Air Mass Analysis PGE12 SAFNWC     Table of contents 1.- Goal of AMA product  2.- AMA algorithm summary description  3.- List of inputs for AMA  4.- Coverage and resolution 5.- Description of AMA outputs 6.- Example of AMA visualisation Access to "User Manual for the PGE12 of the SAFNWC/MSG Scientific Part" for a more detailed description.

 

1.- Goal of AMA product

The AMA products of the SAFNWC aim to support nowcasting applications by providing several products describing air mass characteristics. The sub-products are:

• A classification of air masses as function of total precipitable water, surface temperature and stability information, derived from satellite measurements.
•  An objective determination of the ridge lines of the equivalent-potential temperature field (indicating warm and moist air in which severe local convection is most likely to occur).This parameter is based on ECMWF model data at 850 hPa.
• An objective determination of pre-frontal regions in northerly/westerly direction of the ridge line of equivalent-potential temperature (leading edge of frontal cloud bands has high probability for the initiation of convection). Based on ECMWF model data + IR satellite data.
• An objective determination of black stripes in water vapour imagery (dry air in the upper troposphere is an air mass pattern often linked to the initiation of convection).


2.- AMA algorithm summary description
 

• Air mass classification product
 

The air mass classification is a decision-tree-like classification scheme based mainly on work by Baum et al. (J. Appl. Meteorol., 36, 1519-1540, 1997). Four basic classes are distinguished on the basis of surface temperature thresholds, slightly depending on the time of the day, as follows:
  

Air mass type	Surface Temperature (ºC)    Daytime Twilight        Nighttime
Equatorial Tropical Polar Arctic	>32 >31 >30 between 22 and 32 between 18 and 31 between 14 and 30 between –16 and 22 between –18 and 18  between –20 and 14 <-16 <-18   <-20

 Astronomical twilight is given when the sun's zenith angle is between 90° and 108°. The surface temperature is derived from a split window algorithm using the 10.8 micrometer channel and the 12.0 micrometer channel, therefore being available only in cloudfree areas.

Once the basic class is determined, it is investigated whether the moisture content is within the usual range. This range of typical precipitable water values for each of the four basic air masses is:
 
 

Air mass type	Precipitable water (cm)
Equatorial	> 6
Tropical	between 3 and 6
Polar	between 1 and 3
Arctic	< 1

If the value of "Total Precipitable Water" is larger than the upper limit of the "normal" range, the attribute "moist" is assigned to the affected pixel; if it is below the lower limit, the assigned attribute is "dry".

Finally, a stability index imagery is taken into account to distinguish between stable and unstable air masses:
 
 

Air mass type	Criterion
Stable	stable according to SAI
Unstable	unstable according to SAI

• WV dark stripe product
   

  The detection of dark stripes has the following principal steps:

  • polarstereographic projection of the WV6.2 image.
  • For each pixel, it is checked whether the pixel is darker than an empirical threshold.
  • For each candidate pixel it is verified that it is darker than most of its neighbours.
  • The contiguous areas of dark pixels where the checked conditions are given have to exceed a certain threshold of size. Therefore, a contouring algorithm is applied to encircle these pixels, with subsequent elimination of too short contour lines (empirical threshold). The contour lines passing the test indicate the large-scale black stripes and form the final output.

• `Ridge line' product and `Gradient zone of equivalent-potential temperature' product
   

  To determine the ridge line of the equivalent-potential temperature at 850 hPa, the following steps are carried out:

  •  Computation of a "pseudo curvature vorticity" which describes the curvature of the NWP 850 hPa equivalent-potential temperature isolines (standard formulas used for deriving curvature vorticity from the geopotential height field are applied)
  • For each point, determination of number of neighbours with higher values of "pseudo curvature vorticity" ("less anticyclonic" neighbours). This number is a measure of the qualification to lie on a ridge line.
  • Tracking of lines connecting the most suitable points.

  
  Gradient areas of equivalent-potential temperature to the north and the west of the ridge lines are considered relevant if they satisfy two criteria:

  • distance from the ridge line below a certain threshold,
  • gradient above a certain threshold.

  
  The main interest is on the areas ahead of fronts where there is comparatively high risk of convection development. Hence, the product is "cleaned" in such a manner that large coherent areas of bright pixels in IR 10.8 – which are likely to represent frontal cloud bands – are identified and the gradient zone markers are withdrawn in these regions.
   

3.- List of inputs for AMA

• .Air mass classification product
  • MSG radiance values:
    The following SEVIRI channels are required at full MSG resolution:
    • R12.0 micrometer:     Mandatory
    • R10.8 micrometer:     Mandatory
  • SAFNWC/MSG products used as inputs:
    • PGE06(TPW): Mandatory.
    • PGE08(SAI): Mandatory.
• WV dark stripe product
  • MSG radiance values:
     The following SEVIRI channels are required at full MSG resolution:
    • R6.2 micrometer: Mandatory
• `Ridge line' product and `Gradient zone of equivalent-potential temperature' product
  • MSG radiance values:
     The following SEVIRI channels are required at full MSG resolution:
    • R10.8 micrometer: Mandatory
  • ECMWF NWP data:
    • Temperature and humidity information at 850 hPa: Mandatory.

• Air mass classification

The product format is the SAFNWC/MSG image format. The number of air mass classes in the current version is 20, the coding is as following:
 
 

Code	Name
0	unclassified
1	(reserved for future use)
2	Arctic, normal, stable
3	Artic, moist, stable
4	Polar, dry, stable
5	Polar, normal, stable
6	Polar, moist, stable
7	Tropical, dry, stable
8	Tropical, normal, stable
9	Tropical, moist, stable
10	Equatorial, dry, stable
11	Equatorial, normal, stable
12	(reserved for future use)
13	(reserved for future use)
14	Arctic, normal, unstable
15	Arctic, moist, unstable
16	Polar, dry, unstable
17	Polar, normal, unstable
18	Polar, moist, unstable
19	Tropical, dry, unstable
20	Tropical, normal, unstable
21	Tropical, moist, unstable
22	Equatorial, dry, unstable
23	Equatorial, normal, unstable
24	(reserved for future use)

 The thresholds used to separate the individual air masses are given in section 2.1.
 
  WV dark stripe product

This product yields output coded in BUFR format. The graphical visualisation is to be done by the user. The detailed template of the BUFR records can be found in the Software User Manual for PGE12, section 2.4.2.
 

• `Ridge line' product and `Gradient zone of equivalent-potential temperature' product
    

  These products are output in separate BUFR files. The graphical visualisations are to be done by the user. The detailed template of the BUFR records can be found in the Software  User Manual for PGE12, section 2.4.3.

6.- Example of AMA visualisation

The figure 1 shows the visualisation of the products which are computed over the MSG-N area: WV stripes, ridge lines of equivalent potential temperature and associated gradient zones. Figure 2 gives an example how the air mass classification product may be presented to the user.