Lower atmospheric drying, stability, and increased wildfire activity Graham Mills Bureau of Meteorology Research Centre

Outline Massive drying was observed at screen level during two major recent fires in Australia - Canberra - 18 January 2003 - Eyre Peninsula - 11 January 2005 Why? Was this that unusual? How? Predictability?

Locality map #1

Canberra 18 January 2003 (Mills, 2005, AMM)

Meteorological Context

Dewpoint

The northern dry band? GMS-5 Water Vapour Imagery 3-hourly intervals from 2030 UTC (~ 0500 LDST) (first surface dewpoint drop ~ 0300 UTC)

Eyre Peninsula 11 January 2005

Meteorological context

Strong association with 300 hPa IPV

A general study? (encouraged by Potter, McCaw) Stations spread across the southern fire-vulnerable regions Td < -5C (want low moisture content, not just relative humidity) 1999-2000 to 2004-5 fire seasons (limit of half-hourly data archive) WV imagery? - Front? - Dynamics?

Localities

So far……

How does dewpoint change? Moisture equation: - Horizontal advection - Vertical advection - Diabatic processes: - evaporation - turbulent mixing

Early results? 68% have WV dry slot VERY evident 67% have front/trough passage - can be pre- or post-frontal Climate control - marked interannual variability - dry soil > deeper mixed layers? OR wet soil > shallow inversion? Daytime mixing - majority occur during day Coastal marine layer erosion (YPLC) Topographic interactions

High topography – lowering inversion overnight (Cabramurra)

Major circulation changes: Richmond – 2001-2002 fires

THEN - Upper tropospheric dry air – how does it reach the surface? There’s usually a deep mixed layer, so maybe it doesn’t need a lot of help

Processes? Upper-jet circulations > dry intrusions Mixing by cross-frontal circulations - surface-based fronts - mid-tropo. fronts / sloping b-l “lid” Destabilisation by dry-slot (low e) air - (cf Haines Index concepts) Positive feedbacks between upper IPV and lower frontal circulations? ……………?

Where next? Monitoring WV imagery an immediate (fire) forecast /weather watch component Complete station analysis and categorise What are the dynamic controls on these events? NWP model predictability (process good, results poor?) Link to fuel moisture models