A variety of convective weather forecasts are produced operationally and used by the aviation community as decision aids for rerouting air traffic around convective weather. These forecasts, which include the National Weather Service (NWS) Convective Collaborative Forecast Product (CCFP) and Convective Significant Meteorological Advisories (C-SIGMETs), describe convective activity at different spatial and temporal scales and differ slightly in the characteristics that are included in the forecast area.
A critical challenge in evaluating the quality of these forecasts is determining how to appropriately match the forecasts to the observations so that statistical results are representative of the forecast characteristics, the forecast spatial and temporal scales, and the forecast's operational relevance. This process has been particularly difficult for evaluating forecasts from the CCFP and C-SIGMETs that are required to meet minimum size thresholds as well as specific criteria for coverage of convection, cloud top height, and cell movement.
Historically, observations used to evaluate the CCFP were expanded from a 4-km grid to a 40-km grid to approximately match the scale of the forecast. Matching the forecast scale was difficult to determine, since the impact of the convective activity on the operational flow of enroute air traffic was not well defined. Moreover, the coverage attribute was excluded from the verification approach because the application of the attribute was not clearly understood.
This article presents new methods for defining the observation fields used for evaluating the CCFP and C-SIGMET forecasts that consider the effects of convection on the flow of air traffic such as Convective Constraint Areas (CCAs) and incorporate the observed coverage.
Data: Forecasts and Observations
Forecasts – The CCFP forecasts are issued by
the NWS Aviation Weather Center (AWC), but are produced through a collaborative process with AWC forecasters, airline and Center Weather Service Unit
(CWSU) meteorologists, and MSC (Meteorological Service
of Canada) meteorologists. CCFP forecasts are
required for areas of intense convection and thunderstorms
every 2 hours, with lead times of 2, 4, and 6 hours after
the forecast delivery time. The CCFP comprises
polygons that are at least 3,000 mi2 in size and contain a coverage of at least 25% convection with echoes of at least 40 dBZ composite reflectivity, and also a coverage of at least 25% with echo tops of 25,000 feet and greater.
The C-SIGMET, generated by forecasters at the AWC, is a text forecast of convective activity that is issued hourly but valid for up to 2 hours (as outlined in the National Weather Service Operations Manual D-22). These forecasts are intended to capture severe or embedded thunderstorms and their hazards (e.g., hail, high winds) that are either occurring or forecasted to occur within 30 minutes of the valid period and cover at least 40% of the 3,000 mi2 or larger forecast area.
Observations – The National Convective
Weather Forecast Hazard Product (NCWF-H) is used to
describe intense convection as it applies to the CCFP that is
a threat to aircraft. It is defined by the video integration
and processor (which contours radar reflectivity, in dBZ,
into 6 VIP levels) values of 3 or greater, and/or 3 or
more strokes of lightning in 10 minutes within 8 kilometers
of a grid point, on a 4-km grid. For further information
see http://cdm.aviation weather.noaa.gov/ncwf/ncwf_wt/ncwf_wt_haz.htm.
Techniques for Defining Observations
The techniques for defining the observations for evaluating the CCFP and the C-SIGMET are separated into two parts: 1) developing a definition for Convective Constrained Areas (CCA) and 2) producing observed fields that reflect the attributes of the CCFP, particularly the size and coverage criteria.
Convective Constraint Area (CCA) – This provides the basis for measuring the "scale" of convective activity that impacts the flow of enroute air traffic. Rhoda et
al. (2002) determined that pilots tend to deviate around strong precipitation until they get quite close to the
arrival airport. However, they were unable to determine
the typical distance of the deviations. Therefore, the
CCA concept applied here follows guidance provided by
the Aeronautical Information Manual (AIM), http://www1.faa.gov/ATPubs/AIM/index.htm, which suggests that pilots should remain at least 20 nm
away from intense convection in order to minimize
safety concerns that are related to convection. However,
in practice, this distance is often too large when air
space becomes congested. Therefore, to take this
operational consideration into account, we defined the CCA here as
an area of intense convection (identified by the 4-km NCWF-H grid) plus a 10-nm radius surrounding
the convection. The 10-nm radius is measured from
the center of each 4-km NCWF-H grid box. Figure 1
shows the raw NCWF-H in which the green areas represent
the grid boxes with intense convection. Once the
10-nm radius criterion is applied to the observations in Figure
1, the areas grow slightly (Figure 2) to represent the
CCAs. The CCAs in Figure 2 should not be thought of as
areas "closed" to enroute air traffic. Rather, they should be considered as areas where the flow of enroute air traffic is reduced because of the influences produced by the intense convection.