Doppler on Wheels (or DOW) is a fleet of X-band and C-band radar 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 Illinois and led by Joshua Wurman, with the funding partially provided by the National Science Foundation (NSF). The DOW fleet and its associated Mobile Mesonets and deployable weather stations (Pods and Poles) Doppler on Wheels (DOW) has been deploying in hazardous/challenging weather, driving into the eye of the storm to gather scientific data about wind, rain and snow that are missed by stationary radar systems. They are part of the NSF's "Community Instruments and Facilities" (CIF) program.

The first DOW was invented and deployed in 1995, substantially changing the design paradigm of targeted meteorological studies. Ground-breaking extremely fine-scale data were collected in tornadoes ^[1] and hurricanes,^[2] 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 expanded and evolved to include the first mobile dual-Doppler radar network, the first mobile rapid-Scan radar (the Rapid-Scan DOW, RSDOW), the first quickly-deployable 1-degree C-band radar, the C-band on Wheels (COW). DOWs and integrated instrumentation were core instrumentation in many field projects, including VORTEX, VORTEX2, COPS, MAP, ASCII, IHOP, SCMS, CASES, ROTATE, PAMREX, SNOWD-UNDER, FLATLAND, HERO, UIDOW, UNDEO.^{[citation needed]}, LEE, PERILS,^[3] WINTRE-MIX,^[4] RELAMPAGO,^[5] GRAINEX,^[6] and others.

In late 2018, the DOW Facility debuted a new quickly-deployable C-band radar (or COW) featuring a larger antenna and 5 cm wavelength (as compared to the 3 cm 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.^[7]

The DOW fleet has collected data in 250 tornadoes and inside the cores of eighteen hurricanes. DOWs have been deployed to Europe twice,^[8] 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.

As of 2024, the operational DOWs/COWs include DOW6, DOW7, DOW8, the Rapid-Scan DOW (RSDOW), the COW and the mini-COW. DOW6, DOW7, DOW8, and the Rapid-Scan DOW (RSDOW). They are part of the Flexible Array of Radars and Mesonets (FARM) at the University of Illinois , supporting research and education. DOW6, DOW7, and COW are dual-polarization dual-frequency radars. The mini-COW is dual-polarization single frequency. DOW6 and DOW7 are the most powerful mobile X-band systems with 250 kW transmitters, and the COW and mini-COW are the most powerful mobile/quickly-deployable C-band radars with 2x 1 megawatt (COW) and a single 1 megawatt transmitter (mini-COW). The Rapid-Scan DOW is capable of 7-second rapid volumetric updates, employing a multiple-beam passive phased array (slotted waveguide) antenna.

DOWs are frequently deployed with the tightly integrated surface instrumentation network of the FARM.^[9] 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.^[10]

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 the most intense winds ever recorded (Bridge Creek tornado, 3 May 1999),^[11] and the largest tornadic circulation ever documented (also 3 May 1999 in Mulhall, OK),^[12] 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.^{[citation needed]}

The DOW fleet, PODS, and mobile mesonets have been featured on television, including Discovery Channel's reality series Storm Chasers,^[13] 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.^{[citation needed]}

DOW data has led to the discovery of the descending reflectivity core, a microscale phenomenon that may aid in tornadogenesis.

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.^[14]

• Joshua Wurman
• Bistatic radar
• Pulse-Doppler radar
• Storm chasing