Thunderstorm Forecasting - WMO Commission for Aeronautical
Transcription
Thunderstorm Forecasting - WMO Commission for Aeronautical
Aviation Hazards: Thunderstorms and Deep Convection TREND Empirical thunderstorm forecasting techniques © Copyright Commonwealth of Australia 2006, Bureau of Meteorology Contents ¾ Necessary conditions for convection: ¾ Instability ¾ Low-level moisture ¾ Trigger mechanism ¾ Forecasting thunderstorms for aviation: ¾ Analysis and diagnosis ¾ Symbiosis with other forecasting ¾ The thunderstorm forecasting paradox © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 2 Thunderstorm Fundamentals ¾ ¾ Composite Analysis of Concurrence of the Three Necessary Conditions The Three Necessary Conditions i. Instability ii. Low-level moisture iii. Lifting mechanism (“trigger”) © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 3 Concurrence of the three necessary conditions Low-level moisture Instability Concurrence of All 3 conditions TS ? Low-level convergence © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 4 Thermodynamic Instability ¾ a) Parcel Theory: Instability categorisation via aerological diagram / comparing environment and parcel Dry / Saturated Adiabatic Lapse Rate slopes (ELRs and DALR & SALR) ¾ b) Lifted Index, Li, a fairly universal standard of thermodynamic instability? ¾ Thermodynamic instability indices reference: ¾ Severe Weather Indices, by Ryan Knutsvig, Grand Forks, North Dakota, USA: ¾ http://www.geocities.com/weatherguyry/swx2.html © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 5 a) Parcel Theory ¾ Instability categorisation via an aerological diagram … ¾ Compare Environment (ELR) to: ¾ Parcel’s Dry / Saturated Adiabatic Lapse Rate (DALR) slope ¾ Parcel’s Saturated Adiabatic Lapse Rate (SALR) slope © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 6 Stability of atmospheric layers with differing E.L.Rs to small parcel displacements ELR (B) SALR ELR (C) isotherm DALR ELR (D) ELR(A) Absolutely unstable Conditionally stable/unstable Absolutely stable © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 7 b) Lifted Index ¾ LI = T500 - TP500 ¾ Where LI (°C) is the lifted index, T500 is the 500hPa environmental temperature (°C), TP500 is the 500hPa temperature (°C) which a parcel will achieve if it is lifted dry-adiabatically from the surface to its lifted condensation level (LCL) and then, moist-adiabatically to 500hPa. ¾ (In the lifting process, the lower 1 km mean mixing ratio is used as well as the observed/forecast surface temperature.) © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 8 Interpreting Lifted Index ¾ For Li < -2.0 ¾ Thunderstorms are possible ¾ Note that local conditions may qualify this threshold ¾ The need for a climatological study to determine meaningful threshold of Li © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 9 Skew-T / LogP Diagram: 23 UTC 17/01/01 Li = -5.2º C ⇒ (Severe) Thunderstorms are indicated © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 10 Tropical conditional instability ¾ Tropics are generally conditionally unstable ¾ Upper air divergence enhances low-level convergence ¾ Land ¾ Daytime surface heating ¾ Increased instability ¾ Peak of thunderstorm activity in late afternoon ¾ Ocean ¾ Night-time cloud-top cooling ¾ Increased instability ¾ Peak of thunderstorm activity in early morning © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 11 Low-level moisture ¾ Picture next page: climatology of thunderstorms in Australia ¾ Isobronts – lines of equal occurrence of thunderstorms ¾ Observation: Concentration of TS near coasts, in tropics ¾ Molecular weight comparison: ¾ ¾ ¾ ¾ H20 < [Dry Air] average ⇒ ρ [moist air] < ρ [dry air] ⇒ moist air is more buoyant than dry air! ⇒ moist air enhances convection ¾ Dew-point temperature (in º C) is a common measurement of moisture content ¾ Low-level dew-point analysis by way of isodrosotherms © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 12 Australian thunder-day climatology © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 13 What low-level flow characteristics will enhance localised ascent? Preferred instigators of thunderstorm activity … – – – – Orography (e.g., mountains) Interacting thunderstorm out-flows Convergence of synoptic and local scale winds Discontinuities i. ii. iii. Front Dry line: a sharp gradient in low-level moisture Water vapour dry / moist boundary – Trough line: associated with low-level cyclonic flow and ascent – Tropical Cyclone: large scale low-level convergence © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 14 Mountains and Convection © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 15 Interacting thunderstorm out-flows © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 16 Sea-breeze front and sea breeze convergence over an island © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 17 Cold and Warm Fronts ¾ Temperature discontinuities on a synoptic scale © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 18 Example: Cold Front © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 19 Trough line ¾ A trough of low pressure is associated with low-level convergence ¾ Cyclonic flow is associated with low-level convergence ¾ Especially on eastward side / pole-ward flow © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 20 Prefrontal Trough © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 21 Convection along a Trough Line © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 22 Moisture Discontinuities ¾ Dry Line ¾ e.g., in northern Australia ¾ Isodrosotherm gradient becomes intense ¾ Indicates moisture / dry air boundary ¾ Moist air is more buoyant than dry air ¾ Sets up local circulation ¾ Can initiate convection ¾ Water Vapour Dry / Moist Boundary ¾ Moist air is more buoyant than dry air ¾ Sets up local circulation ¾ Can initiate convection ¾ Often associated with severe thunderstorms © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 23 Forecasting Thunderstorms for Aviation ¾ Observations ¾ Analyses ¾ Public Weather / Severe Weather / Aviation Weather interactions ¾ Thunderstorm Diagnostics ¾ Thunderstorm Forecasting Methodologies ¾ Time Scales and Thunderstorm Forecasting ¾ The Thunderstorm Forecasting Paradox © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 24 Observations ¾ Observations ¾Satellite Data ¾RADAR Imagery ¾Upper winds / Temperature sondes ¾Surface observations ¾ METARs / SPECIs and Synoptic Observations ¾AMDARs ¾Alerts derived from Observations ¾ Input into Weather Watch ¾Do Aviation products require amendment or not? © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 25 Analyses ¾ Hand-analysis of synoptic plots ¾ Every 3 hours, say ¾ Enormous amount of information is inculcated, particularly changes to weather fields, each time the chart is newly analysed ¾ Synoptic scale analyses (NWP or hand-drawn) are the forecaster’s bread and butter © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 26 Thunderstorm Diagnostics ¾ Thunderstorm Forecasting Methodologies - Expert Systems or Decision Trees ¾ Example - Refer to the article ¾ J.R. Colquhoun. 1987: A Decision Tree Method of Forecasting Thunderstorms, Severe Thunderstorms and Tornadoes. Weather and Forecasting: Vol. 2, No. 4, pp. 337–345 ¾ And, also after John Colquhoun’s work: Figures 1a and 1b >>> © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 27 © Copyright Commonwealth of Australia 2006, Bureau of Meteorology © Copyright Commonwealth of Australia 2006, Bureau of Meteorology © Copyright Commonwealth of Australia 2006, Bureau of Meteorology © Copyright Commonwealth of Australia 2006, Bureau of Meteorology Time Scales and Thunderstorm Forecasting ¾ Thunderstorm Climatology ¾ A description of thunderstorm behaviour, and so provides an expectation of thunderstorms ¾ Setting of local thresholds e.g,, Lifted Index ¾ Aviation Forecasts ¾ Short-term forecast period (e.g., TAFs) ¾ Now-casting – Weather watch ¾ e.g., tracking storms with RADAR ¾ Outlook Period: Supported by NWP guidance ¾ Provides heightened anticipation of thunderstorm activity © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 32 Forecast Symbiosis ¾ Aviation weather forecasting can benefit from interactions with ¾ Public and Marine Weather ¾ Severe Weather ¾ And, vice versa ¾ Consider time / space scales ¾ Are they inclusive? Do they overlap? © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 33 Public Weather, Severe Weather, Aviation Weather Interactions Public & Marine Forecasting Severe Weather Aviation © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 34 Public Weather – Severe Weather – Aviation Weather Symbiosis Public & Marine Forecasting Aviation Forecasting Severe Weather Forecasting © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 35 The thunderstorm forecasting paradox ¾ The forecaster intends not to miss a thunderstorm event, if it relates to an Aviation product for which he or she is responsible ¾ This generally leads to over-forecasting thunderstorms: a high “false alarm rate” ¾ The aim is then, with experience, to endeavour not to forecast thunderstorms, when ostensibly overwhelming evidence points to a thunderstorm event! © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 36 Summary ¾ Thunderstorm Forecasting: ¾ A skill which requires practice ¾ Largely experience based ¾ Often false-alarm rate high ¾ Shared data with Severe Weather specialists / Public Weather forecasters ⇒ Optimum net value from team effort © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 37 Forward to NWP guidance for thunderstorm forecasting © Copyright Commonwealth of Australia 2006, Bureau of Meteorology 38