WAFSOPSG/2-WP/14
Appendix A
APPENDIX A
I. Macadam and D.J. Turp
Met Office
FitzRoy Road
Exeter
Devon
EX1 3PB
15 December 2004
The Met Office is developing a suite of ICAO thinned GRIB icing and turbulence forecast products that will be automatically generated from the output of a
Numerical Weather Prediction (NWP) model. Software that derives fields of various icing and turbulence related quantities from archived NWP data has already been written. Some example fields have been compared with significant weather charts with encouraging results. Various algorithms for calculating fields of icing and turbulence potential will be evaluated further and a selection will be introduced into the model. Evaluation of the algorithms selected is expected to continue following the planned introduction of prototype products in late 2005.
1.1 ICAO has expressed a desire for the WAFCs to develop the capability to provide global forecasts of icing and turbulent conditions in the form of fields of numerical data.
The Met Office has embarked on a project that will give WAFC London this capability.
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1.2 The WAFCs currently use the ICAO thinned GRIB format to distribute global forecast fields of winds and other basic meteorological quantities. This format will be used for a suite of products that will provide users with comprehensive forecasts of airspace likely to contain icing and turbulent conditions. The thinned GRIB grid is defined such that all of the gridboxes within a field measure approximately 140x140km in the horizontal. The products will be automatically generated from higher resolution fields of meteorological quantities output by the operational global Numerical Weather
Prediction (NWP) configuration of the Met Office’s Unified Model (UM).
1.3 The Met Office is developing prototype ICAO thinned GRIB icing and turbulence products that will be distributed by WAFC London on a trial basis from late
2005. This document describes the progress that has been made on defining the products and developing the software that will generate them. Work that is likely to take place in the future is also described.
2.1 The Met Office has been working with the International Air Transport
Association (IATA) to produce an initial description of the prototype products. It is recognised that this description may evolve in response to further input from ICAO,
IATA and other stakeholders.
2.2 Regional forecasts of icing and turbulent conditions are currently provided by the WAFC’s in the graphical format of medium level significant weather charts.
Airspace in which icing and turbulent conditions not associated with Cumulonimbus clouds (Cbs) are forecast is marked explicitly on these charts. Airspace forecast to contain Cbs is also shown and it is noted that the presence of such cloud implies the presence of conditions conducive to moderate or severe turbulence, icing and hail. The distinction made between icing and turbulent conditions not associated with Cbs and those associated with Cbs will be maintained in the suite of ICAO thinned GRIB products. Significant weather charts also make a distinction between In-Cloud
Turbulence (ICT) not associated with Cbs and Clear Air Turbulence (CAT). This distinction will also be maintained in the suite of thinned GRIB products. The suite will therefore consist of an icing product, an ICT product and a CAT product that will provide information on conditions not associated with Cbs, plus a Cb product that will provide information on where Cbs are likely to be present.
2.3 The proposed prototype icing, ICT and CAT products will comprise forecast fields of an icing potential index, an ICT potential index and a CAT potential index respectively. The value of an index assigned to a gridbox will indicate the likelihood that icing or turbulent conditions not associated with Cbs will be encountered in that gridbox. For each product and forecast validity time an average and maximum potential will be given for each gridbox in order to provide users with some indication of the spatial variability of conditions within the gridbox.
2.4 For each forecast validity time a field of average icing potential and a field of maximum icing potential will be provided for each of the layers 750 to 650hPa, 650 to
550hPa, 550 to 450hPa, 450 to 350hPa and 350 to 250hPa. A value indicating that an
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icing potential index cannot be defined will be assigned to gridboxes occupied by terrain.
2.5 For each forecast validity time a field of average ICT potential and a field of maximum ICT potential will be provided for each of the layers 750 to 650hPa, 650 to
550hPa, 550 to 450hPa, 450 to 350hPa and 350 to 250hPa. A value indicating that an
ICT potential index cannot be defined will be assigned to gridboxes occupied by terrain.
These fields will contain information on turbulence associated with large scale cloud layers.
2.6 For each forecast validity time a field of average CAT potential and a field of maximum CAT potential will be provided for each of the levels 400, 300, 250, 200 and
150hPa. These fields will contain information on turbulence associated with wind shear and mountain waves.
2.7 The prototype Cb product will provide information on the horizontal and vertical extent of convective clouds with tops colder than -20˚C. For each forecast validity time five fields will be provided: a) A field indicating the maximum percentage of the area of a gridbox forecast to be occupied by Cbs at any level in the vertical. A value of 0 will be assigned to gridboxes containing no Cbs, 1 to gridboxes with a maximum Cb cover of between
1 and 50% inclusive, 2 to boxes with a maximum Cb cover of between 51 and 75% inclusive and 3 to boxes with a maximum Cb cover of greater than 75%. b) A field giving the pressure altitude at the base of any Cbs in each gridbox. c) A field giving the pressure altitude at the top of any Cbs in each gridbox. d) A field giving the pressure altitude at the base of the lowest layer of cloud in which
Cbs are embedded in each gridbox. e) A field giving the pressure altitude at the top of the highest layer of cloud in which
Cbs are embedded in each gridbox.
The pressure altitudes will be expressed in hpa and metres and will be calculated using the ICAO Standard Atmosphere. A value indicating that a pressure altitude cannot be defined will be assigned to gridboxes in the Cb pressure altitude and embedded Cb pressure altitude fields not forecast to contain Cbs or embedded Cbs respectively.
2.8 It is proposed that the four prototype products are routinely generated for the
T+0, T+6, T+12, T+18, T+24, T+30 and T+36 hour forecast ranges.
Icing Product
3.1 A simple method of diagnosing icing conditions from NWP model output is to make the assumption that all cloud in the -20 to 0˚C temperature range contains some
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liquid water. This method is often used by forecasters at WAFC London in the production of medium level significant weather charts. Software that derives UM resolution fields of cloud fraction for large scale cloud layers in the -20 to 0˚C temperature range from archived UM analyses has been written. Some example fields have been generated and visually compared with corresponding significant weather charts for Europe, a mid-latitude region, and southern Asia, a tropical region (see
Figures 1 and 2). In general the airspace containing -20 to 0˚C cloud in these fields approximately coincides with airspace forecast to contain icing conditions marked on the charts. However some airspace forecast to contain icing conditions associated with convective cloud is not represented in the fields. Hence fields of cloud amount derived from output of the model’s convection scheme are also being investigated.
3.2 A cloud scheme that explicitly represents the processes that transfer atmospheric water from one phase to another was recently introduced into the UM. The model is now able to partition the condensate in large scale layers of cloud between the liquid and ice phases in a more sophisticated manner than a simple temperature range method of cloud condensate partitioning. Software that derives UM resolution fields of supercooled liquid water content and cloud fraction for large scale liquid and mixed phase cloud layers from archived UM analyses has been written. Some example fields have been generated and visualised.
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FIGURE 1 - Fields of -20 to 0˚C cloud fraction for 12Z on 12/11/2003
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FIGURE 2 - Significant weather charts for 12Z on 12/11/2003
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Turbulence Products
3.3 Fields of a CAT index output by the UM are often used by forecasters at WAFC
London in the production of significant weather charts. Values of the index are calculated from values of horizontal and vertical wind shear. Fields of different wind shear related CAT indices, the Ellrod indices, are used by forecasters at WAFC Washington. Software to calculate UM resolution fields of the WAFC London CAT index and the WAFC Washington
Ellrod indices from archived UM analyses has been written. Some example fields have been generated and visually compared with corresponding significant weather charts (see Figures 3 and 4). In general the areas in these fields that contain large values of the CAT index and
Ellrod indices approximately coincide with each other and with airspace forecast to contain turbulent conditions marked on the charts.
FIGURE 3 – Fields of CAT index and Ellrod indices for 12Z on 13/11/2003
FIGURE 4 – Significant weather chart for 12Z on 13/11/2003
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FIGURE 5
– Field of mountain wave turbulence index for 12Z on 08/01/2004
3.4 An algorithm for calculating fields of a mountain wave turbulence index is already currently included in the UM. An example field of this mountain wave turbulence index is shown in Figure 5. In general the areas in these fields that contain large values of the index approximately coincide with mountainous regions.
3.5 A database of aircraft turbulent encounters is currently being created using vertical acceleration data from flight data recorders. In this database the turbulent encounters are being grouped according to the most likely cause of the turbulence. The database will be used to evaluate different turbulence algorithms.
Other Software Development
3.6 Some example ICAO thinned GRIB icing and turbulence related fields have been generated from UM resolution fields using the software that the Met Office uses to generate
WAFC London’s existing thinned GRIB products. The fields have been visualised and it is clear that the interpolation method used by the software to generate thinned GRIB fields from the higher resolution UM fields is not appropriate for the generation of thinned GRIB fields of icing or turbulence potential. The software will therefore be modified before it is used in the generation of the thinned GRIB icing and turbulence products.
3.7 A brief investigation of the fields output by the UM that relate to the horizontal and vertical extent of convective clouds has been conducted. Although none of these fields relate to Cbs in isolation, it should be possible to use simple algorithms to generate the Cb product from them.
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4.1 Algorithms for calculating fields of icing and turbulence potential will be selected and introduced into the UM. In preparation for this, various icing and turbulence related fields derived from archived UM output will be evaluated using relevant icing and turbulence related data, including medium level significant weather charts, pilot reports and measurements from aircraft.
4.2 To assist with the adoption of suitable algorithms for diagnosing icing and turbulent conditions from UM output, the Met Office, in conjunction with ICAO and WMO, will hold a scientific workshop at the Met Office’s headquarters in Exeter in February 2005. International experts on the diagnosis and forecasting of icing and turbulent conditions will be invited to attend. Relevant research will be discussed and opinions on suitable algorithms will be sought.
4.3 Following the Exeter workshop the description of the prototype products will be reviewed and the feasibility of introducing information on the likely intensity of any icing and turbulent conditions forecast will be considered.
4.4
The software that the Met Office uses to generate WAFC London’s ICAO thinned
GRIB products will be modified so that it can be used to generate thinned GRIB fields from
UM resolution fields of icing and turbulence potential in an appropriate manner.
4.5 Software will be developed to generate the ICT product.
4.6 Software will be developed to generate the Cb product.
4.7 Following the introduction of the prototype products the fields comprising the products will be routinely compared with medium level significant weather charts and observational data.
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