AMOSSG/4-SN No. 16

26/8/03

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FOURTH MEETING

Montreal, 15 to 18 September 2003

Agenda Item	5:	Aerodrome observation requirements
5.3:	Use of remote sensing techniques

CONVECTIVE ACTIVITY DETECTION (TCU & CB) USING RADAR IMAGERY AND ITS CODING IN AUTO METAR: PRELIMINARY ASSESSMENT

(Presented by Michel Leroy)

SUMMARY
Coding convective activity in METAR uses CB and TCU symbols. AUTO METAR was recently brought in use in France, but to date the AUTO METAR messages do not report convective activity. A study, based on radar imagery and human reporting, was undertaken to assess the feasibility of such a reporting. It shows that the minimal threshold to detect significant convective activity is 33 dBZ and that the detection should be made 30 km around the aerodrome in order to be comparable with human observation. At this stage, AUTO METAR could indicate that convection is detected or not, for example by coding "CB" or not. Further studies will be conducted in order to reduce false-alarm and no-detection rates and to assess the risk level of automatic reporting of convection compared with human reporting. With the help of lightning impacts, a radar reflectivity threshold will have to be found to determine more precisely the level intensity of the convection, in order to distinguish between strong (CB) and moderate (TCU) convection.

1.	INTRODUCTION

1.1	AUTO METAR have recently been brought in use in France on some airports where there was no or irregular observations. This was considered as an improvement of the service by users. Nevertheless, up to now, AUTO METAR cannot report any information on convective activity when there is a clear request from the aeronautical users for an information on convective activity.

1.2	Météo-France has undertaken some studies in order to identify an optimum answer to this requirement, with the following assumptions:

a)	an automatic observation will probably show some differences from a fully human one. These differences are acceptable provided that:

1)	the fact that observation is an automatic one is clearly announced (which is the case with AUTO METAR);

2)	the information provided is as far as possible relevant to the user, while being compliant with Annex 3 4.1.13 and in particular the limitation of the observation techniques; and

3)	the difference between human observation and automatic one are minimized to the best extent possible, while keeping in mind that human observation of convective activity should not be considered as being the absolute truth (for example at night or in case of embedded CB; moreover, one will see further that the existing observation procedures can introduce some uncertainties); one of the goals of the study described hereunder was to get some views on this aspect. In this regard, the risk level of automatic observation (in case of non detection) should not differ significantly from the human observation. Such an aim will allow aeronautical users to make use of AUTO METAR, in particular when they define their operational procedures, in a very similar way they use human METAR; and

b)	the information should be delivered by using the existing messages and codes and with a minimum change of the later, would it be necessary; in this regard, it was considered from the beginning that we should try to use the available codes TCU and CB1 to characterize relevant (for the user), conventional and acknowledged intensity levels of convection, even if one cannot be sure that a human observer would indicate TCU or CB clouds properly said. Thus, CB would be use for "strong" convection, TCU for "moderate" convection.

1.3	Simultaneously, France was implementing the ASPOC tool for ATC, providing a relevant and useful information on convective activity to the ATC. One reminds that this tool was developed mainly by studying the best manner to fulfil ATC needs with the available meteorological radar information. It was considered that, ASPOC being implemented in operations, it was possible to adopt a similar approach with the automatic observation at airport and that the indications delivered in the AUTO METAR had to be consistent with the ASPOC indications.

1.4	At WMO/CaeM/PROMET October 2001 meeting, France proposed objective criteria to identify TCU and CB using radar imagery, in order to report convective phenomena . In this paper, an automatic process is searched to identify such dangerous phenomena, bearing in mind that regular reporting in METAR (TCU and CB coding) would later be used.

1.5	With the aim to search the best compromise between the various constraints described above, studies were undertaken. First, several hypothesis were tested to compare information from radar imagery with human observer reporting. Namely, statistical studies were performed, about reflectivity thresholds, cells area, distance between cells and aerodromes, and so on. These studies dealt with Trappes radar (located about 30 km southwest from Paris) and Roissy and Orly aerodromes, because they have night and day human observations. A full year (2002) of data were considered. At this stage, only detection of convective activity was studied and no attempt was made to identify separately CB or TCU.

1.6	The human observation lasts ten minutes before the reference time. Therefore, radar images acquired on H-5 and H hour were considered. A subset of the ASPOC2 software was completed by a few automatic steps to:

a)	first detect convective cells, list them and build a SIGNORA image (for « SIGNalisation d'ORAge » which means thunderstorm warning) as defined in ASPOC; and

b)	then decide to take them into account depending on their distance to the aerodrome reference point.

In this study, a convective cell is identified by the latitude and longitude coordinates of its centre of gravity, calculated with no weighting — namely cell pixels reflectivity values don't matter. Its area is defined as the number of neighbouring pixels it contains, those pixels values being higher than some thresholds. Thresholds on 30, 33, 40 and 45 dBZ levels were tested (but this is tuneable). Year 2002 METAR were available on a half-hourly basis at Orly and Roissy aerodromes.

As an example, figure 1 below displays Trappes radar imagery, superimposed with lightning impacts and human reporting of convective activity (if any), on July, 30th 2002, 12 UTC. Not only Roissy and Orly are shown, but also all of the aerodromes located in the Trappes radar survey area.

Figure 1: Trappes radar imagery, superimposed with lightning impacts and aerodromes human reporting of convective activity (if any), on July, 30th 2002, 12 UTC

2.	DISCUSSION

2.1	Methodology

2.1.1	METAR coded by human observers, when reporting TCU/CB, were compared with convective cells defined on radar imagery by using a dBZ threshold. In this study, a distance d is defined as the length between the aerodrome reference point and the cell centre of gravity, whatever value of dBZ reflectivity takes this centre. Tests have been performed of varying d distance around each aerodrome reference point, between 0 to 100 km, namely using 5, 10, 15, 20, 30, 40 , 50 and 100 km. For each d value, a contingency table was established between radar imagery thresholds and human observed convectivity. For example, with a max distance d of 30 km between the aerodrome and the cell centre, Orly full year 2002 observations gave the following contingency table:

Table 1	No cell	≥ 30 dBZ	≥ 33 dBZ	≥ 40 dBZ	≥ 45 dBZ
Neither TCU nor CB	15515	483	468	87	10
TCU	168	39	72	29	9
CB	52	24	62	59	49
TCU and CB	0	1	2	3	2

2.2	Table cells were grouped in 2 classes for both kinds of data following:

a)	TCU/CB are reported or not by human observation; and

b)	a reflectivity threshold is used to define 2 classes of radar imagery data.

Then, using a threshold value of 33 dBZ and a max d of 30 km, the former contingency table concerning Orly 2002 became:

Table 2	RADAR
METAR	Cell ≥ 33 dBZ	No cell
TCU and/or CB	287 (a)	284 (b)
Neither TCU nor CB	565 (c)	15998 (d)

2.3	Statistical tests

2.3.1	To determine which reflectivity threshold should be used to detect convective activity and which distance should be considered in the radar imagery automatic process, statistical tests were performed on these contingency tables and indexes were calculated.

2.3.2	In addition, false-alarm and no-detection rates qualify the over- or under-detection of convective activity by the radar imagery process, compared with the human observations. They are defined as :

no detection rate = NDR = b/(a+b)

false alarm rate = FAR = c/(a+c)

where a, b, c and d are contingency table cells values in table 2.

2.4	Main results

2.4.1	Concerning Orly aerodrome, figure 2 shows FAR and NDR indexes as a function of d distance, using a reflectivity threshold of 33 dBZ to discriminate convective activity on radar imagery.

2.4.2	Analysis of this graph, and similar graphs about other aerodromes in the Trappes radar survey area alike, led to the conclusion that, in this process, 30 km seems to be the most pertinent distance to use.

2.4.3	Figure 3 displays false-alarm and no-detection rates as a function of reflectivity, and shows that 33 dBZ should be chosen as a convective activity discriminative threshold.

2.4.4	These results concerning the convective activity discriminative threshold and the most pertinent distance to use could be changed according to the weight of FAR or NDR chosen by user. In this study, we give the same weight to FAR and to NDR. Nevertheless, before doing this, we consider that it worth it to try to better understand and probably reduce FAR and NDR; some indices are given hereunder based on some case studies.

2.4.5	Finally, extra sensitivity tests have shown that the area of the so-called "convective" radar imagery cells appears not to be linked with the false-alarm rate (FAR).

2.5	Case studies

2.5.1	In order to better understand the discrepancies between the human observations and what would have been reported with an automatic detection of convective activity based on the method described above, four situations showing differences between radar imagery and human observation were more deeply examined to explain their false-alarm and no-detection rates. Therefore, ground lightning data were superimposed on both radar and infrared satellite imagery. METAR emitted from other aerodromes in the Trappes radar survey area have also been used. Those case studies highlight advantages and drawbacks of information deduced from radar compared to human observation.

2.5.2	January 15th, 2002: Analysis of radar imagery from 10 to 12 UTC shows reflectivity values above 45 dBZ although none of the aerodromes reports any convective activity. Moreover, there are no ground lightning impacts and infrared temperatures displayed on satellite images are medium-valued, not being typical of convective situation. In fact, those high-level reflectivities correspond to strong precipitation rates in an active perturbation. Then, AUTO METAR would have reported convective activity where there were only high rainfall. It is expected that an appropriate combined use of radar imagery, lightning detection and possibly satellite imagery could reduce false alarm in such cases; this will be investigated further. If an AUTO METAR would have been coded, it would have reported a false-alarm, but this would not have been detrimental to safety.

2.5.3	June 3rd, 2002: On 3:30 UTC radar imagery, a 45+ dBZ cell is located 53 km from Orly (reporting CB in METAR) and 59 km from Roissy (no TCU/CB reporting). Together with many ground lightning impacts, very low infrared temperatures on satellite imagery are associated with radar defined cells. All of these are signs of high top convective clouds. In this case, AUTO METAR wouldn't report any convective activity, as the distance between the aerodromes and the cell is more than 30 km. Despite of the distance of this potentially dangerous phenomenon, the on duty human observer at Orly aerodrome preferred to report it, leading to a difference between METAR and AUTO METAR. This case shows that the existing practice according to which a human observer has to report CB without consideration of its distance from the airport can generate differences between human and automatic reporting of convective activity, both being valid in their context.

2.5.4	July 30th, 2002 :As shown on figure 1, a well defined squall line lies along all of the aerodromes in the area (Roissy, Orly, Melun, Beauvais). Only Orly METAR doesn't report convectivity. Yet a high reflectivity cell lies 3 km from Roissy and an other one is 20 km east from Orly. Many lightning impacts are superimposed on the squall line. Infrared temperatures confirm the convective situation. In this case, the observer doesn't mention any convective activity, even if convective cells on radar imagery are located within a 30 km distance from the aerodrome. In some clearly convective situations, radar information could thus be more pertinent to pilots than the human observer reporting, which doesn't seem to see convection, for some unknown reason. Such a case illustrates that the use of modern techniques can improve an information which is important for the safety of flights.

2.5.5	November 4th, 2002: On 12:30 UTC, Roissy METAR reports CB and Orly reports TCU. Only a low reflectivity cell (<33 dBZ) can be seen 32 km apart from Roissy and 23 km from Orly. An other cell with a reflectivity between 30 and 33 dBZ appears 16 km from Orly alone. No convective activity can be figured out from the infrared satellite image. It seems that both observers overestimated the convective clouds they saw during this showering situation. In this case, AUTO METAR wouldn't mention any convectivity. Here, the human observation is in favor of safety, but was it relevant? More investigation have to be conducted in such case, using other sources of information in order to better assess the level of convective activity. Anyway, user's feedback would be also helpful.

3.	CONCLUSIONS

3.1	To report convective activity in AUTO METAR from radar reflectivities, the most pertinent distance to use, in order to match METAR and AUTO METAR, is 30 km, and a 33 dBZ threshold discriminates potentially dangerous cells, as far as aeronautics is concerned. Based on these assumptions, it is already possible to report convective activity relevant to aeronautical users in AUTO METAR, in a coherent manner with ASPOC ATC tool, and with a good reliability. But we can only report "there is significant convection" or "there is no significant convection". This could be done by coding CB in AUTO METAR when significant convective activity is identified, but this would not mean that Cb clouds are present, while most probably there are (this work mode is for us fully in line with Annex 3, 4.1.13). At this stage, TCU would not be used because we are not able yet to identify which level of radar signal (if any) would be relevant for aviation users Further studies are necessary in order to discriminate between "strong" convection (which would be coded CB) and "moderate" convection (which would be coded TCU). As above for Cb clouds, the coding of TCU in AUTO METAR would not imply existing TCu clouds, but only a given level of convection activity, which is acceptable because the user is well informed of the nature of the observation. A more precise and quantified expression of the users needs would be welcome in this regard.

3.2	Extra studies, taking into account other sources of remote sensing information, will be performed to:

a)	decrease false-alarm; and

b)	explain no-detection rates (in comparison with human observation) and identify if the risk level of automatic detection can be compared with those of human observation.

3.3	We will also try sorting out TCU from CB. A reflectivity threshold will be searched, using samples when at the same time METAR report convectivity and >33 dBZ cells can be detected on the radar imagery. Ground lightning impacts are expected to be of great help in the process of determining if CB are present, and they will be used in the future.

4.	PROPOSAL FOR COMPLEMENTING METAR BY CONVECTIVE INFORMATION

4.1	For France, the most convenient way to introduce, in AUTO METAR, automatically deduced convective information consists in find on a global level, which aerodrome is concerned by thunderstorms and TCU/CB. Therefore, lightning impacts and mosaic radar imagery would be used to code TS/VCTS and/or CB/TCU. This information would then be dispatched to each local aerodrome to produce AUTO METAR and other legal local weather reports.

4.2	In METAR coding, the type of convective clouds must be linked with their cover (NNN) and base height (hhh). However, convective activity detection as described above, and based on the existing radar network in France and its mode of current operations, cannot give any clue on those items. Nevertheless, convection in AUTO METAR could be coded as: /////TCU or /////CB.

4.3	It should also be noted that, at present, the METAR code does not allow to clearly indicate that the reporting of convective activity (CB and TCU) is NOT possible. When reading an AUTO METAR, no difference can be made by the user between "It is technically impossible to report convective activity in this message, either there is or not, and that the reason why you do not read CB or TCU in this message" and "There is no convective activity at the airport, and that's why you do not see CB and TCU in this message". ICAO should draw attention of WMO on this matter so that it can be fixed. The distinction is necessary because one could well imagine that automatic weather stations will be implemented at some airports not covered by a weather radar (making impossible to report any convective activity by automatic means), while others will be. Amendment 73 of Annex 3 could provide a solution.

4.4	Similarly to the coding of CB/TCU in AUTO METAR, one can consider the coding of TS based on automatic detection. Depending on their distance to the aerodrome, thunderstoms are coded with TS or VCTS. Then a thunderstorm located between 0 and 8 km would be coded TS while VCTS would be used if the thunderstom is detected in the 8-16 km range. The detection would be based on lightning detection systems.

5.	ACTION BY THE GROUP

5.1	The group is invited to note the content of the paper; all views about its content are welcome.

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