Description of GEO-iSHAI outputs

The main outputs provided are parameters informing about the water vapour content in the vertical column (total and in selected layers) and about the atmospheric thermal instability. These parameters are computed using the final retrieved temperature and humidity profiles.

Since NWC/GEO release 2016, also the Skin Temperature (SKT) and Total Ozone (TOZ) are additional outputs.

The main outputs of GEO-iSHAI product are written in the iSHAI netCDF output file. The main outputs are:

1. Total Precipitable Water (TPW): Precipitable water in layer from surface pressure to top of atmosphere.
2. Layer Precipitable Water (LPW). Corresponding to the precipitable water in three layers:
2.a BL: Precipitable water in low layer [Psurface to 850 hPa]
2.b ML: Precipitable water in middle layer [850 to 500 hPa ]
2.c HL: Precipitable water in high layer [500 hPa to top of atmosphere]
3. Stability indices: they are calculated from the retrieved profiles of temperature and humidity. The calculated indices are:
3.a. Lifted Index (LI)
3.b Showalter Index (SHW)
3.c. K-index (KI)
4. Skin Temperature (SKT)
5. Total Ozone (TOZ). Note: This parameter is optional and must be activated by the users and the NWP GRIB files must contains ozone fields.
6. Besides the main outputs, the differences between the above parameters calculated from the retrieved profiles of temperature and humidity (and ozone profile if activated) with the parameters calculated from the spatial, temporal and vertical interpolated profiles from background NWP are written as other outputs (following 2007 Madrid Workshop recommendation). Thus the parameters diffTPW, diffBL, diffML, diffHL, diffLI, diffSHW, diffKI, diffSKT, diffTOZ are also written in the netCDF output file.
7. Quality Flags: iSHAI_quality, iSHAI_status_flag, iSHAI_conditions fields.
8. Configurable IR channel BT degraded to 7 bits only in cloudy pixels

GEO-iSHAI main outputs.

Precipitable water parameters

An evaluation of humidity distribution is critical in order to determine the possibilities of convection, severe weather and heavy precipitation. Storms usually develop where humidity is already high or where some mechanism makes it to increase. Precipitable water is a measurement of the water vapor mass contained in a vertical column of atmosphere of unit section and thickness limited between two pressure levels. The units are kg/m2 (mm are commonly used in operational meteorology)

Four different precipitable water parameters, defined as function of two selected pressure levels, are considered of interest:

Total Precipitable Water, TPW: pmax = psurface and pmin = 0

Low Layer Precipitable Water, BL: pmax = psurface and pmin = 850 hPa

Medium Layer Precipitable Water, ML: pmax = 850 hPa and pmin = 500 hPa

High Layer Precipitable Water, HL: pmax = 500 hPa and pmin = 0

Most of the contribution to TPW comes from a layer between surface and 500 hPa.

The internal units of precipitable water fields (BL, ML, HL and TPW) are kg/m2 (equivalent in value to mm).

Stability indices parameters

Stability indices were developed in the past to support storms and severe weather forecast using as main input local radiosonde profiles of temperature and humidity. It is important to remark as introductory note that the use of these indices needs to be supported by previous experience in the use of the more appropriate ones to each specific region and weather regimes. Among all the stability indices, Lifted Index (Galway, 1956) and (Miller, 1972), the Showalter Index (Showalter, 1947) and the K-Index (George, 1960) have been chosen to be codified since the 2010 version. These indices are widely used in operational meteorology.

There are several stability indices defined as the difference between the air environment temperature at 500 hPa and the temperature of an air parcel representative of the properties of low levels, lifted up to the 500 hPa. These indices have negative values when the lifted parcel arrives warmer than the environment as it means buoyancy and hence instability. The units are Kelvin (but ºC is commonly used also in operational meteorology).

Thermal Stability Index = Tenv500 - Tparc500

In the case of Lifted Index (LI), the temperature and humidity of the virtual lifted parcels are the average of the lowest 100 hPa (or 1000 m).

The following table provides empirical relation between LI values and the stability degree:


LI value


V > 0

Stable

-3 <= v < 0

Slightly Unstable

-6 <= v < -3

Unstable

-9 <= v < -6

Very Unstable

v < -9

Extremely Unestable


In the case of Showalter Index (SHW), the lifted parcel represents the environment at 850 hPa. The following table provides empirical relation between SHW values and the stability degree:


Showalter value


v > 0

Stable

-3 <= v < 0

Marginal instability

-6 <= v < -4

Large instability

v < -6

Extreme instability


K Index (KI) measures the thermal stability in function of the vertical gradient of temperature and humidity content in low levels.

KI = ( T850 - T500 ) + ( Td850 - DD700 )

where: DD700 = (T700 - Td700 )

The following table provides empirical relation between KI values and storm probability:

K-Index value

Storm probability

v < 15

0%

15 < v <= 20

< 20%

21 < v <= 25

20 – 40 %

26 < v <= 30

40 – 60 %

31 < v <= 35

60 – 80 %

36 < v <= 40

80 – 90 %

v > 40

> 90 %

The internal units of stability indices fields (LI, Showalter Index and KI) are K (equivalent in value to ºC because they are calculated always as differences of temperature between levels).

Skin temperature (SKT) and diffSKT

SKT and diffSKT=(SKTretrieved – SKTnwp) were calculated internally in previous version of PGE13 SPhR but they were not available to users as outputs and no validation was made. In any case, GEO-iSHAI SKT does not pretend to be an alternative to SST or LST products because more temporal and spatial controls are required. They have been introduced in order users and forecasters to be able to use them as nowcasting parameters. The temporal evolution and spatial gradients of SKT and the diffSKT could be used in several nowcasting applications as monitoring of pixel not well screened out by cloud mask, detection of fogs, advice for very fast increase of SKT as triggering convection mechanism, advice users of bad SKT, etc

The unit is Kelvin (K)

Total Ozone (TOZ) and diffTOZ

TOZ and diff(TOZret – TOZnwp) have been also introduced as optional parameters since release 2016. That means that user must activate them and provide NWP GRIB file with ozone profiles.

TOZ and diff(TOZretrieved – TOZnwp) have been also introduced as optional parameters. That means that user must activate them editing keyword TOZ_CALCULATION to 1 and including ozone profile in the GRIB files supplied to NWC/GEO in fixed pressure levels or hybrid levels.

For this reason the IR9.7 channel is also now a mandatory channel. The retrieval of TOZ is based on statistical retrieval using non-linear regression from ozone profile, SEVIRI IR channels (including IR9.7 channel) and the GEO-iSHAI T profile and GEO-iSHAI SKT. Temporal changes and spatial gradients of TOZ are nowcasting interest parameters due to the relation between TOZ fields and the detection of tropopause breaks and vorticity maxima.

diffTOZ is useful to detect the disagreement between NWP TOZ and retrieved TOZ.

netCDF files description

In this version, the following fields are calculated for clear pixels or FOR:

It is important to highlight that, as GEO-iSHAI retrieves the complete profiles of temperature and humidity, it is possible in future versions to implement new stability indices after user requirements or to modify the limits and thickness of the atmospheric layers where precipitable water is computed.

The format of the file is netCDF and it is fully described in the “Data Output Format for the NWC/GEO” document (NWC/CDOP2/AEMET/SW/DOF available in the NWCSAF Web page).

In releases 2018 and 2016, the outputs are not image like fields and are data fields. It is the responsibility of the user to use adequate tools to generate images.

The netCDF format files scales have the following minima and maxima values. They are fixed in the $SAFNWC/import/Aux_data/iSHAI/S_NWC_PRODIO_iSHAI.cf


Parameter

minimum

maximum

iSHAI_bl

0

35

iSHAI_ml

0

45

iSHAI_hl

0

8

iSHAI_tpw

0

70

iSHAI_li

-15

40

iSHAI_ki

0

60

iSHAI_shw

-15

40

iSHAI_skt

243

343

iSHAI_toz

100

500

iSHAI_diffbl

-2.5

2.5

iSHAI_diffml

-3.5

3.5

iSHAI_diffhl

-1.0

1.0

iSHAI_difftpw

-5

5

iSHAI_diffli

-2.5

2.5

iSHAI_diffki

-7

7

iSHAI_diffshw

-2.5

2.5

iSHAI_diffskt

-15

15

iSHAI_difftoz

-30

30

Residual

0

600

IR_band

0

127

Table 1: Maximum and Minimum values for each parameter


The name of the iSHAI netCDF output file is:

S_NWC_iSHAI_satid_nameregion-resolution_yyyyMMDDThhmm00Z.nc

where:

Examples of iSHAI netCDF output filenames:

MSG:               S_NWC_iSHAI_MSG3_LargeIberica-VISIR_20160810T120000Z.nc

Himawari:        S_NWC_iSHAI_HIMA08_FullDiskHimawari-NR_20171231T000000Z.nc

GOES-R:          S_NWC_iSHAI_GOES16_Conus-NR_20171231T000000Z.nc


The netCDF output files are generated and stored in $SAFNWC/export/iSHAI directory.