# Doppler on Wheels

Doppler on Wheels (or DOW) is a fleet of X-band and C-band mobile and quickly-deployable truck-borne radars which are the core instrumentation of the Flexible Array of Radars and Mesonets affiliated with the University of Alabama Huntsville and led by Joshua Wurman, with the funding partially provided by the National Science Foundation (NSF), as part of the "Community Instruments and Facilities," (CIF) program. The DOW fleet and its associated Mobile Mesonets and deployable weather stations (PODs & Polenet) have been used throughout the United States since 1995, as well as occasionally in Europe and Southern America. The Doppler on Wheels network has deployed itself through hazardous and challenging weather to gather data and information that may be missed by conventional stationary radar systems.

## History

The first DOW platform was created and deployed in 1995, substantially changing the design paradigm of targeted meteorological studies. Ground-breaking, extremely fine-scale data was collected in tornadoes and hurricanes, as well as other phenomena. DOWs, by virtue of providing especially fine-scale targeted observations, have been central to various scientific discoveries, "firsts", and pioneering observations, e.g. the first concrete documentation of specific impacts of weather modification cloud seeding, the first mapping of multiple-vortices in tornadoes, the quantification of tornado low-level wind structure, etc. The DOW program rapidly expanded and evolved to include the first mobile dual-Doppler weather radar network, the first mobile rapid-scan radar (the Rapid-Scan DOW, RSDOW), and the first quickly-deployable 1-degree C-band radar, the C-band on Wheels (COW). The DOWs, Mobile Mesonets, PODs & POLEs, as well as many other devices were crucial for instrumentation in numerous field projects, including VORTEX, VORTEX2, COPS, MAP, ASCII, IHOP, SCMS, CASES, ROTATE, PAMREX, SNOWD-UNDER, FLATLAND, HERO, UIDOW, UNDEO, LEE, PERILS, WINTRE-MIX, RELAMPAGO, GRAINEX, and others.
In late 2018, the DOW Facility debuted a new quickly-deployable C-band radar (or COW) featuring a larger antenna and 5cm wavelength (as compared to the 3cm wavelength of the DOWs). Due to the larger size of the antenna, the truck features a built-in crane allowing for the radar to be assembled on site. The COW was first deployed as part of the RELAMPAGO field campaign in Argentina in late 2018.
The DOW fleet has collected data in 250 tornadoes and inside the cores of eighteen hurricanes. DOWs have been deployed to Europe twice, for the MAP and COPS field programs, and to Alaska twice for the JAWS-Juneau projects, and to South America for RELAMPAGO. DOWs have operated as high as 12,700 feet (3,900 m) on Bristol Head and at 10,000 feet (3,000 m) for the ASCII project at Battle Pass.
The DOW fleet, PODS, and Mobile Mesonets have been featured on television, including Discovery Channel's series Storm Chasers (joined by the Tornado Intercept Vehicles and the Dominator SRV vehicles), National Geographic Channel's specials Tornado Intercept and The True Face of Hurricanes, and PBS's Nova episode "The Hunt for the Supertwister," and others.

## Capabilities

As of May 2024, the current operational Doppler on Wheels vehicles include the CROW (which consists of the DOW8/RSDOW/Mini-COW), and the COW (C-band On Wheels, occasionally referred to as the COW1).
The COW consists of a C-band dual-polarization dual-frequency radar system utilizing two 1 MW transmitters set to a 5cm wavelength configuration. The CROW consists of three separate configurations, the DOW8, which utilizes a single-polarization 250 kW X-band transmitter, the RSDOW, which consists of a 7-second rapid-scan passive phased array antenna, utilizing a TWT 40 kW X-band transmitter system, and the Mini-COW, utilizing a singular 1 MW C-band transmitter capable of 50-second dual-polarization updates.
As of May 2024, the DOW6 and DOW7 are currently undergoing overhauls with new equipment, including the vehicles themselves, the transmitters, and the computing systems, as well as the integration with the new GURU software. The previous iterations of the DOW6 and DOW7 utilized dual-polarization dual-frequency 250 KW X-band transmitters, and were the most powerful mobile X-band systems at the time.
DOWs are frequently deployed with the tightly integrated surface instrumentation network of the FARM. Several instrumented mobile mesonet pickup trucks host in situ weather instrumentation on 3.5-metre (11 ft) masts to complement the remote sensing radars. These mobile mesonets also carry approximately twenty instrumented "PODS", which are ruggedized quickly deployable weather stations designed to survive inside tornadoes, tropical cyclones, and other adverse environments, and a Polenet comprising instrumentation deployed on poles, railings, fences, etc. during hurricane landfalls. An array of up to seven upper air and swarm sounding systems can also be deployed with the DOWs. The DOW fleet is sometimes accompanied by a Mobile Operations and Repair Center (MORC), a large van containing workstations for in-field coordination, data management, and equipment repair.

## Findings

DOW data led to the discovery of sub-kilometer hurricane boundary layer rolls, which likely modulate wind damage and may play a key role in hurricane intensification. DOW data revealed some of the most intense tornadic winds ever recorded (the Bridge Creek–Moore tornado, 3 May 1999, the El Reno tornado, 31 May 2013, and the Greenfield, Iowa Tornado, 21 May 2024), and the largest tornadic circulation ever documented (the Cimarron City–Mulhall–Perry Tornado, which also occurred on 3 May, 1999), and made the first 3D maps of tornado winds and sub-tornadic vortex winds, and documented intense vortices within lake-effect snow bands. About 70 peer-reviewed scientific publications have used DOW data.
DOW data has led to the discovery of the descending reflectivity core, a microscale phenomenon that may aid in tornadogenesis.

## Future instrumentation

There are currently two major projects planned to expand the FARM's capabilities. The first is the creation of an S-band on Wheels Network (SOWNET) featuring four quickly-deployable S-band radars with 10 cm wavelengths capable of seeing through intense precipitation. These smaller truck-mounted radars would replace a single large S-band radar, allowing for dual-Doppler analyses and quicker deployment times. The second planned project is the Bistatic Adaptable Radar Network (BARN) which will be integrated with existing DOWs and the COW to provide high resolution wind vector observations without the need for multiple, expensive transmitters. These bistatic receivers will consist of small antennas that can be deployed like Pods or mounted onto a Mobile Mesonet or similar vehicle.