloud line

A wet-season line

CORAL SEA Burketown GULF OF CARPENTARIA CAPE YORK PENINSULA

<= 03 Oct 0000 Z 04 Oct 0000 Z => GLEX 2002

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Smith, R. K., Recent References

The dry line and inland trough 28 Sep 00Z 29 Sep 00Z

The dry line and inland trough 08 Oct 00Z 09 Oct 00Z

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Some conclusions  MM5 with 3 km horizontal resolution does a very good job in capturing many of the observed cloud lines and low-level convergence lines, at least during the dry season.  What about other regions (e.g. the ‘Top End’) and other seasons?  In particular, what about cases that initiate deep convection?

0525 Z 19 Nov 2004

0625 Z 19 Nov 2004

0725 Z 19 Nov 2004

0825 Z 19 Nov 2004

17 Oct 2004 div

17 Oct 2004 qvqv

VIS 19 Nov 0326 Z – 21 Nov 0826 Z M

0700 Z 0725 Z 19 Oct 2003

0700 Z 0725 Z 20 Oct 2003

Katherine Meteograms

Further work  Examine some cases during the TWP-ICE experiment in the Darwin region in  Examine some cases in Thailand – planned joint work with the Thai Meteorological Department.

Schematic diagram of a steady gravity current z  hhead nose d cold air warm air mixed region c Studies of gravity currents Relevant to sea breezes and thunderstorm outflows

Haboob

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Wi  meier & Goler’s studies of severe storms  Need to stratify the data according to synoptic flow Easterly flow Weak gradient Monsoon westerlies.  Try to understand in terms of MM5 simulations  Concerns a numerical study of tropical thunderstorms.  Motivation The tropical tropopause is much higher than in mid- latitudes 16 km compared with 11 km. The vertical distribution of lifted parcel buoyancy is different for a given CAPE. The criteria (vertical wind shear and CAPE) developed to establish when storms will split and become severe were developed for the middle latitudes.  Aim: To establish such criteria for the tropics.

Small circles  single or multicell storms Large circles  splitting occurs

Small circles  single or multicell storms Large circles  splitting occurs

Preliminary conclusions  Storm splitting to produce supercell storms in the tropics requires a higher vertical wind shear relative to the storm updraft than for similar storms in the mid-latitudes.  Supports the forecaster’s experience that the Bureau of Meteorology’s Thunderstorm Guidance Tool, which is used operationally, but was developed for mid-latitude storms, over-forecasts supercell storms in the tropics.

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