Conditional climatology of persistence of ceiling and visibility

      To forecast cloud ceiling and visibility for airport forecasts (TAFs), aviation meteorologists may use conditional climatology (CC) combined with complementary data and techniques. The data include: in situ observations (surface observations from airports, or METARs), remotely sensed observations (e.g., radar and satellite imagery), and fields from numerical weather prediction (NWP) models and nowcasting systems (Isaac et al. 2012). The techniques are diverse, each of varying applicability, depending on the prevailing conditions, or weather regime. Most of the techniques are beyond the scope of this paper (for an introduction, see COMET 2010). NWP has difficulty modeling fog and stratus; CC's site climatology of these phenomena complements NWP in forecasting.

      The main focus here is on conditional climatology of persistence (CCP): CC combined with the assumptions that: the synoptic situation will remain basically the same for the next few hours, and ceiling and visibility trends will resemble those of matching past cases at the airport in question. CCP is an extension of CC. CC answers a question such as: At airport X in the month of June at 0000 UTC with easterly winds, what percentage of the time are ceiling and visibility in the IFR category (less than 3 miles)? Suppose it is 10%. For forecasting, this could be interpreted as a 10% probability of IFR visibility at X in June at 0000 UTC. Adding conditions will sharpen probabilities in many cases. The CCP question becomes for example: Given that the ceiling and visibility is currently IFR how should one expect this category of visibility to evolve in time? Or: When is the category likely to change due to diurnal cooling? CCP makes the assumption that the synoptic factors of wind, precipitation persist. Analysis of the airport's climatology, by matching past observations to the current one, then following the evolution in hours immediately following those past matches, shows the probability of the current category starting at 100% (by construction) typically dropping over the following hours, with diurnal variations (see Hansen et al. 2010, Ling et al. 2010, and the Visibility Trend figure of MDL 2011).

      The applicability of CCP in operational aviation forecasting has been limited mainly by the amount of manual effort and time required to produce CCP analyses. A system has been developed to automatically and rapidly produce CCP analyses for forecasters, results of which are presented here. The system is designed to be part of a larger First-guess TAF (FGT) and nowcast system being developed by Environment Canada to help aviation forecasters produce TAFs more efficiently for approximately 200 sites across Canada, and to highlight those ones in most need of expert attention. The role of CCP in the FGT system will be to provide real-time, situation-specific, quality-control checking of the expected reliability and longevity of questionable persistence-based forecasts. Whenever persistency-based forecasting is not applicable, its contribution as predictor in objective systems can be suppressed.

Supplementary website: http://collaboration.cmc.ec.gc.ca/science/arma/ams2013

References

COMET, 2010: Writing TAFs for Ceiling and Visibility. COMET MetEd Website, University Corporation for Atmospheric Research. Available at http://www.meted.ucar.edu/dlac2/mod5, Accessed 31 July 2012.

Hansen, B., I. Gultepe, and A. Ling, 2010: Conditional persistence of ceiling and visibility. 44th Annual Congress of the Canadian Meteorological and Oceanographic Society, Ottawa, Canada, 31 May–4 June 2010. Available at http://collaboration.cmc.ec.gc.ca/science/arma/cmos2010, Accessed 31 July 2012.

Isaac, G. A., M. Bailey, F. S. Boudala, W. R. Burrows, S. G. Cober, R. W. Crawford, N. Donaldson, Ismail Gultepe, B. Hansen, I. Heckman, L. X. Huang, A. Ling, J. Mailhot, J. A. Milbrandt, J. Reid, and M. Fournier, 2012: The Canadian Airport Nowcasting System (CAN-Now). Meteorol. Appl. (2012). Available at http://onlinelibrary.wiley.com/doi/10.1002/met.1342/abstract.

Ling, A., B. Hansen, and I. Gultepe, 2010: Regime-based persistence probability of ceiling and visibility. 14th Conference on Aviation, Range, and Aerospace Meteorology, AMS, Atlanta, GA, 18–21 Jan. 2010. Available at https://ams.confex.com/ams/90annual/webprogram/Paper157353.html, Accessed 31 July 2012.

MDL, 2011: Aviation Forecast Preparation System ver. OB9.2., User's Guide, Climatology Tools, Ceiling and Visibility Trend. Meteorological Development Laboratory (MDL), NWS, NOAA. 27 October 2011. Available at http://www.nws.noaa.gov/mdl/pgb/AvnFPS/OB9.2/html/climate-new.html, Accessed 31 July 2012.