OGIMET

Versión española

Meteosat-9/metar

GFS world weather maps

Summary

• What is a GRAMET
• About the data
• Gramet sections
• The meteogram
• The detailed data
• The forecasted air profiles
• Examples:
• Gramet aero for an ICAO site.
• Gramet meteo for a WMO site.
• Gramet aero for a route.
• Gramet meteo for a de un corte meteorológico.

0.- What is a Gramet

Gramet namet derive from spanish gráfico meteorológico. It really is a meteogram, ie, show graphically the evolution of meteorological parameters in time and / or space.

There are two types of Gramet. Classic meteogram showing the evolution of meteorological parameters at ground and upper level. There are two basic types of Gramet. The classic meteogram shows the temporal evolution of meteorological variables at a given location, both surface and upper air. Another type of meteogram, called Route here for historical reasons, shows the weather conditions along routes between points defined by the user. In a path may vary the time between initial and final point. When start and end time are the same then the result is a weather cut

In both cases, you can display data and units focused to the aeronautics, or more wide propose. The link for the first type is gramet_aero and the second gramet_meteo

1.- About the data

The basic data used are extracted from NOAA GFS grib fields with 0.5 degrees lat/lon resolution. In addition, there are some small corrections to take into account the differences between topography of model and actual data, as altitude and land fraction.

In any case, you should note the limits of the model. There will be some zones and weather situations in which the model clearly will fail. As example, the model can ignore some local fogs or deep termical inversions in some areas. Weather for small islands in big oceans are badly predicted because the model ignore them in his topography.

Gramet aero example fron EGLL (Heathrow Airport)

2.- Gramet sections

A gramet consists of three sections. The first is the meteogram area, where the meteorological trend is drawn from the ground up to levels high enough above the tropopause. Is the more colorful area, the background is blue or black as night or day on the point. This shows the evolution of some labeled isotherms, the tropopause and isotachs. Also are shown the various cloud layers predicted by the model and areas with significant icing or turbulence. At the bottom, when it comes to a route or cut, the actual altitude of the terrain is drawn.

Below the main area we can see a more detailed information whose content depends on the type of gramet. For meteograms for single site, some parameters are estimated on the surface. You can see an example for EGLL (Heathrow Airport). The displayed parameters depends on the type. A gramet meteo for same place same time is this other example. Note that the indicator used here is the the WMO index. In case of paths or cuts, what is shown in this section is a schematic alphanumeric information on weather conditions in chosen flight level or altitude (depending on aero or meteo mode). See examples 3 and 4

Gramet meteo example from WMO site 03772 (Heathrow Airport)

In the bottom of gramet is displayed information on the standard levels above the point and instant of time associated with the value of the coordinate x (horizontal) in the meteogram. In case of aero type, are selected flight levels, in other case standar isobaric levels

2.1- Meteogram section

Is the most visual section of a gramet. Is the rectangle with blue background color or black as night or day. The vertical axis represents altitude in logarithmic scale. The altitude is indicated in the lateral axes. For meteo types the units are km, and hectofeets for aero ones. Each value on the horizonal axis, x coordinate, is associated with a moment of time and / or location. For meteograms, all values of x refer to the same place but at different times, making it the x-axis equal to the time axis.

In case of paths or cuts, the x-axis varies the position and time. The temporal variation is assumed linear with the x. The position associated with each x also varies linearly with the distance from the origin along the lines connecting the points in a orthodromic route

What is shown in this section:

• Cloud layers. GFS model provides information for 5 cloud layers: High, medium, low, convective and boundary layer, in addition to the total cloudiness. The amount and position of clouds plotted in meteogram are managed by the model. Below an amount threshold no clouds are displayed. Clouds in a convective layer are displayed as CB or TCU. High clouds are ploted simulating Cirrus. Boundary layer, low and medium clouds are plotted trying to simulate the real aspect
• tropoause. In a red a yelow line the predicted tropopause spatial and temporal evolution is plotted
• isotherms. Most important isotherm, 0ºC, are plotted as red. For aero type no other isotherm are displayed. For meteo type other isotherm are displayed with a 20ºC interval
• isotachs. In dashed brown lines the isotachs are plotted above 50 Kts with 25 kts interval.
• Turbulence. The upper air zones with moderate or heavy predicted turbulence are plotted. The E parameter is used to predict turbulence

  E = ( (5 * HWS) + (VWS)**2 + 42 )/4
  where HWS = Horizontal Wind Shear 10**5 s**-1 (m/s per 100 Km)
  and   VWS = Vertical Wind Shear 10**3 s**-1 (m/s per 1 km)
  

  A computed E between 80 and 160 is considered as moderate turbulence, higher values are classified as heavy turbulence
• Icing Zones with moderate or severe icing are plotted. The algorithm to obtain an icing estimator developed by author is:

    IENG = (lyr * IENG_LYR + cnv * IENG_CNV) / 200;
  
    IENG icing index
    IENG_LYR icing index for layered clouds. 
    IENG_LYR icing index for convective clouds. 
    lyr = amount of LYR clouds in %. 
    cnv = amount of convective clouds in %
  
    IENG_CNV = 200 * (Lwc(bottom_of_cloud) - lwc) / lwc(293K)) * sqrt((T - 253.15)/20.0);  si 253.15<=T<= 273.15
    IENG_LYR = 100 * t *(t + 14)/ 49; where t = (T - 273.15); when  -14 <= t <= 0.
  
    where:
    lwc(293K) = 0.017281 Kg / m3  (water vapor density at 20ºC in a satured air)
    lwc = es(T) / (Rv * T) (water vapor density at air cell conditions)
  

  A result between 30 and 80 is considered as moderate, over 80 as severe icing.
• Precipitation. When the predicted precipitation intensity is above a threshold, is plotted in the meteogram. For an altitude which is supossed that the precipitaction is solid, small snow icons are ploted instead of rain drops. The concentration of snow or rain icons is proportional to precipitation intensity
• Terrain profile. When cut or path mode, the actual terrain profile is plotted in the bottom of meteogram, with y coordinates according to their altitude. The darkest zone is the actual altitude of terrain just in the path, the ligth one is the maximum altitude for 5 nautical miles at both sites of main orthodromic path line.

Gramet aero example for route from EGLL to LSZB

2.2-Surface or upper level data

Below meteogram area more detailed data are displayed

For a single site meteogram some common surface parameters are plotted. In case of aero type, hourly temperature, relative humidity, wind (Knots), precipitation type, one hour accumulated precipitation amount, and QNH are stimation. (See example 1). In case of meteo type (example 2) wind units are Km/h and sea level pressure are showed instead of QNH (hPa)

For cut or path gramet what is plotted also depends on type. For aero type, (Example 3) some parameters in selected flight level are displayed. The 'x' coordinate where is plotted is related with the time and position where the conditions are described.

• density altitude (green font) in hectofeets. Is the altitude in a standard atmosphere where the density is the same than in the current conditions at selected flight level.
• Temperature in celsius degrees (red or blue, depend on negative or positive temperature)
• Lateral wind component in knots with respect the main path. From the right side, assuming we are moving from start to end of path, there is a 'R'as prefix. From the left side a 'L'L
• Head wind component in knots, assuming we are moving along the path from start to end. A 'H' as prefix means is head, and plotted red. 'T' prefix is tail component and plotted as green

When gramet is a meteo path or cut type (example 4) the following parameters are showed at the altitude level selected

• Air pressure (hPa) (green)
• Temperature Celsius (red or blue, depending on negative or positive temperature)
• Wind ditection (degrees), meteorological convention
• Wind intensity (knots)
• Amount and type of clouds in level.Amount indicated as CLEAR, SCT, BKN ó OVC. Type CNV or LYR

Gramet meteo cut from 08419 to 08001

2.3-Upper air data

In the bottom of gramet are plotted wind data (direction and intensity) and temperature in som standard levels. For aero type, the selected flight levels are FL050, FL100, FL150, FL200, FL300, FL400. For meteo type the isobaric lavels are 925, 850, 700, 500, 300 y 200 hPa