Large-scale influences during ACTIVE

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Large-scale influences during ACTIVE Rossby
waves and their effects on tropical convection

• Grant Allen1 G. Vaughan1 P. May2 D. Brunner3, W.
  Heyes1, P. Minnis4
• (grant.allen_at_manchester.ac.uk)

1Centre for Atmospheric Science, University of
Manchester 2Bureau of Meteorology, Melbourne,
Australia 3EMPA, Switzerland 4LARC, NASA Final
ACTIVE meeting Manchester, 21st May 2009
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Scope

• What are tropical tropopause folds?
• Why are they important?
• A case study of a series of folds during the
  ACTIVE campaign
• Implications for tropical convection

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Isentropic descent Tropopause folds
Plot taken from Holton, 1995 Isentropes solid
lines Isotherms dashed lines Shaded area
lowermost stratosphere

• The thermodynamic structure of the tropical
  troposphere links the sub-tropical lower
  stratosphere with the tropical troposphere no
  barrier to transport (isentropic)
• One mechanism of such transport is due to
  breaking Rossby waves and consequent tropopause
  folding along the STJ

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A Region of Interest (ROI)
Left MTSAT WV image dark areas are dry, blue
represent tall/high clouds) NW Australia Domain
defined for analysis Lat 20 ?12 OS Lon120-135
OE

• Region of interest (cross-hatch area above)
  defined here for further analysis
• Region defined by the prevalence of an
  eastward-advancing dry slot (dark area in MTSAT
  WV image above)

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Convective (and other) properties of the ROI

• Two tropical drought periods are defined by
  grey shaded time periods.
• Time-series of cloud fraction shows
  anti-correlation with PV during periods of
  large-scale descent (?)
• Rainfall and mid-level RH are also correlated
  during the dry periods
• Together, these trends show that rainfall and
  cloudiness were much reduced during the passage
  of a series of tropopause folds (evident as
  periods of high PV)
• Concentrate here on the second drought period as
  a case study

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Cloud Cover and Height

• Derived from MTSAT-1R brightness temperatures
  using a split-window technique of Minnis et al.,
  1995
• During the drought period, convection is shallow
  and limited to the coast (where forcing is likely
  to be strongest)

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Rossby waves
Arrows wind direction scaled to speed Colours
Potential vorticity
Selected images of ECMWF PV (at 325 K) and wind
data show the passage of a series of breaking
Rossby waves (red protrusions) over northern
Australia during the dry period The double
breaking event on 22 Nov (panel b) facilitated
the northward propagation of the wave by 23 Nov
(panel c). By 27 Nov, a quiescent jet was
re-established to the south
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Tropopause folds

• A finger of high PV defines the fold, which
  enters the ROI on 22 Nov carrying very dry air
  from the tropopause
• The PV signature associated with the fold
  persists until 25 Nov
• The dry air advected by the fold persists in the
  ROI even after the PV signature is lost on 27 Nov

Colours PV Black contours RH
Dashed lines Equivalent potential
temperature Vertical dashed lines define the
latitude limits of the ROI
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Ozone and humidity profiles
Deep dry, ozone-rich layers present between 22nd
27th Nov 13th and 30th Nov are typical
pre-monsoon examples Spot the odd one out
transition between the influence of the two
separate folds

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Back trajectories

• 4-day cluster back trajectories from the enhanced
  ozone layers observed at Darwin show they
  originated in or near to the breaking Rossby
  waves
• By 30 Nov (panel e), after the drought period,
  trajectories return to a more climatological
  tropospheric origin.

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Promotion of a large tropical cloud band
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(No Transcript)
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• Left Plots of CAPE and CIN diagnosed from ECMWF
  profiles at 0930 local time on 20 Nov 2005 over N
  Australia
• The tropical cloud band developed in a region of
  high CAPE (gt 2000 J/kg), but low CIN (lt 100 J/kg)
• The region of low CAPE and high CIN to the SW of
  the line indicates the position of the advancing
  dry slot seen in the satellite images

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Dynamics at the point of convective initiation

• A cross-section of wet-bulb potential temperature
  (Theta-W above) through the plume shows a region
  of reduced theta-W above the boundary layer at
  14O S (where convection originated)
• This reduction of theta-w with height over a
  moist boundary layer represents a region of high
  potential instability

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Conclusions

• A series of breaking Rossby waves over northern
  Australia during November 2005 led to strong
  modulation of convection over a wide area of the
  tropics
• Dry air which descended due to tropopause folding
  led to a marked reduction in cloud-cover and
  rainfall for a period of 6 days in total
• At the edge of one such advancing dry layer,
  generation of potential instability led to an
  eruption of a synoptic scale cloud system.
• Further evidence that mid-latitude tropopause
  level processes can influence tropical convection
  and rainfall.