AMOFSG/8-SN No. 40 5/1/10

AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

EIGHTH MEETING

Melbourne, Australia, 15 to 18 February 2010

Agenda Item 5: Observing and forecasting at the aerodrome and in the terminal area 5.1: Aerodrome observations

REPORTING OF DUSTSTORMS AND SANDSTORMS

(Presented by Bill Maynard and Bryan Boase)

SUMMARY

This paper proposes that a clear definition for the meteorological observation and reporting of duststorms and sandstorms be determined.

1. INTRODUCTION

1.1 The International Civil Aviation Organization (ICAO) formed the Aerodrome Meteorological Observation and Forecast Study Group (AMOFSG) to assist the ICAO (MET) Secretariat in the development and review of the international standards and recommended practices related to meteorological information, as established in ICAO Annex 3 — Meteorological Service for International Air Navigation.

1.2 The minutes of the third meeting of the 178th session of the Air Navigation Commission include the following discussion related to duststorms and sandstorms (in part, paragraphs 18 to 33 refer):

…[a] Commissioner was reluctant to agree that items b) (severe squall lines), e) (widespread sandstorm/duststorm) and j) (accidental release of radioactive materials into the ) should be deleted. This reluctance was shared by several Commissioners, one of whom understood that in some areas of the world it was important to provide information on sandstorms and duststorms….

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…On the subject of the proposed deletion of three items from paragraph 1.3.3, based on an informal poll, it was decided that the list of items would be retained as currently reflected in Appendix 2 of Annex 3…

1.3 It should be noted that there is a parallel, and potentially overlapping, effort by the Meteorological Warnings Study Group (METWSG) to clarify the criteria for issuance of SIGMET for duststorms and sandstorms (note Action 2-2 for that group). The World Area Forecast System Operations Study Group (WAFSOPSG), based on 1.2, must also consider how to include information related to these phenomena in significant charts (SIGWX). It is therefore critical to clearly define criteria for meteorological observations and reports relating to duststorms and sandstorms.

2. DISCUSSION

2.1 The World Meteorological Organization (WMO) defines duststorms and sandstorms in WMO No. 407 - Manual on the Observation of and other Meteors and in WMO No. 306 – Manual on Codes, as follows:

An ensemble of particles of and energetically lifted to great heights by a strong and turbulent (WMO No. 407)

A restriction "less than 1,000 metres" shall be applied to [code figures] 30-35. (WMO No. 306, Paragraph 12.2.6.5.10)

30 to 32 - Slight or moderate duststorm or sandstorm 33 to 35 - Severe duststorm or sandstorm (WMO No. 306, Code Table 4677)

Code Symbol Description Slight or moderate duststorm or sandstorm 30 (has decreased during the preceding hour) Slight or moderate duststorm or sandstorm 31 (no appreciable change during the preceding hour) Slight or moderate duststorm or sandstorm 32 (has begun or increased during the preceding hour) Severe duststorm or sandstorm 33 (has decreased during the preceding hour) Severe duststorm or sandstorm 34 (has no appreciable change during the preceding hour) Severe duststorm or sandstorm 35 (has begun or increased during the preceding hour)

2.2 It should be noted that if the visibility is observed to be 1000m or greater it would be reported as either blowing dust or blowing sand (BLDU or BLSA). It should also be noted that WMO does not currently distinguish between duststorm and sandstorm in terms of their code form or symbol.

2.3 The Australian definitions of duststorms and sandstorms are as follows:

Dust or sand storm - caused by turbulent raising large quantities of dust or sand into the air and reducing visibility to less than 1000 metres. Severe dust storm (or sandstorm) reduces visibility below 200 metres. The difference between a dust storm and

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a sandstorm is the size of the particles. A dust storm consists of fine particles, often raised to great heights (3000 metres), which may be carried great distances from the source. A sandstorm consists of coarse particles, which are not usually raised to a great height or carried far from the source. (Bureau of Meteorology Surface Observations Handbook)

Slight or moderate duststorm/sandstorm has visibility < 1000m but not < 200m. Severe duststorm/sandstorm has visibility < 200m. (Bureau of Meteorology Codes Handbook)

On this assumption the METAR/SPECI in Australia uses DS/SS when the visibility is 200m to < 1000m and +DS/+SS when the visibility is < 200m.

2.4 The Canadian definitions for dust and sand phenomena are:

Blowing Dust (BLDU) or Blowing Sand (BLSA). Dust or sand, raised by the wind to moderate heights above the ground. If the visibility is reduced to 1/4 of a statute mile or less, blowing dust and blowing sand will be reported as heavy (+BLDU, +BLSA). The visibility at eye level is sensibly reduced.

Duststorm (DS). Dust raised to great heights by a strong turbulent wind. The forward portion of the storm may have the appearance of a wide high wall. The visibility at eye level is reduced to 1/2 of a statute mile or less. If the visibility is reduced to 1/4 of a statute mile or less, the phenomena will be reported as heavy (+DS).

Sandstorm (SS). Sand raised to great heights by a strong turbulent wind. The forward portion of the storm may have the appearance of a wide high wall. The visibility at eye level is reduced to 1/2 of a statute mile or less. If the visibility is reduced to 1/4 of a statute mile or less, the phenomena will be reported as heavy (+SS).

Intensity shall be indicated only with , showers, precipitation associated with , blowing dust, sand or , duststorm or sandstorm. In Canada tornadoes or shall be reported using +FC. Canada will not use the "-" indicator for BLDU, BLSN, DS or SS; all occurrences of these phenomena shall be reported with no indicator or a + indicator i.e. BLDU or +BLDU.

2.5 The American Meteorological Society glossary defines duststorms and sandstorms as follows:

Duststorm. An unusual, frequently condition characterized by strong winds and dust-filled air over an extensive area. Prerequisite to a duststorm is a period of over an area of normally arable land, thus providing the very fine particles of dust that distinguish it from the more common sandstorm of regions. A duststorm usually arises suddenly in the form of an advancing dust wall that may be many kilometers long and a kilometer or so deep, ahead of which there may be some dust whirls, either detached or merging with the main mass. Ahead of the dust wall the air is very hot and the wind is light. In U.S. weather observing practice, if blowing dust reduces visibility to between 5/8 and 5/16 statute mile, a “duststorm” is reported; if the visibility is reduced to below 5/16 statute mile, it is reported as a “severe duststorm.” Duststorm winds can also be associated with outflows and gust fronts. While these are

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often shorter-lived than synoptically forced duststorms, they can be quite intense, with an impressive leading edge of the dusty gust front, sometimes called a dust wall.

Sandstorm. A strong wind carrying sand through the air. The diameter of most of the particles ranges from 0.08 to 1 mm. In contrast to a duststorm the sand particles are mostly confined to the lowest 3.5 m (10 ft), rarely rise more than 15 m (50 ft) above the ground, and proceed mainly in a series of leaps (saltation). Sandstorms are best developed in desert regions where there is loose sand, often in , without much admixture of dust. Sandstorms are due to strong winds caused or enhanced by surface heating and tend to form during the day and die out at night.

2.6 There is also a question related to how to address the current definition that a duststorm has a great vertical extent. It is suggested that consideration be given to adding a sky condition criteria (presumably confirmed by a METAR / SPECI with VV) to any revised definition to address this issue. This will also ensure that users are aware that the ground is not likely to be visible from the air.

2.7 It should be stressed that the qualifier LIGHT (-) should not be used as it tends to detract from the potential severity of these phenomena. This is currently a recommendation in Annex 3, Appendix 3, paragraph 4.4.2.6 where it states that ‘… Light intensity should be indicated only for precipitation.’

2.8 It is critical that international consistency regarding the definition of meteorological phenomena be obtained. It is also critical that there is consistency between the terms used for meteorological observations and those used for forecasts and warnings, which in turn will allow verification of the particular weather element.

3. RECOMMENDATIONS

3.1 The following recommendations are made with respect to duststorm and sandstorm coding in aeronautical meteorological observations and reports:

a) given that WMO does not distinguish between a duststorm or a sandstorm in their code form or symbol, sandstorm (SS) and blowing sand (BLSA) should be discontinued and duststorm (DS) and blowing dust (BLDU) should be used in all cases.

b) codes for aeronautical meteorological observations and reports should take the form:

• DS for wind driven dust and/or sand causing visibility less than 1 000 m and fully obscured sky (VV). • +DS for wind driven dust and/or sand causing visibility less than 500 m and fully obscured sky (VV).

c) proposed amendment to Annex 3, Appendix 3, paragraph 4.4.2.3(d):

d) Other phenomema .... Duststorm/sandstorm DS

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– Reported when the visibility is less than 1 000m and the sky is obscured. The qualifier + (heavy) is used when the visibility is less than 500m.

3.2 Liaison with WMO is recommended to ensure that there is a clear definition for internation observing practice between a duststorm/sandstorm and a heavy duststorm/sandstorm.

4. ACTION BY THE AMOFSG

4.1 The AMOFSG is invited to:

a) note the information in this paper; and

b) consider the recommendations provided.

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