Large-scale influences during ACTIVE – Rossby waves and their effects on tropical convection Grant Allen 1 G. Vaughan 1 P. May 2 D. Brunner 3, W. Heyes 1, P. Minnis 4 1 Centre for Atmospheric Science, University of Manchester 2 Bureau of Meteorology, Melbourne, Australia 3 EMPA, Switzerland 4 LARC, NASA Final ACTIVE meeting – Manchester, 21 st May 2009

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

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

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 O S Lon: O E 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)

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 (ω) 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

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)

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

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

Ozone and humidity profiles Deep dry, ozone-rich layers present between 22 nd – 27 th Nov 13 th and 30 th Nov are typical pre- monsoon examples Spot the odd one out – transition between the influence of the two separate folds

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.

Promotion of a large tropical cloud band

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 (> 2000 J/kg), but low CIN (< 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

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 14 O S (where convection originated) This reduction of theta-w with height over a moist boundary layer represents a region of high potential instability

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.