AMOSSG/4-SN No. 8

25/8/03

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FOURTH MEETING

Montreal, 15 to 18 September 2003

Agenda Item	5:	Aerodrome observation requirements
5.1:	Reporting requirements and criteria

USE OF REMOTE SENSING TECHNIQUES

(Presented by Michel Leroy)

SUMMARY
A brief description of the possible use of remote sensing techniques is given (weather radar, lightning network, satellite, wind profilers, lidar). These techniques give the possibility of improving the generation of automatic aerodrome observations, but use of specific products or images would be the best way to get the most from such systems.

1.	INTRODUCTION

1.1	This paper is a tentative list of remote sensing techniques, with some identification of the possibility of improving automatic aerodrome observations.

1.2	The text of this paper is mainly extracted from the draft manual on Automatic Meteorological Observing Systems at Aerodromes.

2.	DISCUSSION

2.1	Some systems offer interesting possibilities for pilots and aeronautical users. However, there is an obstacle to getting the most out of the information they offer and that is the current method used to broadcast meteorological information: local reports and METAR/SPECI messages. The advantage of these reports is that they are simple, clearly defined and therefore recognized by the various people in the aeronautical industry. METAR/SPECI messages can also be broadcast by all transmission media, even the most simple or crude, which is useful when communication infrastructures are poor. The trouble is that information is lost when data from remote sensing systems is reduced to a few characters in an alphanumeric message. Nevertheless, this process is sometimes necessary to access information easily. A typical example of this problem is the detection of convective cells using a precipitation radar from which information can be extracted on the presence of CB/TCU clouds (see cloud chapter), but a radar image prepared to show these major reflectivity zones contains more detailed information on the extent of the zone, and its movement and severity. This would therefore be very useful for both the pilot and control officer, who could anticipate possible diversions. Such observation methods are sometimes used but are not currently standardized.

2.2	Weather radar images

2.2.1	A weather precipitation radar detects the presence of precipitation (and sometimes even clouds), and quantifies the intensity of precipitation. Very convective cloud cells are clearly visible and result in high reflectivity levels. A proposal is being examined by the WMO to define CB and TCU clouds, or more precisely convective clouds that are bothersome or dangerous to aircraft, using reflectivity levels.

2.2.2	A disadvantage of this method is that high levels of reflectivity can also exist during disturbance periods with heavy precipitation, without the presence of CB/TCU clouds. The combination of radar images with infrared satellite images can fine--tune the diagnostic seen by satellite, since CB and TCU clouds have a large vertical extent so the temperature at their tops is low.

2.2.3	Development is underway in several countries to extract information on convective clouds from radar and satellite images and integrate it into METAR/SPECI messages and local reports. There are however coding difficulties : how to mix an information about CB/TCU without knowing their height of base within a cloud group, which should have information on extend, height and cloud type.

2.2.4	An objective definition of the area around the airport, in which CB/TCU clouds must be indicated, is also necessary. This area should perhaps be related to the area where a thunderstorm might be detected (TS or VCTS). Since a definition does not exist at present, it is very possible for the presence of CB clouds to be indicated by a human observer when he sees lightning, even if far away (a distance of 100 km has been seen as possible).

2.2.5	A radar image or a radar image composite is not always perfect and can contains errors such as echoes from stationary objects, extremely widespread echoes or a bright band (liquid/solid transition). Techniques used to reduce these errors cannot be described here; researchers around the world are looking for solutions. A "raw" image must often be interpreted by a professional meteorologist and does not always meet aeronautical operational needs. Rather than using a "raw" image, it is more effective to prepare an image that can more easily be used as is. France is experimenting and using such a product with four reflectivity thresholds and a smoothing of convective zones, with the possibility of superimposing lightning information. This product makes it possible for control officers, through their knowledge of high reflectivity zones, to anticipate the possible deviation of aircraft, which often have their own onboard radar providing analogue information. This sort of image therefore contains much more extensive information than the presence or absence of CB clouds in a METAR, though local reports and METAR/SPECI messages are still important in identifying cloud type.

2.3	Lightning networks

2.3.1	There are networks that detect the impact of lightning based on its electromagnetic signature. The time of detection of a lightning bolt's electromagnetic pulse is measured by a sensor network, which locates the lightning. The most widely-used technology detects cloud-ground lightning with a precision of location that depends on the density of the network and its topology, and that can, for example, reach 1 km, with sensors spaced at 200 km. More localized cloud-ground and intra-cloud lightning detection systems also exist.

2.3.2	This type of network is the best way of determining the electric activity associated with thunderstorms and is a valuable complementary tool for meteorologists, situation analyses and forecasts. It makes it possible to locate electrical activity in real time with excellent precision of location (at airport level). This information makes it possible to report to an automatic system the presence of a thunderstorm at the airport or in its vicinity. The technical difficulty lies in transmitting the information in real time from the central computer of a lightning network to an automatic observation system at the airport. Luckily, the evolution of communication media is offering more and more technical possibilities. Developments consisting in using information from a lightning network to report present weather TS (or VCTS) and possibly the presence of CB clouds in local and METAR reports are underway in several States.

2.4	Satellite images

2.4.1	Satellite images are very useful for meteorological analyses and forecasts. At the local level, they make it possible to better determine the meteorological situation. Infrared images allow the measurement of the temperature at the top of clouds and, combined with high radar reflectivities, the identification of thunderstorm cells. This is because CB (and TCU) clouds extend a long way vertically, so their cloud top is high and cold. Instruments that automatically identify thunderstorm cells have been developed in some countries and can be used, along with radar images, to identify cloud type.

2.4.2	Using satellite images with various wave lengths also makes it possible to detect low cloud zones and/or fog (it is difficult to distinguish between low clouds and fog). These images are an important tool in helping to write forecasts (TAFs), but for the moment are not used directly for local observations or METAR messages.

2.5	Wind profilers

2.5.1	These measure the vertical profile of upper wind. They can be useful for sites subject to upper wind shear.

2.5.2	There are two types of wind profiler: ultrasound (SODAR) or electromagnetic waves (UHF radar). An antenna system emits pulses in several vertical directions. Part of the signal emitted is back-scattered by small inhomogeneities in the atmosphere (such as variations in the refractive index), to the antenna system, now serving as a receiver. The time the signal takes to return determines the distance. The frequency of the signal shifts according to the radial movement of the atmospheric zone that back-scattered the signal (Doppler effect). The combination of radial speeds in the different pulse directions (at least three) makes it possible to calculate the horizontal speed in different altitude bands.

2.5.3	These instruments can calculate high-frequency profiles, every 10 minutes for example, ensuring a follow-up in real time. Profiles can contain errors caused by parasitic signals, so filtering algorithms are required. These algorithms mostly use the preceding profiles to monitor the temporal coherence of successive profiles. The output of such systems is typically a temporal succession of wind profiles represented as vectors. It is possible to set wind shear thresholds to extract a synthetic indication that can be used locally by the control officer and possibly included in a METAR/SPECI as an additional observation.

2.6	Lidar

2.6.1	A lidar emits a laser light pulse and analyses the return signal back-scattered by the atmosphere in one or more directions, from which it can deduce the wind, the extinction coefficient and other parameters. A slant measurement from a distance would make it possible to measure the wind above the runway or in the approach zone. Unfortunately, the optical signal must not be reduced by clouds or fog, in which case the instrument is blinded and its usefulness diminished. Slant visibility could possibly be measured, but with limitations and performances difficult to assess. This type of instrument is expensive and is used only for research. A laser ceilometer is a specific lidar designed to detect clouds, so being blinded behind a cloud is not a problem.

3.	CONCLUSION

3.1	There exist possibilities to use remote sensing techniques to derive information for improving the generation of automatic aerodrome observations, mainly convective activity and thunderstorms.

3.2	But such systems are capable to get a much more complete information, generally graphic displays, which are not identified as standard or recommended products in Annex 3. Nevertheless, such graphic displays would give a very rich and useful information, e.g. mapping of convective activity at and around the aerodrome.

4.	ACTION BY THE GROUP

4.1	The group is invited to:

a)	note the content of the paper;

b)	discuss the opportunity to develop proposals for an amendment to Annex 3, taking into account the future guidance material (manual on the use of automated observing equipment at aerodromes);

c)	discuss the possibility to mention in Annex 3 the usefulness of some specific products presentation (displays), not following the current form of METAR/SPECI or local reports.

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