Airborne Cloud Radar (ACR) Reflectivity, Wakasa Bay, Japan


This data set includes 94 GHz co- and cross-polarized radar reflectivity. The Airborne Cloud Radar (ACR) sensor was mounted to a NASA P-3 aircraft flown over the Sea of Japan, the Western Pacific Ocean, and the Japanese Islands.

Citing These Data

Stephens, G. L. and R. T. Austin. 2004. Airborne Cloud Radar (ACR) Reflectivity, Wakasa Bay, Japan. [indicate subset used]. Boulder, Colorado USA: NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: 10.5067/3YXTPM48L5GY.

Overview Table

Category Description
Data format netCDF
Spatial coverage and resolution 30° to 42°N, 132° to 151°E
60 or 120 m vertical resolution, 0.8° degree beamwidth
Temporal coverage and resolution 14 January 2003 to 3 February 2003. Averaged profiles were collected every 3 seconds.
Tools for accessing data Use an appropriate application to view netCDF data.
File naming convention Files use the convention "" where acr=instrument, date, start time, and version.
File size 70 KB to 2.6 MB
Parameter(s) Co-polarized and cross-polarized reflectivity and vertical velocity
Procedures for obtaining data Data are available via FTP and HTTPS.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s) Name and Title

Graeme L. Stephens
Richard T. Austin

Colorado State University

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services


The investigators would like to thank the following University of Massachusetts scientists for their work with these data: Jorge Roman-Nieves, Harry Figueroa, and Stephen Sekelsky.

2. Detailed Data Description

The University of Massachusetts and NASA Jet Propulsion Laboratory (JPL) collaborated to build and operate the ACR, which is a 94 GHz radar that provides measurements of vertical cloud structure in addition to radar backscatter and radiative properties of different cloud types. It has flown in four NASA-sponsored experiments on the NASA P-3. This system was developed to determine the feasibility of a future spaceborne radar program.


Data are provided in netCDF format. The netCDF headers contain dimensions, units, and other information. Files contain the co-polarized reflectivity, cross-polarized reflectivity, and vertical velocity for a given flight leg. Reflectivity and velocity data are structured as two-dimensional arrays, where altitude and time are the two dimensions. For more information on the netCDF file format, please see the NetCDF Resources at NSIDC Web site.

File Naming Convention

Files use the convention "" where acr=instrument, date, start time, and version. For example, the file "" contain data from 15 January 2003 starting at 03:41:19 UTC, and is a version 2 file. The "nc" extension stands for the netCDF file type.

File Size

File sizes range from 70 KB to 2.6 MB.

Spatial Coverage

Spatial coverage includes the Wakasa Bay coordinates shown below.

Southernmost Latitude: 30° N
Northernmost Latitude: 42° N
Westernmost Longitude: 132° E
Easternmost Longitude: 151° E

Spatial Resolution

Data were acquired in 60 or 120 m vertical resolution; 0.8 degree beamwidth.

Temporal Coverage

14 January 2003 to 3 February 2003. Flights did not occur every day; the specific dates of coverage were: 14, 15, 19, 21, 23, 26-30 January , and 1 and 3 February 2003.

Temporal Resolution

Averaged profiles were collected every three seconds during P-3 flight legs.

Parameter or Variable

Parameter Description

The parameters for these data are co-polarized and cross-polarized reflectivity (more strictly, the equivalent radar reflectivity factor).

Unit of Measurement

Reflectivity is given in units of mm^{6} m^{-3}. Note that some clear-air regions have reflectivity values that are slightly negative, due to the noise characteristics of the signal when no target is present. These cases must be noted when converting reflectivity to logarithmic (dBZ) units..

Sample Data Record

The sample image below was taken from the file "" The sample shows the equivalent reflectivity factor (Zhh):

3. Data Access and Tools

Data Access

Data are available via FTP and HTTPS.


Total data volume is approximately 67 MB.

Software and Tools

For a list of tools for reading/viewing netCDF files, please see the NetCDF Resources at NSIDC: Software and Tools Web page.

The netCDF format is an interface for array-oriented data access with an interface library. The netCDF library also defines a machine-independent format for representing scientific data. Together, the interface, library, and format support creating, accessing, and sharing scientific data. The netCDF software was developed at the Unidata Program Center in Boulder, Colorado. More information about netCDF and software for manipulating and displaying NetCDF is available from Unidata.

Related Data Collections

4. Data Acquisition and Processing

Data Acquisition Methods

The ACR measured 94 GHz co- and cross-polarized radar reflectivity below the NASA P-3 aircraft during the Wakasa Bay Experiment. The radar was pointed in the nadir direction. The ACR was operated whenever the P-3 was above approximately 2,438 m (8000 ft) above ground level (AGL), and flight legs were usually flown at around 6,400 m (21,000 ft) above mean sea level (MSL). There were very few gaps in the data during the flights. The ACR has a beamwidth of 0.8° and was usually operated with a vertical resolution of 120 m, although 60 m vertical resolution was used for some flight lines. Vertical profiles were recorded every 0.3 seconds and then averaged in post-processing to one profile every 3 seconds. Altitude coordinates were determined from the ocean surface echo in the ACR Zhh reflectivity profile. (Radar determination of the P-3 aircraft altitude was necessary due to the inaccuracy of the pressure altitude supplied by the navigation system.) Start time and latitude/longitude information were obtained from the onboard GPS navigation system. The data set also includes measurements of oceanic backscatter collected while the P-3 was flown at a specified bank angle.

Sensor or Instrument Description

An ACR consists of a radio frequency/intermediate frequency (RF/IF) subsystem, a digital signal processor, and data handling and control electronics. The RF/IF subsystem uses a combination of frequency mixing and multiplication to generate the transmitted signal at one of four frequencies in a 50 MHz band centered at 94.92 GHz. The power of the transmitted signal is supplied by an extended interaction amplifier (EIA). The transmit RF pulses can be routed to either the V or H port of an orthomode transducer (OMT) using a ferrite switch matrix. The OMT then feeds a 30 cm lens antenna. A small portion of the transmitted power is coupled though an attenuator into the receiver for calibration purposes. The ACR system was designed to test the feasibility of a spaceborne cloud radar system, now under development as CloudSat.

5. References and Related Publications

Sadowy, G. A., R. E. McIntosh, S. J. Dinardo, S. L. Durden, W. N. Edelstein, F. K. Li, A. B. Tanner, W. J. Wilson, T. L. Schneider, and G. L. Stephens. 1997. The NASA DC-8 airborne cloud radar: design and preliminary results. Proceedings of IGARSS '97. vol. 4, 1466-1469.

Related Documents
Document Description URL
University of Massachusetts ACR Web University of Massachusetts/NASA JPL Airborn Cloud Radar Web site.
NetCDF Resources at NSIDC Gives a brief summary of netCDF files, a list of tools for accessing these files, and a list of data sets in netCDF format at NSIDC.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

ACR Airborne Cloud Radar
AGL Above Ground Level
AMSR-E Advanced Microwave Scanning Radiometer - Earth Observing System
EIA Extended Interaction Amplifier
FTP File Transfer Protocol
JPL Jet Propulsion Laboratory
MSL Mean Sea Level
netCDF Network Common Data Format
OMT Orthomode Transducer
RF/IF Radio Frequency/Intermediate Frequency

Document Creation Date

February 2004

Document URL