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History of modifications

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Version	Date	Description of modification	Chapters / Sections
1.0	31/03/2021	Initial version	1–6, References
1.1	07/09/2021	Clarification of PNPR-CLIM input data	2.1.2 – Table 10

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	table1
	table1

Table 1: AMSU-B and MHS radiometers' characteristics. QV = Quasi-vertical; polarization vector is parallel to the scan plane at nadir; QH = Quasi-horizontal; polarization vector is perpendicular to the scan plane at nadir.

AMSU-B	Advanced Microwave Sounding Unit-B
MHS	Microwave Humidity Sounder Unit
Satellites	AMSU-B: NOAA-15, NOAA-16, NOAA-17MHS: NOAA-18, NOAA-19, MetOp-A, MetOp-B
Mission	Humidity sounding in almost all-weather conditions and precipitation rate
Instrument type	Absorption-band MW radiometer/spectrometer - 5-channels
Scanning technique	Cross-track: 90 steps of 16 km at sub satellite point, swath 2180 km (2250 km) for MHS (AMSU-B) Along-track: one 16-km line every 8/3 s
Coverage/cycle	Near-global coverage twice/day
Resources	Mass: 63 kg – Power: 93 W – Data rate: 3.9 kbps
MHS/AMSU-B Central frequency (GHz)	MHS/AMSU-B channel bandwidth (MHz)	MHS/AMSU-B channel polarisation	MHS/AMSU-B channel radiometric accuracy (NEΔT)
89.0	2800/1000	QV/QV	0.22/0.37 K
157.0/150.0	2800/1000	QV/QV	0.38/0.84 K
183.31 ± 3.0	2000/2000	QH/QV	0.42/0.60 K
183.31 ± 1.0	1000/500	QH/QV	0.57/0.70 K
190.311/183.31 ± 7	2000/1000	QV/QV	0.45/1.06 K

Other datasets used to develop and test the NN algorithm are:

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	table2
	table2

Table 2: Summary of channel characteristics of SSM/I instruments as described by Wentz (1991)

Centre Frequency [GHz]	Polarization	Pixel Integration Time [ms]	Samples per scan	Inter-Scan Period [s]	Along-track Sample Spacing [km]	3 dB Footprint Size (Along-track × Cross-track) [km]
19.35	v/h	7.95	64	3.8	25	69 × 43
22.235	v	7.95	64	3.8	25	50 × 40
37.0	v	7.95	64	3.8	25	37 × 28
37.0	h	7.95	64	3.8	25	37 × 29
85.5	v/h	3.89	128	1.9	12.5	15 × 13

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	table3
	table3

Table 3: Summary of spacecraft's and SSM/I instrument's properties

Altitude	860 km
Inclination	98.8 °
Orbit Period	102 min
Swath Width	1394 km
Effective Scan Angle	102.4 °
Local Zenith Angle	53.1 °
Calibration Method	On board; each scan; fixed cold space reflector and reference black body hot load

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section1_2_2
section1_2_2
1.2.2 The SSMIS Instrument

The SSMIS is a 24-channel microwave radiometer using multiple frequencies and therefore is able to replace different former instruments namely SSM/I, Special Sensor Microwave Temperature (SSM/T), and Special Sensor Microwave Humidity Sounder (SSM/T-2). For precipitation retrieval the channels 12–18, with centre frequencies at 19.35, 22.235, 37.0 and 91.655 GHz, are of main interest here. Thus, SSMIS continues the measurements of the SSM/I instrument with a shift of SSM/I highest frequency channel from 85.5 to 91.655 GHz. Further channel characteristics are given in table 4.

Its first usage was aboard the F-16 spacecraft of the DMSP in October 2003. Further missions followed by F-17 (2006), F-18 (2009) and F-19 (2014). Like earlier satellites of the DMSP (see section 1.2.1) these ones are on a near-polar, sun-synchronous, circular orbits with a period of 101.8 minutes. Their local equator crossing occurred during 2–11 a.m./p.m.

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	table4
	table4

Table 4: Summary of SSMIS channel characteristics exploited in the precipitation retrieval

Channel	Centre Frequency [GHz]	Polarization	Pixel Integration Time [ms]	Samples per scan	Inter-Scan Period [s]	Along-track Sample Spacing [km]	3 dB Footprint Size (Along-track × Cross-track) [km]
12	19.35	h	8.44	90	3.8	25	73.6 × 46.5
13	19.35	v	8.44	90	3.8	25	73.6 × 46.5
14	22.235	v	8.44	90	3.8	25	73.6 × 46.5
15	37.0	v	8.44	90	3.8	25	45.0 × 31.2
16	37.0	h	8.44	90	3.8	25	45.0 × 31.2
17	91.655	v	4.22	180	1.9	12.5	15.5 × 13.2
18	91.655	h	4.22	180	1.9	12.5	15.5 × 13.2

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	table5
	table5

Table 5: Summary of spacecraft's and instrument's properties for SSMIS

Altitude	833 km
Inclination	98.9 °
Orbit Period	101.8 min
Swath Width	1707 km
Effective Scan Angle	143.2 °
Local Zenith Angle	53.1 °
Calibration Method	On board; each scan; fixed cold space reflector and reference black body warm target

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section1_2_3
section1_2_3
1.2.3 The AMSR-E Instrument

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	table6
	table6

Table 6: Summary of channel characteristics for the AMSR-E instrument

Centre Frequency [GHz]	Bandwidth [MHz]	Polarization	Pixel Integration Time [ms]	Scan cycle [s]	Sampling Interval [km]	IFOV [km]
6.925	350	h/v	2.6	1.5	10 × 10	74 × 43
10.65	100	h/v	2.6	1.5	10 × 10	51 × 30
18.7	200	h/v	2.6	1.5	10 × 10	27 × 16
23.8	400	h/v	2.6	1.5	10 × 10	31 × 18
36.5	1000	h/v	2.6	1.5	10 × 10	14 × 8
89.0	3000	h/v	1.3	1.5	5 × 5	6 × 4

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	table7
	table7

Table 7: Summary of spacecraft's and instrument's properties for AMSR-E

Altitude	705 km
Inclination	98.2 °
Orbit Period	98.8 min
Swath Width	1445 km
Effective Scan Angle	122 °
Earth Incidence Angle	55 °
Calibration Method	On board; each scan; fixed cold space reflector and reference black body warm target

The IFOVs of AMSR-E and SSM/I / SSMIS were harmonized by averaging the brightness temperatures of each three neighbouring AMSR-E scan positions.

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	table8
	table8

Table 8: Summary of channel characteristics for the TMI instrument

Channel	Centre Frequency [GHz]	Polarization	Pixel Integration Time [ms]	EFOVs per scan	EFOV (Along-track × Cross-track) [km]	Inter-Scan Period [s]	IFOV (Along-track × Cross-track) [km]
1	10.65	v	6.6	104	63.2 × 9.1	6.6	59.0 × 35.7
2	10.65	h	6.6	104	63.2 × 9.1	6.6	60.1 × 36.4
3	19.35	v	6.6	104	30.4 × 9.1	6.6	30.5 × 18.4
4	19.35	h	6.6	104	30.4 × 9.1	6.6	30.1 × 18.2
5	21.3	v	6.6	104	22.6 × 9.1	6.6	27.2 × 16.5
6	37.0	v	6.6	104	16.0 × 9.1	6.6	16.0 × 9.7
7	37.0	h	6.6	104	16.0 × 9.1	6.6	16.0 × 9.7
8	85.5	v	3.3	108	7.2 × 4.6	3.3	6.7 × 4.1
9	85.5	h	3.3	108	7.2 × 4.6	3.3	6.9 × 4.2

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	table9
	table9

Table 9: Summary of spacecraft's and instrument's properties for TMI

Altitude	402 km
Inclination	35 °
Swath Width	758.5 km
Effective Scan Angle	130 °
Earth Incidence Angle	52.8 °
Calibration Method	On board; each scan; fixed cold space reflector and reference black body warm target

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section2
section2
2 Input and Auxiliary Data

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section2_1
section2_1
2.1 PNPR-CLIM

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	table10
	table10

Table 10: Additional algorithm input variables. Sea ice information and snow cover information are Boolean masks derived from daily sea ice cover and snow depth variables, respectively.

Variable	Data source
Sea ice information (daily)	ECMWF ERA5
Snow cover information (daily)	ECMWF ERA5
2 m temperature (monthly)	ECMWF ERA5
Freezing level (monthly)	ECMWF ERA5
Sea ice cover (monthly)	ECMWF ERA5
Snow depth (monthly)	ECMWF ERA5
Total column integrated water vapor (monthly)	ECMWF ERA5

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section2_2
section2_2
2.2 HOAPS-v4

The HOAPS v4 Level-2 data have been generated on the basis of the CM SAF SSM/I / SSMIS FCDR (Fennig et al., 2017; Fennig et al., 2020; CM SAF’s HOAPS v4.0 ATBD [D1], section 2.2). The initial dataset was extended to cover the full period until the end of 2017 for the present merged CDR. It is assumed that the inter-calibration coefficients remain applicable over the period 2015–2017.

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	table11
	table11

Table 11: Summary of the characteristics of the GMP Dual Precipitation Radar. The GPM KuPR minimum threshold is closer to 12–13 dBZ than the official 18 dBZ in the table (from Tang et al., 2017).

Instrument	GPM DPR
Instrument	KaPR	KuPR
Launch time	27 Feb 2014	27 Feb 2014
Altitude (km)	407	407
Inclination angle (°)	65	65
Frequency (GHz)	35.547 and 35.553	13.597 and 13.603
Horizon resolution at nadir (km)	5	5
Swath width (km)	120	245
Vertical resolution (m)	250/500	250
Minimum detectable Ze (dBZ)	12 (KaHS) 18 (KaMS)	18
Measurement accuracy (dBZ)	< ± 1	< ± 1

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section3_2_2_2
section3_2_2_2
3.2.2.2 The NN training dataset

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	table12
	table12

Table 12: Characteristics of the NN training dataset built from MHS-DPR coincidences

Period	1/01/2015 – 31/12/2016
Geographical area	67 °S – 67 °N, 180 °W – 180 °E
Number of MHS orbits	36,856
Number of pixels with precipitation	3,000,000
Horizontal resolution (Km)	16 km × 16 km (nadir) 26 km × 52 km (scan edge)
Reference precipitation product	2B-CMB level-2 GMI/DPR combined V06A on Ku-band radar swath (NS)
MHS BTs	FIDUCEO FCDR v4.1

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	table13
	table13

Table 13: List of variables in the NN training dataset built from MHS-DPR coincidences.

Variable in the database	Data source
Latitude (MHS pixel)	FIDUCEO FCDR v4.1
Longitude (MHS pixel)	FIDUCEO FCDR v4.1
Mean Time (of DPR pixels within the ATMS pixel)	2B-CMB level-2 GMI/DPR combined V06A
Surface precipitation rate	2B-CMB level-2 GMI/DPR combined V06A
Precipitation liquid fraction information	2B-CMB level-2 GMI/DPR combined V06A
Time of MHS pixel	FIDUCEO FCDR v4.1
MHS Scan position	FIDUCEO FCDR v4.1
Sea ice information	ECMWF ERA5
2 m temperature	ECMWF ERA5
Total column integrated water vapor	ECMWF ERA5
Freezing level	ECMWF ERA5
Snow depth	ECMWF ERA5
Land/Sea Mask	ESA

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section3_2_3
section3_2_3
_heading h.17dp8vu
3.2.3 Algorithm Flowchart

The PNPR-CLIM algorithm high-level flowchart is shown in figure 3.

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	table14
	table14

Table 14: PNC module input variables.

Input Variable	Variable dimensions	Variable type
BT	5	Instantaneous
monthly T2m	1	Average
monthly FL	1	Average
monthly TPWV	1	Average
daily SD	1	Average
daily SIF	1	Average
ANG	1	Static
OLC	1	Static

The static and average variables determine a sort of a priori status of the atmosphere, allowing the NN to correctly interpret the radiative signal (after applying a zenith correction, if necessary): for example, a depression in the BT around the 157 GHz window channel is commonly due to the scattering of precipitating particles. However, if the atmosphere is sufficiently dry, the emission signature of a radiatively cold surface may lead to the same effect on the measured BT. Through the use of the ancillary data the NN aims to discriminate these different states.

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	table15
	table15

Table 15: Optimal NN module input variables.

Variable Type	Input Variable	Variable dimension	Variable type
BT	89.0 GHz 157.0 GHz 183.31 ± 7.0 GHz	3	Instantaneous
BT Differences	183.31 ± 3.0–183.31 ± 1.0 GHz 183.31 ± 3.0–183.31 ± 7.0 GHz 183.31 ± 1.0–183.31 ± 7.0 GHz 183.31 ± 3.0–157 GHz 183.31 ± 7.0–157 GHz	5	Instantaneous
Model Derived Ancillary Variables	monthly T2m (K) monthly FL (m) monthly TPWV (kg m^-2) daily SD daily SIF	5	Average (from ECMWF ERA5 reanalysis)
Ancillary Variables	ANG OLC	2	Static Map

The performance analysis of the PRE module was carried out using the 2016 dataset, which is an independent part of the observational MHS-DPR coincidence database, not used in the training and design phase of the algorithm (about 1.5 million points) (see section 3.2.2.2).

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	table16
	table16

Table 16: PNPR-CLIM precipitation retrieval module statistical indexes.

	Ocean	Land
BIAS[mm/h]	0.02	0.01
CC	0.75	0.70
RMSE[mm/h]	1.02	1.08

Another result of the verification study is shown in figure 7. It shows the 2D histogram of the surface precipitation rate estimates from the NN and the corresponding values in the 2B-CMB dataset over ocean and land. Only pixels for which both the neural network and the 2B-CMB provided rainfall estimates ≥ 0.1 mm/h (TP pixels) were considered. In the scatterplot, the logarithmic axes represent the precipitation rate (NN vs. GPM 2B-CMB referred to as DPR), while the colour represents the number of points in the dataset for each 2D precipitation rate bin. Most of the points are close to the main diagonal for both ocean and land, with slight overestimation of very low precipitation (precipitation rate < 0.5 mm/h) over land by the NN.

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	table17
	table17

Table 17: Contribution of the conditions occurred in the BQF index.

BQF	Occurred Condition
0	Precipitation Probability Index
1	Radiometer Scan index
2	Quality of input data
3	Background surface index (Snow)
4	Background surface index (Sea Ice)
5	Orography index
6	High Latitudes Index
7	Calibration Index
8	Invalid Data

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	figure11
	figure11

Figure 11: Distribution of the QF index (percentage over all the available observations) for the year 2017.

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	table18
	table18

Table 18: Semi axes in km for the two instrument classes. For AMSU-B / MHS, the semi axes depend on the scan position sp = 1, …, 90 with 45 and 46 being the central positions in the scan line, close to the sub-satellite point. It is nb = sp for sp ≤ 45 and nb = 91 - sp else

AMSU-B / MHS (PNPR-CLIM)

SSM/I / SSMIS (HOAPS v4)

AMSR-E² (HOAPS v4 extended)

TMI (HOAPS v4 extended)

Along-scan

Mathdisplay
0.5 \cdot 79.08 + 2.84 \cdot nb - 14.78 \cdot nb^{0.666}

15.5

9

Cross-scan

Mathdisplay
0.5 \cdot 28.72 - 0.90 \cdot nb + 0.094 \cdot nb^{1.5}

22.5

16

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	note2
	note2

2 AMSR-E brightness temperatures from three neighbouring scan positions are averaged to match the SSM/I and SSMIS resolutions, see section 1.2.

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	table19
	table19

Table 19: Valid versions of COBRA products

Version	Description
1.0	Precipitation rates are derived from the original algorithm as described in this document. The version number is valid for daily and monthly files.

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section4_1
section4_1
4.1 Daily merged COBRA data

Files containing daily data of accumulated rain rates are named after following syntax:

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	table20
	table20

Table 20: List of variables used in any file of daily precipitation rates

Variable Name	Dimension(s)	Unit	Description
Coordinates
lat	1	°N (degrees North)	Latitude of grid cell centre
lat_bnds	2	°N (degrees North)	Boundaries of top (northern) and bottom (southern) grid cell edge
lon	1	°E (degrees East)	Longitude of grid cell centre
lon_bnds	2	°E (degrees East)	Boundaries of left (western) and right (eastern) grid cell edge
time	1	Seconds since 1970-01-01	Time stamp of the current day
time_bnds	2	Seconds since 1970-01-01	Boundaries of the time interval covered by time variable
platform_id	1	N/A	An integer used for internal platform assignment
instrument_id	1	N/A	An integer used for internal instrument assignment
Data Variables
precip	3 (time, lat, lon)	mm/d	Daily accumulated precipitation rates that are represented by a single multi-platform composite
precip_stdv	3 (time, lat, lon)	mm/d	Daily mean of intra-platform standard deviation derived from hourly values
Quality Variables
quality_flag	3 (time, lat, lon)	N/A	Mean of PNPR-CLIM quality flags, whose assigned data was used in composite creation
num_obs	4 (time, lat, lon, instrument_id)	N/A	Total number of observations separated by instrument type
num_covered_hours	3 (time, lat, lon)	N/A	Accumulated number of hours, for which data of at least one platform is available on the respective day
Ancillary Variables
platform_name	2 (time, platform_id)	N/A	Names of all platforms that are used for composite creation for the respective day. The names are allocated to the platform identifier. Platform names are saved as char array. Thus, there is an additional dimension in the netCDF file describing the length of the longest string.
instrument_name	2 (time, instrument_id)	N/A	Assigned names of the specific instrument identifier. Instrument names are saved as char array. Thus, there is an additional dimension in the netCDF file describing the length of the longest string.

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section4_2
section4_2
4.2 Monthly merged COBRA data

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	table21
	table21

Table 21: List of variables used in files with monthly precipitation rates

Variable Name	Dimension(s)	Unit	Description
Coordinates
lat	1	°N (degrees north)	Latitude of grid cell centre
lat_bnds	2	°N (degrees north)	Boundaries of top (northern) and bottom (southern) grid cell edge
lon	1	°E (degrees east)	Longitude of grid cell centre
lon_bnds	2	°E (degrees east)	Boundaries of left (western) and right (eastern) grid cell edge
time	1	Seconds since 1970-01-01	Time stamp of the current month
time_bnds	2	Seconds since 1970-01-01	Boundaries of the time interval covered by time variable
platform_id	1	N/A	An integer used for internal platform assignment
instrument_id	1	N/A	An integer used for internal instrument assignment
Data Variables
precip	3 (time, lat, lon)	mm/d	Monthly mean precipitation that is represented by a single multi-platform composite
precip_stdv	3 (time, lat, lon)	mm/d	Monthly mean of intra-platform standard deviation derived from hourly values
Quality Variables
quality_flag	3 (time, lat, lon)	N/A	Mean of PNPR-CLIM quality flags, whose assigned data was used in composite creation
num_obs	4 (time, lat, lon, instrument_id)	N/A	Total number of observations separated by instrument type
num_covered_hours	3 (time, lat, lon)	N/A	Accumulated number of hours, for which data of at least one platform is available in the respective month
Ancillary Variables
platform_name	2 (time, platform_id)	N/A	Names of all platforms that are used for composite creation for the respective month. The names are allocated to the platform identifier. Platform names are saved as char array. Thus, there is an additional dimension in the netCDF file describing the length of the longest string.
instrument_name	2 (time, instrument_id)	N/A	Assigned names of the specific instrument identifier. Instrument names are saved as char array. Thus, there is an additional dimension in the netCDF file describing the length of the longest string.

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annex
annex
Annex

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Intermediate_PNPR-CLIM_output
Intermediate_PNPR-CLIM_output
Intermediate PNPR-CLIM output

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FIDUCEO_FCDR_L1C_<RADIOMETER><SATELLITE><START TIME, YYYYMMDDHHMMSS><END TIME, YYYYMMDDHHMMSS{_}>_EASY_v4.1_fv2.0.1.nc

PNPR-CLIM output:

PNPR-CLIM_<FIDUCEO input filename>.nc

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	table22
	table22

Table 22: Variables within the PNPR-CLIM output file

Name	Long Name	Type
CHAN	Channel	1D
LAT	Latitude	2D
LON	Longitude	2D
POS	Scan position	1D
PP	Probability of Precipitation	Geo2D
PR	Precipitation Rate	Geo2D
QUALITY_PIXEL_BITMASK	FIDUCEO quality pixel bitmask	Geo2D
SCAN	Scan line	1D
UPR	Unmasked Precipitation Rate	Geo2D
UTIME	Scan time	1D
QF	Quality Flag	Geo2D
BQF	Quality Bit Flag	Geo2D

A list of global attributes of PNPR-CLIM netCDF output files is reported, as an example, in the following paragraph:

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1.2.2 The SSMIS Instrument

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1.2.3 The AMSR-E Instrument

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2 Input and Auxiliary Data

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2.1 PNPR-CLIM

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2.2 HOAPS-v4

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3.2.2.2 The NN training dataset

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3.2.3 Algorithm Flowchart

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4.1 Daily merged COBRA data

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4.2 Monthly merged COBRA data

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1.2.2 The SSMIS Instrument

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section1_2_3
1.2.3 The AMSR-E Instrument

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section2
2 Input and Auxiliary Data

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section2_1
2.1 PNPR-CLIM

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section2_2
2.2 HOAPS-v4

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section3_2_2_2
3.2.2.2 The NN training dataset

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3.2.3 Algorithm Flowchart

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section4_1
4.1 Daily merged COBRA data

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section4_2
4.2 Monthly merged COBRA data

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Annex

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Intermediate_PNPR-CLIM_output
Intermediate PNPR-CLIM output