TRMM LBA Rain Gauge/Disdrometer Draft Operations Plan

Please direct any questions, comments, or suggestions to Larry Carey at carey@radarmet.atmos.colostate.edu

I. Rain Gauges/ Disdrometers

1. Introduction

The purpose of the rain gauge and disdrometer deployment during TRMM-LBA is 1) to obtain independent measurements of surface rain rate for calibration and comparison with radar derived estimates in a variety of rainfall regimes and at various ranges from the two radars, 2) to investigate the temporal and spatial variability of rainfall in Rondonia, and 3) to determine how this variability impacts the accuracy of the ground validation (GV) radar products.

In addition to scientific goals, there are also important logistical issues. The chosen site locations must be 1) available private land, 2) easily accessible during the wet season (e.g., preferably on paved vs. dirt roads if possible), and 3) reasonably close to one of the two radars for purposes of data collection and instrument maintenance. In addition, each of the 2-D video disdrometers (2DVD) requires shelter and air conditioning for their accompanying PCs. The rest of the instruments utilize data loggers which are designed to withstand the elements. To accomplish these scientific and logistical goals, the rain gauges and disdrometers listed in Sec. I1 above will be deployed at five separate locations in the vicinity of Ji Parana.

2. Shipping and Deployment Plan

Thirty Qualimetric tipping bucket rain gauges, and 30 UNIDATA data loggers will be air shipped from NASA Goddard Space Flight Center to Ji Parana for installation in November 1998. One 2DVD, one Joss impact disdrometer, and three APL impact disdrometers with supporting instrumentation have been included for shipment in the NOAA / profiler seatainer which will be shipped to Ji Parana in October 1998. A second 2DVD will be air shipped to Ji Parana from the University of Iowa in late November or early December. Six additional APL impact disdrometers will be shipped to Ji Parana at approximately the same time. Routing of the air shipments will depend on time and cost requirements, unless otherwise dictated because of import licensing requirements.

Final instrument locations and exact network configurations will depend on the results of a final site survey to be conducted prior to instrument set-up. The final site survey will be conducted during October 1998 and instrument set-up is expected to occur primarily during November 1998. During the final site survey, necessary site preparation will occur. For example, wooden platforms will be constructed on which the rain gauges and disdrometers will be placed. If necessary, fencing will be constructed to keep cattle away from the instruments. The final set-up will involve the physical placement, programming, and calibration of the gauges and disdrometers according to TRMM procedures. It is anticipated that the rain gauges and lower maintenance disdrometers (APL and Joss) will begin data collection by 1 Dec 98. The 2-D video disdrometer will require more elaborate set-up, calibration, and maintenance and therefore will likely be deployed at the beginning of the IOP (e.g., first week of January 1999).

One 2-D video and an APL disdrometer will be operated at the TOGA radar site (Abracos Hill). A location at intermediate distances (30 - 40 km) from both of the radars would have been ideal from a scientific and data analysis perspective. However, two logistical issues took precedence in this siting location: 1) The accompanying data processing and display PC will be housed in one of the TOGA seatainers. If the instrument is not sited at the TOGA radar, then a shelter with air conditioning will have to be identified. 2) Collocation with the TOGA radar will allow radar scientists to maintain and monitor the 2DVD carefully and frequently. Otherwise, a remote site causes logistical and manpower issues in arranging frequent site visits. The drop size distribution (DSD) data from this disdrometer will be compared to the S-pol radar observations located approximately 62 km to the southeast. Given the choice between the two radars, the TOGA seemed to be the obvious site choice. The S-pol is a 10-cm (minimal attenuation problems), polarimetric radar while the TOGA radar is at an attenuating wavelength (5 cm) and is non-polarimetric. The rest of the instrumentation will be deployed at 4 separate networks (Fig. 1).

Fig. 1 Updated road map containing the approximate locations of the four rain gauge networks relative to the S-pol (S) and TOGA (T) radars, major roads, and cities and towns. This color map will be updated for the final operations plan.

Although rain gauge data will be used as a reference for the radar estimation of rainfall, it is critical to establish first the sampling error of the gauges. The design and proposed locations of the first two gauge/disdrometer networks has this goal in mind. The configuration of these networks is optimized to study the spatial variability of tropical rainfall at a variety of distance classes from 10 meters to 4 km, permitting the estimation of the shape of the spatial correlation function of rainfall. This information can then be utilized to estimate the gauge sampling error throughout the entire rain gauge deployment (given the local gauge configuration, number of gauges, and the individual rain gauge measurement error).

The triangle design of the rain gauge cluster utilized in networks #1 and #2 was developed to capture the small scale variability of rainfall with the minimal number of gauges. There is a need to understand the small scale variability of rainfall to reduce the errors in the radar-rainfall estimation. Radar-rainfall errors are a combination of radar errors due to sampling, noise of the system, incorrect calibration, and natural variability of rainfall. If the spatial correlation of rainfall can be determined for distances corresponding to the size of a radar-rainfall grid (ie. 2 km), the natural variability of rainfall can be removed from the radar-rainfall error. To determine the spatial correlation function, gauges must be placed over a range of intergauge distances, but there must be a number gauges at the same distance to improve the sample statistics. A triangle cluster allows for both. For networks # 1 and 2, there will be three samples at each vertices of the triangle. Then there will be at least 3 more samples at each vertex of a sub triangle of gauges placed in the large network (See Figs. 2 and 3). Ideally, the gauges should deployed at distances over powers of 10 so scaling of rainfall can also be examined. For instance, gauges could be located at ranges of 1m, 10 m, 100 m, 1 km, out to 2 km. Finally, the remaining gauges should be dispersed outside the rain gauge cluster to increase the sample area.

Fig. 2 Idealization of gauge/disdrometer network # 1 arranged in two nested triangles. There are a total of 12 Qualimetric rain gauges and 6 disdrometers. See the text for more details.

Network #1 (Fig. 2) will consist of twelve rain gauges and four disdrometers (one 2DVD, one Joss, and two APL) arranged in two nested, equilateral triangles with sides of 2 and 4 km. The inner (2km) equilateral triangle will contain nine rain gauges and all four of the disdrometers. Two vertices of the inner triangle will each contain a tight cluster of 3 rain gauges which will be separated by approximately 10 to 100 meters. As shown in Fig. 1, one of these two vertices will be collocated with the profiler at the Ji Parana airport which is situated approximately 30 km (50 km) to the north-northeast (east-southeast) of the S-pol (TOGA) radar. The Joss, one 2-D video, and one APL disdrometer will be located at the profiler site. The purpose of collocating the three disdrometers is to provide cross-validation between the three instruments. The PC used to process and display the 2-D video disdrometer data will be housed in one of the profiler seatainers. The second vertex possessing 3 closely clustered gauges will be accompanied by an APL disdrometer. The third vertex of the inner triangle will contain a single rain gauge and an APL disdrometer. Along two sides of the inner triangle, a rain gauge will placed approximately 0.5 km from a vertex. Finally, the outer (4 km) equilateral triangle will consist of one rain gauge at each vertex with an APL collocated at one of the vertices. This multi-instrumented (profilers, disdrometers, gauges) ground validation network should provide unprecedented observations of DSD, rainfall rate, and precipitation vertical structure in tropical continental rainfall and will be crucial to achieve improved understanding of the ground radar (both polarimetric and Doppler) measurements and the TRMM satellite measurements.

Fig. 3 Idealization of gauge/disdrometer network # 2 arranged in two nested triangles similar to network #1. There are a total of 12 Qualimetric rain gauges and 4 disdrometers. The primary difference between the layout of the two networks is the number and location of disdrometers. The gauge layout is the same. See the text for more details.

Network # 2 (Fig. 3) will be centered approximately 55 km to the north-northwest of the S-pol radar along BR 364 (Fig. 1) and will consist of twelve rain gauges and three APL disdrometers arranged in a similar nested triangle configuration (e.g., equilateral triangles measuring 2 and 4 km on a side). One APL disdrometer will be placed with a rain gauge at each of the three vertices of the inner triangle. There will also be an APL disdrometer at one of the vertices of the outer triangle. The purpose of separating 24 gauges and 10 disdrometers into two separate networks is to maximize the probability of capturing a significant rainfall event on any given day while still allowing the investigation of rainfall spatial variability on small scales.

Fig. 4 Idealization of gauge network # 3 and 4 arranged in a triangle configuration. See the text for more details.

Rain gauge networks # 3 and #4 (Fig. 4) will be located 80 and 120 km to the northwest of the S-pol radar along BR 364 as shown in Fig. 1. Each of these networks will contain three gauges arranged in a triangle measuring approximately 1 to 1.5 km on a side. The purpose of these two networks is to provide information regarding the range behavior of the radar estimation of rainfall at distances far from S-pol, which is critical for the development of TRMM GV rainmap products. The combination of the four gauge networks provide reference data for the S-pol radar at ranges from 30 to 120 km . In addition, reference data will be available at 20 to 50 km in range to both the southeast and northwest of the TOGA radar. The gauges will be clustered at scales less than a typical radar rainfall grid (say 2 km) so that a representative rain gauge sample average over the corresponding grid point can be compared to the radar rainfall estimate with confidence. Of equal importance, each of the 4 networks will be located along a well travelled, fast moving, and paved road (BR 364). This should facilitate data collection and instrument maintenance.

3. Staffing plan

The tipping bucket rain gauges and the APL and Joss disdrometers are stand alone, low maintenance equipment which require only periodic data collection and maintenance visits. As a result, site visits to each gauge or impact disdrometer will typically occur once a week to download data and check on instrument status. These visits will be the responsibility of the "rain gauge-disdrometer scientist (RGDS), who will be designated by the operations director. The RGDS will be responsible for site visits to the gauge/disdrometer networks, necessary maintenance, data collection, and quality control in the field. Based on available manpower, the RGDS may utilize personnel from other platforms in order to complete the site visits.

Based on TRMM's experience during TEFLUN-B, the 2-D video disdrometer is a more delicate and higher maintenance instrument when operating 24 hours a day in a high humidity environment. The RGDS will also be responsible for monitoring and servicing these instruments as needed. Similar to the rain gauge and impact disdrometer tasks, the RGDS is expected to utilize assistance from scientists dedicated to other platforms (e.g., TOGA or S-POL radar crew members) to assist in the maintenance and monitoring of the 2DVDS. Preferably, this scientist should have prior field experience with the 2DVD. This individual can coordinate data monitoring, data collection, QC efforts, and any necessary maintenance.

4. Operations scenario

The data loggers for the Qualimetrics rain gauges have enough memory to record approximately 1500 mm of rainfall. The expected total IOP (January and February) rain accumulation at gauges in Rondonia is from 200 to 1300 mm. However, it is advisable to visit the sites as frequently as possible to check on instrument status. Therefore, all rain gauges and their collocated APL disdrometers should be checked and their data down loaded and saved with a laptop PC approximately once a week. Because the gauges and disdrometers are distributed over a relatively large area and because accessibility may be difficult in the wet season, it is expected that the RGDS will spend each day collecting data from a small subset of the gauges and APL disdrometers. Over the course of a week, it is anticipated that all of the gauge/disdrometer sites can be visited. As stated above, the RGDS may utilize additional personnel to perform the gauge-disdrometer site visits. Thus, the composition of the gauge/disdrometer site visit teams will be determined in cooperation between the RGDS, the operations director, and the chief radar scientists at the S-pol and TOGA radars.

The 2-D video disdrometers will require more frequent attention. Based on recent experience in a hostile tropical environment during TEFLUN-B, site visits should occur on a daily basis at both 2DVD deployment locations. The frequency of these site visits during the experiment will depend on the performance of the instruments in the field and the availability of personnel for the task.

5. Data plan

a. Collection method

Rain gauge and APL disdrometer data will be collected from the UNIDATA data loggers using a portable lap top computer and customized software provided by the TRMM Project Office. This procedure will be accomplished once a week. Three (3) back-up copies of the gauge and disdrometer data should be made immediately on a PC floppy disk or tape drive. In addition, three (3) back up disks or tapes should be made of the 2-D video disdrometer data approximately once every week. The Joss disdrometer data will be automatically archived on a computer housed within the profiler seatainer. Back-up copies of this data will be made on Jaz drive media by personnel responsible for maintaining the profiler equipment.

b. QC plan

Quality control (QC) of the rain gauge and disdrometer data will occur on a periodic basis during the IOP using data analysis software on a laptop PC (or the 2DVD PC) provided by the TRMM Project Office, manufacturers, and/or TRMM PIs. During the field campaign, data from the gauges and APL and Joss disdrometers will be available for QC at least once a week. Ideally, the in-field QC process would cover all data from all instruments. However, it is recognized that the volume of data combined with limitations in manpower will make complete in-field QC a difficult goal to achieve. At a minimum, all data from each gauge and disdrometer should be scanned and partially QC'ed in order to identify obvious and serious instrument problems. Samples of various gauge and disdrometer data should be QC'ed in complete detail. The RGDS will be responsible for leading the in-field QC effort. Data from the 2DVD should be viewed and analyzed on an "as needed" and likely more frequent basis using software provided by the manufacturer and/or TRMM PIs on the 2DVD PC. Final, post-mission rain gauge and disdrometer data QC will be accomplished by the TRMM office in conjunction with other TRMM LBA investigators using the same procedures utilized for primary TRMM GV site data.

c. Availability plan

The TRMM Project Office will make raw rain gauge and disdrometer data gathered and periodically QC'ed in the field available via FTP as soon as practical after conclusion of the IOP. Copies of all data collected from the rain gauges will be provided to the assigned Brazilian collaborating P.I. prior to departure of U.S. scientists.

d. Archive plan

The TRMM Project Office in collaboration with the TRMM TISDIS should archive the raw and QC'ed rain gauge and disdrometer data on mass store. In addition, back-up tape copies of raw and QC'ed data should be maintained by the TRMM Project Office and TRMM PI's.