Return periods of prolonged fog events in Canada

Transcription

1 Return periods of prolonged fog events in Canada 43th Annual Congress of the Canadian Meteorological and Oceanographic Society, Halifax, NS, 31 May 4 June 2009 Authors: Bjarne Hansen (Science and Technology Branch), Ismail Gultepe, Aaron McCay, Alister Ling Thanks to: William Burrows, Stewart Cober, Walter Dnes, William Richards

2 Introduction Climatology is what you expect. Weather is what you get. Weather forecasts help to plan tactically for day 1, strategically for days 2-3 Climatology helps meteorologists forecast more accurately, can help in planning from short-range (routine operations) to long-range (engineering) Knowledge of local climatology, combined with similarities between current and historical events, driving large-scale synoptic factors, can cue operational meteorologists to consider forecasting a high-impact rare event Local climatology often described in terms of basic frequency of events, calculated based on counts of individual hourly observations For example, YVR had fog in 8074 of hours frequency of fog is 1.67% Prolonged events are: Relatively rare and have high impact on safety, transportation, economy, e.g., fog, freezing rain, snow accumulation, blizzard, drought, etc. Often due to stalled synoptic systems Not detected and described by frequency based on individual hour counts

3 Rare, prolonged, high-impact impact events are best described in terms of return periods (RP) How are such events best detected? Problem: how to interpret breaks in prolonged events? *? Two 24-h h periods of fog with a 1-h 1 h break in between are, for all practical purposes, the same as one 48-h h fog event. * McCay, A., 2006: Fog at CYVR Nov th Canadian Aviation Weather Workshop, Edmonton, AB, 19 Oct DRAFT Page 3 May 30, 2009

4 Disregard breaks H hours long. Increment H. Test effect. H = Hourly observations A few prolonged events Condition: observed, not observed May choose to regard this sequence as a single prolonged event Chart legend As the number of observations being grouped together to form events increases from 0 to 3, the number of events decreases, which causes the average duration of event to increase.

5 Vancouver Intl., BC, Low IFR (cig < 500 or vis < 1 SM) Return Period (days) Low IFR often caused by fog. ~ 30 years Delays aviation traffic. ~ 3 years ~ 3 months 10 days Lower curve more frequent the prolonged event Logarithmic axis a factor of 10 per per bold line 1 day Event Duration x (hours)

6 Vancouver Intl., BC, Low IFR (cig < 500 or vis < 1 SM) Return Period (days) 2-day event RP ~ 3 years 1-week event (!), 173-hours no breaks > 2 hours Nov day event RP ~ 5 years * 1-day event RP ~ 1 year Event Duration x (hours) Summary statistics As connection of breaks increases, average durations of events increase.

7 Summary statistics Frequency: Period: Observations: Average duration: 2.78%, or out of the non-missing observations matched the low IFR condition. Period of climate record. If only a subset is used (e.g., January), it is noted in the plot title. 99.8%, or out of maximum possible hourly records in period of record were not missing. Prolonged events which had no breaks, 1-hour breaks, 2-hour breaks, and 3-hour breaks were, respectively, of average duration 3.8, 4.7, 5.3, and 5.8 hours long.

8 Vancouver Intl., BC, Low IFR (cig < 500 or vis < 1 SM) * Month Sunset Sunset Sunrise Sunrise Low IFR conditions are seasonal: Peak in terms of hours in October The longest events tend to occur in November * science/arma/climatology Time (UTC)

9 Vancouver Intl., BC, November, Low IFR (cig < 500 or vis < 1 SM) Return Period (days) 1-day event RP ~ 1 year 2-day event RP ~ 3 years 3-day event RP ~ 5 years one month in 15 one month in 10 one month in 4 Annual / November Statistics / Event Duration x (hours) / DRAFT Page 9 May 30, 2009

10 Study of climatology and synoptic factors helps to increase forecast skill 10 Vancouver Intl., Jan Visibility (km) Fog Freezing Fog Vis Day

11 500 hpa Geopotential Height (m) Composite Mean Jan * Study shows Ω-block is conducive to prolonged fog over west coast. Ω-blocks are forecast quite well by NWP. Ensembles can help. Ω Forecasters see the pattern in medium-range models, and forecast prolonged fog events start, duration, end time with skill days in advance. * NCEP / NCAR Reanalysis data/composites/day

12 Synoptic factors setup for prolonged fog in Vancouver Omega blocking patterns required in the 500 hpa flow Strong subsidence inversion forms, traps moisture in low levels Persistent Low IFR (< 500 feet ceiling and/or < 1 SM vis) lasting 3 days straight: return period of ~ 5 years, events occur only from October to January Conditions improve when block breaks down

13 Return period analyses for Canadian airports Plots are available online for all hourly climate sites for various conditions: fog (vis ½ SM), LIFR, IFR, VFR, blizzard, precipitation (any, freezing, frozen, liquid), no precipitation (drought) Work supports two projects Arctic Ice Fog Forecasting (contact Ismail Gultepe) Aviation Guidance (contact Alister Ling)

14 DRAFT Page 14 May 30, 2009