Ken Kula
Severe Weather in a Center
At the time I wrote this, the exact cause of Air France 447’s demise over the equatorial Atlantic was still uncertain. There’s much speculation that significant weather (many reports point to towering thunderstorms along its route) played a role in the events that led to the crash. The Air France flight was operating in an area without radar coverage in oceanic control airspace. It would have been relying on its own weather radar and PIREPs (Pilot Reports) from other flights to avoid significant storms. If the flight was in radar coverage within domestic U.S. airspace, it would have received specific advisories from controllers. In an ARTCC (Air Route Traffic Control Center – “Center”), there are numerous products that could be used to warn flight crews about severe weather along their route of flight.
Radar is a quick and useful tool to report precipitation, but has limitations. What a controller sitting in a windowless room observes on their display is not exactly what a pilot sees on their airborne weather radar, or out their cockpit windows. A Center controller has a huge amount of information available to them, but their weather radar may not be as precise as, say an approach controller. With a radar refresh rate almost twice as fast than the Center’s, an approach controller’s radar concentrates on the area closer to airports, say within 50 miles of the facility. Radar systems used in enroute applications are of broader focus, with a 200-plus mile range. The weather information from these radars across the country are patched together to make a seamless display that extends beyond the boundaries of each of the 20 coterminous U.S. ARTCCs. This system is known as the Weather and Radar Processor system, or WARP.
WARP takes information from a mosaic of NEXRAD (Next Generation Radar) sites, including the National Weather Service’s WXR-88D Doppler radars, and processes the data into a display that is overlaid on a controller’s radar presentation. Importantly, the displayed data is that of precipitation only, not clouds or air motion/turbulence. Three intensity levels are presented in the enroute world; moderate, heavy, and extreme precipitation. Note that light precipitation is not displayed, nor should one expect to receive any advisories about this level of precipitation. Controllers are required to advise crews if their route of flight will enter an area of displayed precipitation. WARP displays are updated approximately every 4 minutes. Thus a fast-moving squall line may have already moved a mile or more since the display on a Center radar screen was updated. I’ve seen many times where a pilot has deviated around an unseen (for us in the Center) overhang from a thunderstorm, with no returns on our screen, too. On the plus side, WARP will display the entire area of weather, not a reduced picture due to limited range or capacity. Center controllers have the advantage of map/airway overlays on their displays too, and can (sometimes) offer a clear path away from significant radar returns. Ultimately, controllers know that a pilot in command has the final say as to the ultimate course of action for deviating around weather.
Supervisors have additional weather products at their positions. They can call on information not displayed on controller’s screens, such as lightning strikes, enhanced precipitation displays (including data from Canada and Puerto Rico in six levels of intensity with light and frozen precipitation intensities), plus forecasted storm movement and intensity increase/decrease tendencies. Each Center has a Traffic Management Unit (TMU), whose staff keeps up to date on significant weather conditions. They can order reroutes to avoid severe conditions, or order a “ground stop” that will not allow aircraft to depart into severe weather. The TMU works closely with each approach control that the Center overlies to offer alternatives to assist in avoiding severe weather. Another significant asset in each ARTCC is the Center Weather Service Unit (CWSU). This is a National Weather Service (NWS) unit of meteorologists with all of their NWS/NOAA forecasting tools and machines. AIRMETs and SIGMETS are graphically displayed on computerized briefing terminals, and typed messages sent to each concerned Center Area of Operation from the CWSU. In Boston Center, the four meteorologists are very influential with their forecasting of Boston’s locally strong sea breezes, which usually require runway changes throughout the day. As of June, 2009, the 20 local CWSUs were being scrutinized to see if they could be combined into one or two centrally located units in a controversial plan.
There’s one more part of the weather puzzle that is vitally important. PIREPS fill in the blanks of a controller’s situational awareness. Whether you are working with a Center or a Terminal controller, PIREPS are key bits of information. There are certain rules that state when a controller must solicit PIREPs, such as for severe turbulence, icing, or cloud bases, etc. Visual observations from pilots or controllers can be part of a PIREP, too. The reports are sent from a controller to a Center’s TMU directly, or via an operations supervisor. From there the PIREP is sent out for wider distribution. One note that’ll make the system more efficient… when describing icing conditions, an outside temperature is required, and controllers will query for one if they have the time!
Each thunderstorm season, the FAA mounts a significant effort for controllers to review their requirements for dealing with severe weather. Each year we’re reminded about pending litigation resulting from disastrous encounters between aircraft and severe weather, in which controllers were involved. Remember, it’s a team effort; the folks down below want you to avoid severe weather as much as you do.
Ken Kula is a retired 30 year veteran with the FAA, recounting his experiences as a controller, staff specialist and supervisor. His words are not official FAA views, and may not reflect the FAA’s positions.