ADS-B coverage in the U.S. Image courtesy of Google
Pilots and crew now depend on having some kind of datalink service in the cockpit. It’s just about as ubiquitous as GPS is these days. These datalink systems can be certified systems integrated directly into the aircraft’s panel-mounted avionics as well as portable solutions. If you have a datalink solution for flight information services broadcast (FIS-B) in your cockpit, you’ll be happy to know (as we are knee-deep in thunderstorm season) there are six new weather products being broadcast starting mid-year in the lower 48 U.S. states.
Pilots have basically two choices when it comes to datalink weather. The two options currently available are those from a satellite-based system offered through SiriusXM and a ground-based system offer through the FAA’s FIS-B that is part of the “ADS-B In” cockpit advisory services. FIS-B transmits graphical NWS products, temporary flight restrictions and special-use airspace information via universal access transceiver (UAT) equipment on 978 mHz. Both options do require a hardware purchase, but only SiriusXM requires a subscription, making it more expensive in the long run.
Last year I had the pleasure of being a member of the Radio Technical Commission for Aeronautics (RTCA) SC-206 working group. This is just one of many working groups that make up the Aeronautical Information and Meteorological Data Link Services organization of the RTCA.
Initially the working group was set to deliver four new products in late 2017 to include lightning as well as a forecast for cloud tops, icing and turbulence. During the initial phase of study, I requested that the working group also consider adding center weather advisories (CWAs) and graphical AIRMETs (G-AIRMETs). To avoid going through multiple development phases, it was decided to delay the delivery and combine these two efforts. Key Site deployment (first time these products will be broadcasted by external radios) started May 9, with remaining deployment across the country projected to be completed by June 7.
Perhaps the most useful addition to the broadcast is lightning. Any weather that exhibits lightning will also be fraught with dangerous convective turbulence. It’s important to understand that the broadcast includes only cloud-to-ground lightning strikes. So don’t become complacent; many of the storms in the central plains, for example, have a 10-to-1 ratio of intra-cloud strikes to cloud-to-ground strikes where this ratio can become infinite for short periods of time in some severe thunderstorms.
Due to bandwidth limitations, you won’t get the position of every cloud-to-ground lightning strike that occurs. Instead you will get a gridded pattern of lightning strikes at a 2-km resolution with the number of strikes in that grid is being passed along as well in the broadcast.
This is a 1- or 2-hour forecast of the icing condition that includes probability and severity. You may know this as the Forecast Icing Product (FIP) found on aviationweather.gov. The most significant issue with this product is its age. It’s only refreshed once each hour (although it is transmitted every 15 minutes). When the latest update is received, you will stare at that same image for another hour. Icing can be very transient at times, and it’s likely that an hour-old forecast may not be very relevant.
You may know this as the Graphical Turbulence Guidance (GTG) product found on aviationweather.gov. It provides an aircraft-independent forecast called eddy dissipation rate (EDR). An atmosphere that’s dissipating eddies quickly is one that is turbulent. Aircraft class is essential to use EDR. Similar to icing, it’s a 1- or 2-hour forecast, but only extends up to FL240. It’s transmitted every 15 minutes, but will only be updated once each hour and has the same issues with age as the icing forecast.
These are known as G-AIRMETs. They are not just a graphical depiction of an AIRMET. In fact, the textual AIRMET is now a byproduct of the G-AIRMET forecast, which has no associated text, just some metadata. Like the retirement of the area forecast last October, the AIRMET is also on the chopping block. So broadcasting the G-AIRMET in place of the AIRMET is just one step in a forward direction to reduce the dependency on the legacy AIRMET so it can be retired.
Since March 2010, the G-AIRMET has been the primary operational product issued four times a day by forecasters at the Aviation Weather Center (AWC). The legacy AIRMET is a time-smeared forecast valid over a six-hour period. This created some very large and not-so-useful en route advisories for turbulence, icing and IFR conditions.
Conversely, the G-AIRMET consists of five snapshots, each valid at a single time to include the initial snapshot and four forecast snapshots valid in 3, 6, 9 and 12 hours from the valid time of the initial snapshot. Therefore, each snapshot defines the coverage of that particular aviation hazard (e.g., icing) at a particular time creating a better and more useful spatial and temporal resolution. If you are wondering, text for the legacy AIRMET is still automatically generated by taking the union of the first three G-AIRMET snapshots (initial, 3 hour and 6 hour).
Center Weather Advisories
Center Weather Advisories (CWAs) are issued by meteorologists at the Center Weather Service Units (CWSUs) located at the Air Route Traffic Control Centers (ARTCCs) throughout the country. Like SIGMETs, they are not scheduled and are issued only when conditions warrant. I have always been perplexed why CWAs were not already one of the products broadcast since they have such a short lead time and are usually valid for no more than two hours. In other words, they represent more of a NOWcast rather than a forecast and describe conditions that are evolving as you are flying to your destination.
CWAs are issued in concert with existing G-AIRMETs and SIGMETs. They can be issued whenever an area of weather develops that is dangerous, but doesn’t meet national SIGMET or G-AIRMET criteria. Also they can be issued to augment an existing G-AIRMET. For example, a G-AIRMET for IFR conditions advises a pilot about the potential for widespread IFR conditions (visibility below three statute miles and ceilings less than 1,000 feet). However, CWAs are often issued within the bounds of an IFR G-AIRMET when there are a significant number of airports reporting low IFR conditions (visibility at or below one statute mile and ceilings at or below 500 feet).
Knowing the cloud tops is likely the holy grail of aviation weather. But don’t get too excited here. These cloud tops are not based on observations such as those you can get from infrared satellite data. Instead, what you will see is a one-hour forecast from the high resolution rapid refresh (HRRR) model. This may do reasonably well with stratiform or nimbostratus tops, but not so well with areas of moist convection such as a broken field of cumulus clouds or even cumulonimbus clouds.
Update and transmission interval for new FIS-B weather products.
Whether or not you can view one or more of the six new weather products depends solely on the vendor that processes the data transmitted to your ADS-B In receiver. The vendor has to write software to process the data received from the hardware and render it in on your display. Moreover, your software vendor may not render all of the data that is available in the broadcast. For example, the lightning product includes the number of strikes within each grid, but your particular vendor may simply show a single lightning bolt and may choose to ignore how much activity occurred in that grid.
In the end, FIS-B does provide pilots with the opportunity to get near-real time en route weather updates including graphical depictions without the need to call a flight service station specialist on the radio. This solution will suffice until the next major milestone is hit, namely, broadband internet connectivity in the cockpit.
Scott Dennstaedt is a CFI and former NWS meteorologist and co-founder of WeatherSpork.