Turning winds with height rainfall diagnostic Kevin Tory Jeff Callaghan 22 May 2009—CSIRO

Introduction Thermal advection and extreme rainfall examples How does thermal advection contribute to vertical motion? A simple method for diagnosing vertical motion (winds that rotate with height) -- Theory -- Graphical explanation using an idealized example Under what conditions can anticyclonic turning of winds with height be used to diagnose extreme rainfall? How should the diagnostic be used?

48 Hour forecast Thermal advection and extreme rainfall examples

72 Hour forecast Thermal advection and extreme rainfall examples

96 Hour forecast Thermal advection and extreme rainfall examples

Caloundra 491mm Beerburrum 541 mm Mt. Glorious 406 mm Nanango 423 mm Alderley 400 mm Thermal advection and extreme rainfall examples

“South-east Queensland is in the grip of the worst flooding for more than 30 years”

850—500 thickness W C W C TC Abigail C C W W TC Sadie

L W C Extreme rain event: Timor, March 2006.

Sulawasi Floods: June UTC 20 June 2006

Winds backing with height Red 500 Green 700 Blue 850 Sinjai turn anticyclonically with height

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W WAA CAA θ=const u Z x Consider an element of flow (let’s call her Flo). To conserve θ she follows the isentrope. How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const u Z x How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection θ=const u Z x WC W WAA CAA CpCp But what if the isentrope was part of a wave with phase speed C p > u ? How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection u Z x WC W WAA CAA CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection WC W θ=const WAA CAA u Z x CpCp How does thermal advection contribute to vertical motion?

Adiabatic vertical motion can be expressed as Vertical motion Static stability Thermal evolution Thermal advection Thermal advection is only a good indicator of vertical motion in systems where the thermal evolution is small. Vertical motion increases with decreasing static stability. How does thermal advection contribute to vertical motion?

The relationship between turning winds with height and thermal advection Assume gradient wind balance Coriolis Centrifugal Gradient wind ( u,v ) flows parallel to geopotential ( Φ ) contours Mean thermal advection between two pressure levels ( MTA ) can be expressed as a function of the thickness ( Φ 1 – Φ 0 ) gradient

The relationship between turning winds with height and thermal advection Use gradient wind relationship to express the thickness gradient as a function of u, v and F Then the mean thermal advection between pressure levels 1 and 0 becomes

The relationship between turning winds with height and thermal advection Rotation term: MTA increases with increasing Coriolis and increasing curved flow—Thus valid for some near-equatorial flows Rotation term Turning winds with height term Turning winds with height term is proportional to the area between the two wind vectors. TWWH is zero when the vectors are parallel (no turning with height) TWWH is maximum when the vectors vary in direction by 90° TWWH increases with increasing wind speeds TWWH > 0 when the winds rotate anti-clockwise with height In SH Rotation term 0 for anticyclonic rotation with height, thus MTA < 0 and vertical flow is upward.

Winds backing with height Red 500 Green 700 Blue 850 Sinjai turn anticyclonically with height

Anticyclonic turning winds with height from the surface to 200 hPa. - Strongest turning 800 and 500 hPa. Brisbane—Wednesday

θ0θ0 X Z Y θ1θ1 X Y Low pressure projection from upper isentropic dip High pressure from cold dome isobars and isentropes coincident

θ0θ0 X Z Y X Y θ1θ1

θ0θ0 X Z Y X Y θ1θ1

θ0θ0 X Z Y X Y θ1θ1

X Z Y θ0θ0 θ1θ1 X Y WAA CAA Isobars

X Z Y θ0θ0 θ1θ1 X Y WAA CAA Anticyc cyc Anticyclonic rotation of winds with height WAA Cyclonic rotation of winds with height CAA

Under what conditions can anticyclonic turning of winds with height be used to diagnose extreme rainfall? 1Theory assumes gradient flows—e.g., tropical low-pressure systems

Under what conditions can anticyclonic turning of winds with height be used to diagnose extreme rainfall? 1Theory assumes gradient flows—e.g., tropical low-pressure systems 2Thermal evolution term must be small—e.g., slowly evolving, slowly translating systems relative to the system flow

Under what conditions can anticyclonic turning of winds with height be used to diagnose extreme rainfall? 1Theory assumes gradient flows—e.g., tropical low-pressure systems 2Thermal evolution term must be small—e.g., slowly evolving, slowly translating systems relative to the system flow 3Low static stability gives a greater vertical response for a given wind structure

Under what conditions can anticyclonic turning of winds with height be used to diagnose extreme rainfall? 1Theory assumes gradient flows—e.g., tropical low-pressure systems 2Thermal evolution term must be small—e.g., slowly evolving, slowly translating systems relative to the system flow 3Low static stability gives a greater vertical response for a given wind structure 4High intensity, highly curved flow gives a greater vertical response for a given wind structure.

Under what conditions can anticyclonic turning of winds with height be used to diagnose extreme rainfall? 1Theory assumes gradient flows—e.g., tropical low-pressure systems 2Thermal evolution term must be small—e.g., slowly evolving, slowly translating systems relative to the system flow 3Low static stability gives a greater vertical response for a given wind structure 4High intensity, highly curved flow gives a greater vertical response for a given wind structure. 5Vertical motion = extreme rain when humidity is high and atmosphere is conditionally unstable —Warm season oceanic tropics nearly always ideal

How should the diagnostic be used? Using the rotation of winds with height to diagnose ascent and rainfall can really only provide qualitative guidance, because it is only one component of the equation. The conditions listed on the previous page can give forecasters useful quantitative insight. Forecasters in the Brisbane office have developed a sense of when extreme rainfall can be expected, often when model forecasts a predicting significantly less rain. Most importantly the diagnostic can be used to flag regions of isentropic vertical flows, and thus provide a conceptual model of the dynamics leading to extreme rainfall. The diagnostic complements rather than replaces existing rainfall products.

Summary Thermal advection can be identified in many tropical and extra- tropical extreme rainfall events around the world For gradient flows thermal advection can be diagnosed from rotation of winds with height—Conditions apply For rapidly rotating, warm-season, tropical oceanic systems, the anticyclonic rotation of winds with height should provide a warning flag for extreme rainfall The diagnostic should be seen as an extra tool that provides additional insight into dynamical set-up for extreme rainfall