Aviation Hazards: Thunderstorms and Deep Convection

Transcription

1 Aviation Hazards: Thunderstorms and Deep Convection TREND NWP Products for Thunderstorm Forecasting

2 Contents Model choice Identifying parameters important for convection: Low-level convergence High relative humidity Cold air aloft Model trend Qualifying NWP guidance 2

3 NWP Guidance NWP may heighten the anticipation of thunderstorm activity As a future outlook day approaches, NWP guidance generally becomes more pertinent 3

4 NWP Guidance Particularly valuable for Day + 2 and onwards May provide prognostic fields for the concurrence of the three necessary conditions for thunderstorms: i. Instability ii. Low-level moisture iii. Lifting Mechanism ( trigger ) 4

5 Choice of NWP Model If there is a choice of NWP model Build up a study of which model performs best in which synoptic situation generally At forecast policy preparation time, allow time to compare NWP output from the various models before settling for the optimal guidance 5

6 Prognosis of Low-level Convergence Synoptic scale: location and movement of frontal systems (by estimation) and trough lines at the surface Low-level wind NWP guidance Prognose areas of confluence Upper-level divergence is important source of assistance to tropical convection 6

7 NWP Prognosed Relative Humidity and Cloud NWP can be used to define regions where Relative Humidity (RH) is greater than 70% say,. These can indicate regions of expected cloud RH > 80% at 700 hpa level indicates possible middle level cloud and rain 7

8 NWP Guidance Aviation forecasting has an emphasis on first 24 hours or so NWP outlooks are broader scale If a Public Weather shift precedes Aviation shift a good preparation for Outlook Period Trends from NWP Example: prognosed thermodynamic data, e.g., Lifted Index Check NWP guidance against observations for ground-truth 8

9 NWP as Trend Movement of: Synoptic scale systems such as anticyclones, cyclones, etc Middle-level systems such as troughs and ridges Middle-level air fields such as temperature Upper-level divergence (especially in the Tropics) 9

10 Tracking Temperature Trends 500 hpa temperature field May define a cold pool in the mid-levels and its movement Cold air aloft In effect, a measure of instability 10

11 NWP and Composite Analysis Techniques Using various NWP fields Compare with concurrence of three necessary conditions for thunderstorm formation Apply criteria Gives ingredients approach to thunderstorm forecasting into outlook period. 11

12 Qualifying NWP Guidance NWP often does not have the time response or spatial resolution to pick up atmospheric changes of the time-space scale necessary for aviation forecasting: Supplement NWP guidance with observations of the upper troposphere 12

13 NWP-derived Products used in Thunderstorm Forecasting Model Output Statistics, MOS Ensemble forecasts from multiple NWP models These products are particularly useful for prediction of continuous such as temperature and QNH fields in TAFs Statistical approaches do not respond well to outlying or extreme situations 13

14 Journal Reference: NWP use in aviation forecasting A. J. M. Jacobs and N. Maat. 2005: Numerical Guidance Methods for Decision Support in Aviation Meteorological Forecasting. Weather and Forecasting: Vol. 20, No. 1, pp

15 Summary NWP generally does not respond well enough at the aviation forecasting scale of time and space Yet, NWP data is valuable guidance for anticipating likely thunderstorm activity More elaborate techniques using derived products from NWP are found to be as accurate as the experienced aviation forecaster for some forecast parameters NWP verifies favourably over long-term studies, but not as useful on extreme events 15

16 Forward to Case study 16