HIGH RESOLUTION WINDS (NWC/GEO-HRW)

Updated description including developments for NWC/GEO-HRW v6.2
(released inside NWC/GEO v2021 in April 2022), with examples.

1. Goal of HRW algorithm
2. Summary description of HRW algorithm
3. Description of HRW inputs
4. Description of HRW outputs
5. Examples of HRW output visualization

Access to “Algorithm Theoretical Basis Document for the Wind product processors of the NWC/GEO (HRW v6.2)” for a detailed description

1. Goal of HRW algorithm

NWC/GEO-HRW algorithm aims to provide detailed sets of Winds (Atmospheric Motion Vectors, AMVs) and Trajectories from:

Up to seven MSG-1·4/SEVIRI satellite channels (HRVIS, VIS06, VIS08, IR108, IR120, WV062 and WV073).
Up to six Himawari-8·9/AHI satellite channels (VIS06, VIS08, IR112, WV062, WV070 and WV073).
Up to three GOES-13·15/IMAGER satellite channels (VIS07, IR107 and WV065).
Up to six GOES-16-17/ABI satellite channels (VIS06, VIS08, IR112, WV062, WV070 and WV074).

The AMVs and Trajectories are calculated through the displacement of cloudiness features in successive images from all these channels, and of humidity features in successive images from the water vapour channels. The AMVs and Trajectories can be calculated 24 hours a day, considering:

15 minute image cycles ("Nominal scan mode") or 5 minute image cycles ("Rapid scan mode") with MSG-1·4 satellites.
10 minute full-disk image cycles with Himawari-8·9 satellites.
15 minute image cycles (in the "Continental United States" region) or 30 minute image cycles (in the "North America" region) with GOES-13·15 satellites.
10 minute full-disk image cycles with GOES-16·17 satellite.

The product includes pressure level information and a quality control flagging, giving some indication of its error in probabilistic terms. It has been developed by AEMET (the Spanish National Weather Service) in the framework of the EUMETSAT's "Satellite Application Facility on support to Nowcasting and Very short range forecasting" (NWC SAF).

The AMVs and Trajectories can be calculated for up to two different scales at the same time: "Basic scale" and "Detailed scale". The corresponding dimension of the tracers to be tracked can be defined by the user, with a default value of 24 pixels for the "Basic scale". Many other processing parameters can also be configured, in particular the region and the satellite channels to be processed for the AMV calculation.

This product can be useful in near real time applications including:

Nowcasting: Watch and warning of dangerous strong wind situations; Monitoring of the general flow, of convergence and divergence at low and high levels, or of small scale circulation and wind singularities.
Forecasting applications: Assimilation of HRW outputs in NWP models, or in applications where the observed wind or the observed displacement of atmospheric features is needed.

Several different HRW outputs (as BUFR or netCDF bulletins) are provided for the calculated winds and trajectories, with a level 2 of processing. A level 3 of processing (as a grid interpolation of the winds or a meteorological analysis based on the data) is not included.

2. Summary description of HRW algorithm

Eight different steps are considered in the processing of HRW algorithm:

1. Initialization of data: next matrices are read or calculated for the start of the processing, by the NWC/GEO-NWCLIB library, for the running slot and region in which HRW algorithm is calculated:

Normalized reflectances for the MSG, Himawari-8·9 or GOES-13·17 visible channel images, and brightness temperatures for the MSG, Himawari-8·9 or GOES-13·17 infrared and water vapour channel images.
Latitude, longitude, solar and satellite angle matrices.
Temperature, wind and geopotential profiles, and surface pressure fields from NWP data.
NWC/GEO Cloud type (CT), Cloud top temperature and height (CTTH) and Cloud microphysics (CMIC) outputs (if so configured, for a better AMV and Trajectory height assignment).

2. Tracer determination: Two methods are used consecutively:

Gradient method: with a fast and efficient search of well defined cloud edges.
Tracer characteristics method: which covers holes in the search of tracers with "Gradient method", with a longer but still reasonable computing time; two different tests are run based on finding a brightness threshold separating the cloudiness in front from the background, and finding a well defined shape of the feature to be tracked, to avoid too linear elements.

An option inside the HRW algorithm process permits that new tracer centres for the running cycle are defined at the final locations of AMVs calculated for the previous cycle. This way, a set of “Persistent tracers” can be successively tracked in consecutive images, and “Trajectories” are defined through the progressive positions of these tracers throughout the time.

In NWC/GEO-HRW v6.2 (v2021), an option has been implemented to increase the spatial density of AMVs at medium and low levels. For all satellites the proportion of medium and low level AMVs has increased at least a 9%, and for MSG satellites, an optimal distribution of high/medium/low level AMVs of 35%/32%33% is even reached. This is done reducing four times the distance betwen tracers related to medium, low and very low clouds. Additionally, in this version the running time is reduced up to a 30% in the default configuration, with a similar number of calculated AMVs.

3. Tracer tracking and Wind calculation: One of two well known methods can be used for this process:

Euclidean distance.
Cross correlation (default option).

The tracking area in which tracers from a previous image are looked for in the current image, can be defined through the extrapolation of the linearly interpolated NWP wind guess for a quicker running of the algorithm. Nevertheless, this option is not used as default one to reduce the dependence from the NWP winds. The best three tracking centres are kept, from which one is taken at the "Quality Control step".

A “mixed calculation method” is also available since NWC/GEO-HRW v6.0 (v2018), considering at the same time short and long time intervals, through which the tracking process is verified in short time intervals, but the AMVs are calculated considering displacements in long time intervals. This process is useful for the calculation of AMVs with high resolution images, and to improve the quality of the calculated AMVs. However, this option is not used in the default configuration.

4. Height assignment: Two different methods can be used:

Brightness temperature interpolation method: calculates the cloud top pressure and cloud base pressure, interpolating the brightness temperature of the corresponding satellite channel (or MSG/IR120, Himawari-8·9/IR112, GOES-13·15/IR107, GOES-16·17/IR112 channel in the case of visible channels) to a NWP vertical temperature profile. Then, the AMV/Trajectory pressure is defined through one of these two values, taking into account the cloud type related to the tracer.
Cross Correlation Contribution (CCC) method (default option): defines the AMV/Trajectory pressure, considering only the pressure of the pixels contributing most to the image correlation (which is available only for "Cross correlation" tracking and if NWC/GEO-CT and CTTH outputs are available). A correction of the pressure level for cloudy AMVs/Trajectories is also implemented, considering the AMV/Trajectory Liquid water path/Ice water path (obtained from the corresponding NWC/GEO-CMIC output, and related to the depth of the cloud which is being tracked).

5. Quality Control: the “Quality Indicator method”, developed for the AMV calculation at EUMETSAT/MPEF, has been adapted for HRW algorithm. Several consistency tests are if possible computed, considering:

Temporal consistency test: vector consistency with neighbour AMVs in the previous slot.
Spatial consistency test: vector consistency with neighbour AMVs in the same slot.
Forecast consistency test: vector consistency with the NWP forecast wind in the same slot and location.
Two scale consistency test (only for the “Detailed scale”): vector consistency with simultaneous neighbour AMVs from the “Basic scale” in the same slot.

Three individual “Quality Indices” are calculated for each AMV/Trajectory, considering the normalized statistical fitting functions described by the method and the weighted sum of the partial consistency tests:

Quality Index with forecast (QIF): Quality index including the forecast consistency test.
Quality Index without forecast (QINF): Quality index excluding the forecast consistency test.
Common Quality Index (QIC): defined through a common calculation module to be used by all AMV producing centres. The "Common Quality Index" has shown to provide a real skill in filtering AMVs from different AMV algorithms, for a better consistency.

6. Orographic Flag: an “Orographic flag” is also calculated, related to the detection of land tracers, and tracers blocked by orography or related to orographic waves (for which the AMVs/Trajectories are not displacing with the corresponding atmospheric flow), with next possible values:

1: AMV wrongly located below the corresponding surface pressure (basically due to microphysics corrections in the AMV pressure). These AMVs are always skipped.
2: Very important orographic influence at the current location of the AMV.
3: Less important orographic influence at the current location of the AMV.
4: Very important orographic influence at any previous location of the AMV.
5: Less important orographic influence at any previous location of the AMV.
6: No orographic influence has been found at any current or previous position of the AMV.

7. AMV Selection: a comparison is made among all AMVs calculated for the same tracer, when more than an AMV has been calculated, for the final definition of the AMV related to that tracer (only one AMV per tracer can be kept). This comparison is made considering the behaviour of each one of those AMVs for each one of the following criteria:

From the AMV calculation process: tracking correlation.
From the Quality control process: temporal, spatial and forecast consistency tests; two scale consistency test also for the “Detailed scale”.
From the Orographic flag process: orographic flag.

Possible test values for each case are:

3: When the AMV is the best of all for the same tracer for the corresponding criterion.
2: When the AMV is slightly worse than other AMVs for the same tracer for the corresponding criterion.
1: When the AMV is rather worse than other AMVs for the same tracer for the corresponding criterion.
0: When the comparison could not be defined for the given AMV.

The AMV selection is based on the AMV with the best comparison tests. In case of draw, the AMV with the best forecast consistency test is selected, or else the AMV with the best tracking correlation.

8. Autovalidation of AMVs: considering requests from the users, since NWC/GEO-HRW v6.0 (v2018) the algorithm is able to calculate validation statistics for the AMVs with the algorithm itself, using as reference NWP forecast winds (in real time or reprocessing processes) or NWP analysis winds (in reprocessing processes).

The validation statistics are written for each running slot in a specific file S_NWC_HRW-STAT_*.txt, including information per channel, per level or as a whole on the number of AMVs (NC), the mean wind speed (SPD), the normalized bias (NBIAS), the normalized mean vector difference (NMVD), and the normalized root mean square difference (NRMSVD) with respect to the reference NWP forecast or analysis winds.

The reference NWP wind at the AMV level and the best fit pressure level, and the vector differences with both, can also be calculated and stored in the AMV output for further processing and research.

3. Description of HRW inputs

NWC/GEO configuration files: HRW Model Configuration file (*.cfm) and Region Configuration file (*.cfg).
MSG or Himawari-8·9 or GOES-13·17 satellite data: Full Resolution MSG/HRIT, Himawari-8·9/(HSD or EHH), GOES-13·15/GVAR or GOES-16·17/NetCDF original image files, with brightness temperatures for the infrared and water vapour channels to be used and reflectances for the visible channels to be used, for the working region, for the current and previous slot.
NWP data: Temperature profiles and longitudinal and latitudinal component wind profiles for as many as possible of the following pressure levels: 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, 50, 30, 10 hPa. Also geopotential profiles in case the Parallax correction or the Orographic flag is calculated. Also surface pressure fields in case the Orographic flag is calculated.
NWC/GEO-Cloud Type (CT) and Cloud Top Temperature and Height (CTTH) output for the working region and slot (optionally, for a better height assignment).
NWC/GEO-Cloud Microphysics (CMIC) output for the working region and slot (optionally, for a microphysics correction of the AMV pressure level, if CCC method is used for the height assignment). This option is not available for GOES-13·15 satellites.
List of Tracers, Predecessor winds and Trajectories calculated at the previous slot (in case they are available).

4. Description of HRW outputs

Depending on configuration, AMVs and Trajectories are provided in the form of BUFR or NetCDF bulletins, considering separately each processed region and each tracer scale used (“Basic scale” and “Detailed scale”). The different options for the HRW outputs are:

1. Three different HRW AMV BUFR bulletin formats:

One similar to the format used by all previous versions of HRW algorithm (named S_NWC_HRW-WIND_*.bufr). This case is the default option for HRW algorithm. Additionally in this case, if trajectories are calculated, an additional BUFR bulletin (named S_NWC_HRW-TRAJ_*.bufr) is also provided with the content of the corresponding Trajectories.
Other one equivalent to the old format used by other AMV bulletins distributed by other AMV centres through the WMO Global Telecommunications System (named S_NWC_HRW-WINDEUM_*.bufr). This version, NWC/GEO-HRW v6.2 (v2021), is the last one providing this format.
Finally, other one equivalent to the new format ("sequence 310077") defined by the IWWG/International Winds Working Group, to be used by other AMV bulletins distributed by other AMV centres through the WMO Global Telecommunications System (named S_NWC_HRW-WINDIWWG_*.bufr).

2. One HRW NetCDF bulletin format, providing information for both AMVs and Trajectories (named S_NWC_HRW_*.nc).

The content of the default option for HRW algorithm, the first BUFR bulletin format, includes next parameters:

General processing indicators:

Satellite identifier.
Generating centre and application.
Wind computation method.
Origin of first guess.
Satellite instrument used.
Satellite channel central frequency and width.
Year, month, day, hour, minute of final image.
Time increment from initial to final image.
Satellite cycle of final image.
Quality control characteristics.
Horizontal and vertical tracer size in pixels.

Processing indicators for each AMV/Trajectory sector:

AMV/Trajectory identifier.
Predecessor AMV identifier and Number of Trajectory sectors (if any, with option calculating “Persistent tracers”).
Horizontal and vertical tracer size in metres.
Tracer tracking method (“Euclidean distance” or “Cross correlation”).
Initial latitude and longitude.
Latitude and longitude increments.
Wind direction and speed.
Wind temperature and pressure level.
Quality indices (QIF, QINF, QIC).
Type of tracer (Basic tracer; Detailed tracer related to a wide basic tracer; Detailed tracer related to a narrow basic tracer; Detailed tracer unrelated to a basic tracer).
Type of tracer (Cloudy or Clear air tracer) and Height assignment method used.
Number of AMVs computed for the tracer (1 to 3).
Correlation and Quality tests: which or the partial QI consistency tests (two scale, temporal, spatial, forecast, orographic test) have been applied, and in case several AMVs were calculated for the tracer, if the selected one is better, slightly worse or rather worse than the other AMVs for the same tracer for the considered test.
Number of NWP levels used.
Number of predecessor AMVs.
Orographic flag.
Cloud type.
MSG-1·4, Himawari-8·9 or GOES-13·17 satellite channel used.
Correlation (if “Cross correlation” is used for the Tracer tracking).
Pressure error (if “CCC method” is used for the Height assignment).
Pressure correction (if “CCC method” is used for the Height assignment).
Validation NWP reference wind at AMV level: direction, speed, direction difference and speed difference.
Validation NWP reference wind at best fit pressure level: direction, speed, direction difference, speed difference and pressure level.
Validation NWP best fit pressure level.
Validation flag (validation against NWP analysis or forecast).

5. Examples of HRW output visualization

Four different HRW algorithm configuration files (*.cfm) have been defined as default options for the running of NWC/GEO-HRW algorithm, repectively for use with MSG satellites, Himawari-8·9 satellites, GOES-13·15 satellites and GOES-16·17 satellites. All of them are named safnwc_HRW.cfm, and are located in the corresponding $SAFNWC/config subdirectories related to each satellite configuration.

Next figures show the AMV output of HRW algorithm for five specific examples with these satellites and configurations, in which HRW algorithm has been respectively run every 15 minutes with MSG2 in the “European and Mediterranean region”, every 10 minutes with Himawari-8 in the "China/Korea/Japan region", every 15 minutes with GOES-13 in the “Continental United States region”, every 10 minutes with GOES-16 in the “Continental United States region”, and every 10 minutes with GOES-17 in the "Eastern Pacific region".

NWC/GEO-HRW v2021 Basic AMVs for “European and Mediterranean region“ (MSG-2, 14 May 2010, 12:00 UTC).
Colours based on the AMV pressure

NWC/GEO-HRW v2021 Basic AMVs for “China/Korea/Japan region“ (Himawari-8, 2 April 2018, 00:00 UTC).
Colours based on the AMV pressure

NWC/GEO-HRW v2021 Basic AMVs for “Continental United States region“ (GOES-13, 1 July 2010, 17:45 UTC).
Colours based on the AMV pressure

NWC/GEO-HRW v2021 Basic AMVs for “Continental United States region“ (GOES-16, 11 June 2019, 12:00 UTC).
Colours based on the AMV pressure

NWC/GEO-HRW v2021 Basic AMVs for “Eastern Pacific region“ (GOES-17, 29 April 2021, 00:00 UTC).
Colours based on the AMV pressure