Weather Support to De-icing Decision Making (WSDDM) is a concept being introduced at many airports across the world, particularly in the USA.
A WSDDM system, developed by scientists at the National Center for Atmospheric Research (NCAR) in California, is based on a complex system of temperature sensors, weather prediction sensors and radar, controlled and monitored by state-of-the-art software.
The system allows winter weather ‘now-casting’ at airports where it is installed and gives a qualitative assessment of weather conditions on a minute-by-minute basis.
This allows airport managers to make crucial operations decisions based on accurate predictions of weather conditions and to modify their decisions based on a minute-by-minute update on how these conditions are changing.
Knowing what the weather is doing at any given time is of great importance to airport operators. For many airports, the ability to follow changes in severe precipitation and storm weather allows them to operate more safely and efficiently.
Of particular value is information that allows airports to carry out de-icing operations efficiently and accurately. These are crucial to the cold weather operation of many US airports. On average, Denver Airport in Colorado has 90in of snow annually, while Minneapolis St Paul in Minnesota has 100in. Both of these airports have operational WSDDM systems installed.
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A WSDDM system is also of great value to runway operations managers who have to coordinate runway snowplough clearance operations and air traffic control staff involved in the gate-hold planning of flights.
WSDDM systems were developed to provide the weather information – in the form of real-time now-casting and short-term forecasts (up to one hour and from one to six hours) – required by airlines, airport air traffic operations and pilots during winter weather.
The sensors for the system are usually installed up to 30km from the airport in all directions. The deployment of the sensors varies depending on the size of the airport and the direction of its prevailing weather patterns. The more sensors that are included in the system, the more refined the weather data provided will be.
The system is totally modular, which allows further sensors to be
added for wider coverage outside the airport or to refine the system (for example if the airport were to construct a new runway).
The WSDDM system’s sensors measure temperature, atmospheric pressure, dew point, and wind speed and direction. A hot-plate snow gauge measures the liquid equivalent of snowfall. Precipitation gauge equipment is also part of the system.
All the sensors update their parameters every minute (now-casting), whereas typically this type of data would be updated every hour. The system works in conjunction with a weather radar system, which can determine the position of rain / snow bands or heavy precipitation approaching the airport and also track how fast these bands are moving.
The tracking information from the radar is used to produce a one-hour forecast of when any precipitation will reach the airport. The radar system reflectivity is calibrated for the liquid equivalent snowfall rate using sensor data from the ground-based snow gauge.
The radar data correlates automatically with the snow gauge every five minutes to show a true value for the type of precipitation – a surface sensor detects the precipitation type – and the snowfall rate.
All of this information is channelled to an integrated control system display on a PC workstation, which Roy Rasmussen, head of WSDDM research at NCAR, calls ‘a one-minute weather manager’. The display depicts the radar reflectivity and surface weather data in a plan view for the local area surrounding the airport.
From the display, airport personnel can use the data to assist airport operations – the system can be used with as little as two hours’ training by non-meteorologists. The system also makes use of wireless communications to provide weather data to users all over the airport.
During the summer, the system can be used for tracking the progress of thunderstorms and typical summer rain events, which, according to Rasmussen, is actually easier for the system than tracking snowfall.
DE-ICING DECISION MAKING
De-icing operations are a major concern for many airport operators. When an aircraft is de-iced, it has a definite time window (hold-over time) during which it must take off.
The hold-over time will depend most, according to NACAR studies, on the liquid equivalent snowfall rate – the de-icing protection time is inversely proportional to the liquid equivalent snowfall rate – and also on the real-time temperature. The effectiveness of the glycol fluids used for de-icing is reduced by an increase in liquid equivalent values.
WSDDM is able to calculate an accurate value for the crucial hold-over time window through its ability to determine the type of precipitation occurring and its liquid equivalent value. The part of the software system that does this is called ‘check-time’ and it is able to calculate how long it will be before a particular de-icing fluid fails in real-time under the prevailing conditions.
An update is provided for all aircraft on the ground every minute. So every aircraft has a check-time figure associated with it. All operations personnel and pilots are made aware of the value for each aircraft so that they can make decisions based on this accurate data. The system can be integrated into new technology such as Total Airport Management Systems (TAMS).
The WSDDM system also has a future-modelling capability. Roy Rasmussen says, "It is a forecast model, and we bring in radar data to help initialise the model. It produces a 0–6-hour forecast for snow, precipitation, temperature and wind speed at the airport, and it is a high-resolution model."
This facilitates the prediction of some of the most dangerous weather conditions and allows airport operators to take any necessary action.
Currently, quoting a visibility level is the standard method used by the National Weather Service for reporting snowfalls. However, this method has been found to be highly problematic (and responsible for a number of de-icing accidents), as it does not take account of the type of snow and when it is observed – small crystals do not block as much light as large, fluffy snowflakes and snowfalls look different at night.
Visibility-based snow intensities should be used cautiously, since they cannot predict the crucial liquid equivalent snowfall rate, which determines the time during which a de-icing solution will be effective.
The WSDDM system allows a scientific and quantitative assessment of the type of snow falling and the liquid equivalent associated with it. This means that an airport can use definitive data to make crucial decisions, thereby increasing safety levels, preventing accidents and saving resources.
The WSDDM system is currently marketed by Weather Support to Decision Making LLC (WSDM), which was recently acquired by Vaisala, a major US weather instrument company.
Vaisala supplies some of the sensor instrumentation for the system that identify the type of snow falling. NACAR, which has been developing WSDDM since 1996, continues to carry out research to improve and refine the system with funding from the US Federal Aviation Authority.
The WSDDM system has become less expensive to implement in recent years, so hopefully it will continue to be adopted at airports across the USA, and eventually across the world.