COMMISSION FOR BASIC SYSTEMS
MEETING OF EXPERT TEAM ON DATA
REPRESENTATION AND CODES
MUSCAT, OMAN, 5 - 8 DECEMBER 2005
ET DR&C/Doc. 4.3(1)
Submitted by UKMO
Summary and Purpose of Document
This document considers coding issues associated with medium level instability, and the coding of Cb.
The ET DR&C meeting is kindly asked to consider and discuss this document which may lead to adjustment of regulations.
Earlier this year discussions took place on coding issues associated with medium level instability, and the coding of Cb. The Met Office College issued an agreed clarification on the Forecasting Instructions in July, outlined below.
Thunderstorms resulting from Medium Level Instability
Altocumulus Castellanus is often a precursor of thundery activity but does not in itself result in a thunderstorm. Medium level Cumulonimbus can and frequently do develop from Ac Cas in which case the cloud form is described as Cumulonimbus-Altocumulusgenitus. The fact that the base of the resulting Cumulonimbus may be in excess of 6500 feet should not preclude it being reported as Cumulonimbus.
Standard: When thunderstorms are observed/forecast to be occurring, at least 1 okta of Cumulonimbus must be reported/forecast, even if the base of the cloud is in excess of the nominally accepted range of low cloud types (up to 6500 FT).
Lightning seen (SYNOP present weather code 13) does not, of itself, constitute a thunderstorm in progress and does not necessarily require the reporting of a Cb. The standard will apply to all SYNOP and METAR observations, TAF format forecasts and all variants of Cross Section, Area Forecast and Significant Weather products. All Met Office training literature, the Defence Services Manual and the Civil Aviation Services Manual will, where appropriate, be updated accordingly.
In addition, it was stated that discussion and clarification of the rules governing the reporting of Cb in special weather conditions would be addressed. Representatives from Aviation Services, Defence and Surface Networks have agreed the following procedure, which should be adhered to with immediate effect and WMO have been informed.
The cloud type Cumulonimbus must be reported whenever a thunderstorm is in progress
Where it is impossible to determine a separate and distinct amount of Cumulonimbus cloud, due for example to a layer of lower cloud, then that layer of lower cloud will be reported as Cumulonimbus.
Example 1 8 oktas at 1000FT, identified as Stratus prior to commencement of the thunderstorm. Thunderstorm commences, and after attempts to determine an identifiable amount/base of CB fail, the cloud will be reported as ‘OVC010CB’ in METAR and as ‘88910’ in SYNOP.
Example 2 8 oktas at 1000FT, identified as Stratus prior to commencement of the thunderstorm. Thunderstorm commences, and during the Special Observation the Stratus breaks to allow identification of a distinct 2 oktas of Cumulonimbus cloud at 2000 FT. The cloud will be reported as ‘BKN010 FEW020CB’ in METAR and as ‘86710 82920’ in SYNOP.
Example 3 3 oktas 700FT, 8 oktas at 1000FT, each identified as Stratus prior to commencement of the thunderstorm. Thunderstorm commences, and after attempts to determine an identifiable amount/base of CB fail, the cloud will be reported as ‘SCT007 OVC010CB’ in METAR and as ‘83707 88910’ in SYNOP.
Example 4 8 oktas on surface or sky obscured by fog or falling snow. Thunderstorm commences, the cloud will be reported as ‘OVC000CB’ in METAR and as ‘88900’ in SYNOP.
The ONLY exception to this rule is where the sky is obscured by blowing sand, blowing dust, or blowing snow. In this instance the vertical visibility will be reported as VV/// in the Metar and 89/// in the SYNOP. There will be no cloud group
OSSN November 2005