High Resolution Clear-Air Turbulence Observations With Mode S

Clear-Air Turbulence (CAT) is one of the two significant weather hazards for aviation in the upper airspace. Despite this and over 60 years of research we still have not gathered sufficient knowledge about the detailed mechanisms that are important for this phenomenon nor have we found out how to forecast it accurately. Moreover recent research indicates that with the climate warming the frequency and intensity of turbulence outbreaks will increase over one of the most important air traffic corridors - North Atlantic.

One of the causes of this state of affairs is a very poor data coverage. There have not been many field campaigns aimed at measuring CAT in the recent years but more importantly the one systematic source of data about turbulence - the commercial aircrafts have simply not been supplying the kind of data necessary for the turbulence research. The main source of information - PIREPs was proven to be very inaccurate. It has been mostly replaced by AMDAR/ACARS but those were broadcasted every 10-20min during the flights cruise phase - this means roughly every 150-300km. That was, of course not satisfactory and in recent years some aircrafts have been reporgrammed to broadcast dense EDR reports (up to every 1 min). However most of this change is taking place over the continental US. The rest of the world (including Europe) is enjoying very sparse data coverage at best. Moreover it seems that installation of dense EDR reporting aboard sufficiently many aircrafts will be a lengthy process not guarantying good access to the data for the scientific community in the end (see MetOffice and British Airways).

Hence the question remains: is it possible to get good data coverage over Europe with satisfactory spatial and temporal evolution so that research is possible using a source of data that is potentially widely available? The answer is yes. The data source we propose to use here is Mode S - short binary messages broadcasted openly by the aircrafts in order to assist the Air Traffic Control (ATC). These messages have been shown to contain satisfactorily accurate horizontal wind information with temporal resolution of few seconds (that is approx. 1km). Almost every commercial aircraft has a transponder allowing for broadcast of Mode S. In order for the data to be broadcast the only requirement is the presence of an ATC interrogator nearby. This unfortunately renders seas and oceans uncovered but allows very good coverage over most of Europe (total coverage of the continent is due 2020). Therefore one obtains very dense high resolution horizontal wind measurements from this source of data. The other advantage of this data is the fact that one can obtain it with an off-shelf receiver that costs approx. 500EUR and gather information locally, share it with other scientists or set up a network of such receivers to gather data from larger area.

R.D. Sharman and R. Frehlich have shown that given the availability of wind measurements one can estimate the magnitude of the Eddy Dissipation Rate (EDR) - a well established turbulence intensity measure. However they were working on ACARS data that were characterized by very low spatial resolution. We propose to apply their method to the Mode S data. In the present paper we present the results of application of this method to data obtained during the DELICAT project flight campaign and few other flights where a NLR owned Cessna Citation measured CAT.