Abstract The Beobachtung von Ozean und Wolken–Das Trans ITCZ Experiment (BOWTIE) field campaign investigated how convective storm dynamics interact with the ocean surface to shape the structure of the Atlantic intertropical convergence zone (ITCZ). Conducted aboard the German Research Vessel (R/V) Meteor during August and September 2024, the campaign targeted the full meridional extent of the ITCZ while transiting the tropical Atlantic from east to west. The research was driven by evidence suggesting that storm-scale dynamics is pivotal for shaping the broader structure of the ITCZ and its connection to global circulation patterns and energy transport. BOWTIE featured high-resolution atmospheric and oceanic profiling, with a particular focus on the coupled boundary layers. Observations included cloud and humidity profiles, winds, precipitation, sea surface temperature, and upper-ocean physical and biogeochemical properties. A suite of advanced instruments provided vertically resolved cross sections of convective environments and surrounding conditions. BOWTIE was part of the larger international Organized Convection and EarthCARE Studies over the Tropical Atlantic (ORCESTRA) initiative, which coordinated eight campaigns across the Atlantic. During the voyage, the R/V Meteor served as a platform for two additional ORCESTRA campaigns: Soundings and Turbulent eddy measurements in the ITCZ with a Network of Quadcopters (STRINQS), which deployed unmanned aerial vehicles for profiling near-storm environments, and Process Investigation of Clouds and Convective Organization over the Atlantic Ocean (PICCOLO), which brought Colorado State University’s Sea-Pol scanning dual-polarization C-band radar onboard. This article provides an overview of BOWTIE’s scientific goals, campaign design, and observing strategy and presents selected early results from the extensive dataset. The combination of in situ, airborne, and radar measurements offers new insight into how ocean–atmosphere interactions at convective scales shape the ITCZ’s broader structure and behavior. Significance Statement The intertropical convergence zone (ITCZ) plays a central role in shaping tropical rainfall and global circulation, yet the processes governing its structure and variability remain incompletely understood. The Beobachtung von Ozean und Wolken–Das Trans ITCZ Experiment (BOWTIE) field campaign provides a coupled observational view of the Atlantic ITCZ by combining ship-based atmospheric and oceanic measurements with scanning and profiling radar, autonomous platforms, and coordinated aircraft and satellite observations. By sampling the full meridional extent of the ITCZ over 40 days and nights, BOWTIE reveals how convective organization, boundary layer dynamics, and upper-ocean variability interact across spatial and temporal scales. These observations advance understanding of the physical processes that regulate tropical rain belts and their day-to-day variability.

Convective clouds play an important role in Earth's climate system and are a known source of extreme weather. Gaps in our understanding of convective vertical motions, microphysics, and precipitation across a full range of aerosol and meteorological regimes continue to limit our ability to predict the occurrence and intensity of these cloud systems. To improve predictability, the National Science Foundation (NSF) sponsored a large field experiment entitled "Experiment of Sea Breeze Convection, Aerosols, Precipitation, and Environment (ESCAPE)." ESCAPE took place between 30 May and 30 September 2022 in the vicinity of Houston, Texas, because this area frequently experiences isolated deep convection that interacts with the region's mesoscale circulations and its range of aerosol conditions. ESCAPE focused on collecting observations of isolated deep convection through innovative sampling and developing novel analysis techniques. This included the deployment of two research aircraft, the National Research Council of Canada Convair-580 and the Stratton Park Engineering Company Learjet, which combined conducted 24 research flights from 30 May to 17 June. On the ground, three mobile X-band radars and one mobile Doppler lidar truck equipped with soundings were deployed from 30 May to 28 June. From 1 August to 30 September 2022, a dual-polarization C-band radar was deployed and operated using a novel, multisensor agile adaptive sampling strategy to track the entire life cycle of isolated convective clouds. Analysis of the ESCAPE observations has already yielded preliminary findings on how aerosols and environmental conditions impact the convective life cycle. SIGNIFICANCE STATEMENT: The ESCAPE field experiment provided unique observations of coastal convective cloud vertical motions, microphysics, and precipitation across a wide range of summertime aerosol and meteorological regimes. The highest aerosol concentrations occurred near the refineries in eastern Houston but did not contribute to the cloud condensation nuclei and ice-nucleating particles. The airborne measurements included frequent sampling of intense convective updraft dynamics and microphysics. A novel radar-based sampling of convective cells provided unique observations of their 3D structure throughout their life cycle. Mobile trucks equipped with soundings provided a detailed sampling of the sea-breeze structure and evolution. These datasets will be used for improving high-resolution simulations of high-impact events in coastal urbanized areas.

We present findings from radar calibration experiments involving three radars operated by the Colorado State University (CSU) in the US and by the & Eacute;cole Polytechnique F & eacute;d & eacute;rale de Lausanne (EPFL) in Switzerland. The experiments were based on the comparison between measured radar variables and the known properties of artificial point targets electronically generated with a polarimetric radar target simulator (RTS) from Palindrome Remote Sensing. Radars under test included the two magnetron-based radars CHILL and SPLASH (its mobile version) from CSU and EPFL's new solid-state radar StXPol.For the CHILL and SPLASH calibration measurements in Colorado, a mobile lifting platform was employed that elevated the target simulator instrument to approximately 15 m above ground. The creation of virtual targets with polarimetric signatures allowed for a direct calibration of polarimetric variables. While the SPLASH radar exhibited good Zdr and sufficient Zh accuracy, remarkable precision and stability were found in CHILL's reflectivity data time series, where the reflectivity bias compared to the virtual target was less than 0.2 dB over a 1 h time series.Calibration issues that arise with solid-state radar systems were investigated with experiments conducted with the EPFL StXPol radar. This pulse compression system transmits a linear frequency-modulated long pulse as well as a non-modulated short pulse for observations at close ranges. The two pulses are separated in frequency by 50 MHz, and consequently calibration targets were generated independently for the two channels. Excellent stability and accuracy were found for Zdr in both channels. While Zh stability was also very high, a large reflectivity bias in both the long and the short pulse channel was detected.For the first time, the article introduces and analyzes a weather radar calibration procedure that is based on electronically generated radar targets. Experimental data suggest that precise absolute and differential calibrations can be achieved if data are obtained in an environment free from multipaths and if the generated targets are precisely located in the center of the radar's range gate. Experimental shortcomings associated with limited sampling resolution of the radar scan over the targets are also investigated.

Winter precipitation forecasts of phase and amount are challenging, especially in Northeast United States where mixed precipitation events from various synoptic systems frequently occur. Yet, there are not enough quality observations of winter precipitation, particularly microphysical properties from falling snow or mixed phase precipitation. During the winters of 2021-2022, 2022-2023, and 2023-2024, the NASA Global Precipitation Measurement (GPM) Ground Validation (GV) program conducted a field campaign at the University of Connecticut (UConn). The goal of this campaign was to observe various phases of winter precipitation and winter storm types to validate the GPM satellite precipitation products. Over the three winters at UConn, a total of 40 instruments were deployed across two observing sites that captured 117 precipitation events, including 19 phase transition events as indicated by the PARSIVEL(2). These instruments included scanning and vertically pointing radars, along with suites of in-situ sensors. In addition, an unmanned aircraft system has been deployed in 2023-2024. Here, an overview of the different field deployments, instrumentation, and the datasets collected are presented. To showcase the observations, this article features a wide-ranging set of measurements collected from the instrument suite for the 28 February 2023 storm, during which six to eight inches of snow accumulated at the two different observing sites. Also included is a discussion on how these observations can be combined with other datasets to validate ground-based and remote sensing measurements and highlight important atmospheric processes that impact winter precipitation phase and amount. The datasets collected from this GPM GV field campaign are available at 10.5067/GPMGVUCONN/DATA101 (Cerrai et al., 2025).

A comprehensive understanding of various microphysical processes underlying precipitation formation can be achieved through simultaneous measurements from ground-based and airborne radar systems at different frequencies. The study presented in this paper primarily centers on the analysis of collective observations of winter precipitation obtained from various radars deployed during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) conducted by the National Aeronautics and Space Administration (NASA). This work focuses on precipitation observing remote sensing instruments, namely the Dual-frequency Dual-polarized Doppler Radar (D3R) operating at Ku/Ka-bands, airborne radars at X, Ku, Ka, and W-band frequencies aboard NASA's ER-2 flight, and instruments on NASA's P-3 flight, including probes and dropsondes. Coordinated Range Height Indicator (RHI) scans from the D3R along the flight track are used for simultaneous observation of a snowstorm event on 28 th February 2023. Cross-validation procedures are performed, accounting for differences in spatial resolution and viewing geometry through volume matching. The ground based disdrometer is used to project the level of inter-comparison expected between radar measurements at different frequencies. The inter-comparison between the D3R and the aircraft-borne radar systems revealed consistent measurements. Additionally, this study provides an assessment of distinct ice crystal habits observed during the storm based on radar measurements, corroborated by corresponding observations from microphysics probes aboard the P-3 flight.

Advanced Quantitative Precipitation Information (AQPI) is a synergistic project that combines observations and models to improve monitoring and forecasts of precipitation, streamflow, and coastal flooding in the San Francisco Bay Area. As an experimental system, AQPI leverages more than a decade of research, innovation, and implementation of a statewide, state-of-the-art network of observations, and development of the next generation of weather and coastal forecast models. AQPI was developed as a prototype in response to requests from the water management community for improved information on precipitation, riverine, and coastal conditions to inform their decision-making processes. Observation of precipitation in the complex Bay Area landscape of California’s coastal mountain ranges is known to be a challenging problem. But, with new advanced radar network techniques, AQPI is helping fill an important observational gap for this highly populated and vulnerable metropolitan area. The prototype AQPI system consists of improved weather radar data for precipitation estimation; additional surface measurements of precipitation, streamflow, and soil moisture; and a suite of integrated forecast modeling systems to improve situational awareness about current and future water conditions from sky to sea. Together these tools will help improve emergency preparedness and public response to prevent loss of life and destruction of property during extreme storms accompanied by heavy precipitation and high coastal water levels—especially high-moisture laden atmospheric rivers. The Bay Area AQPI system could potentially be replicated in other urban regions in California, the United States, and worldwide.

During three consecutive winter seasons, between December 2021 and April 2024, several ground-based wintry precipitation measurement instruments were deployed at the University of Connecticut’s main campus. The instruments included an assortment of K-band and W-band profiling radars and Ka-Ku band scanning radars, weighing, and tipping bucket pluviometers, laser disdrometers, high-speed and high-resolution cameras for quantitative precipitation measurement, weather stations, and an unmanned aircraft system for environmental variables. The goal of this field campaign is to provide a dataset for validating NASA Global Precipitation Measurement (GPM) products, and to examine the error characteristics of co-located ground-based instruments. In this manuscript, we present the instrument suite and discuss possible uses of this unique set of measurements for remote sensing applications.

Water is a critical resource that causes significant challenges to inhabitants of the western United States. These challenges are likely to intensify as the result of expanding population and climate-related changes that act to reduce runoff in areas of complex terrain. To better understand the physical processes that drive the transition of mountain precipitation to streamflow, the National Oceanic and Atmospheric Administration has deployed suites of environmental sensors throughout the East River watershed of Colorado as part of the Study of Precipitation, the Lower Atmosphere, and Surface for Hydrometeorology (SPLASH). This includes surface-based sensors over a network of five different observing sites, airborne platforms, and sophisticated remote sensors to provide detailed information on spatiotemporal variability of key parameters. With a 2-yr deployment, these sensors offer detailed insight into precipitation, the lower atmosphere, and the surface, and support the development of datasets targeting improved prediction of weather and water. Initial datasets have been published and are laying a foundation for improved characterization of physical processes and their interactions driving mountain hydrology, evaluation and improvement of numerical prediction tools, and educational activities. SPLASH observations contain a depth and breadth of information that enables a variety of atmospheric and hydrological science analyses over the coming years that leverage collaborations between national laboratories, academia, and stakeholders, including industry.

The science of mountainous hydrology spans the atmosphere through the bedrock and inherently crosses physical and disciplinary boundaries: land-atmosphere interactions in complex terrain enhance clouds and precipitation, while watersheds retain and release water over a large range of spatial and temporal scales. Limited observations in complex terrain challenge efforts to improve predictive models of the hydrology in the face of rapid changes. The Upper Colorado River exemplifies these challenges, especially with ongoing mismatches between precipitation, snowpack, and discharge. Consequently, the U.S. Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) user facility has deployed an observatory to the East River Watershed near Crested Butte, Colorado, between September 2021 and June 2023 to measure the main atmospheric drivers of water resources, including precipitation, clouds, winds, aerosols, radiation, temperature, and humidity. This effort, called the Surface Atmosphere Integrated Field Laboratory (SAIL), is also working in tandem with DOE-sponsored surface and subsurface hydrologists and other federal, state, and local partners. SAIL data can be benchmarks for model development by producing a wide range of observational information on precipitation and its associated processes, including those processes that impact snowpack sublimation and redistribution, aerosol direct radiative effects in the atmosphere and in the snowpack, aerosol impacts on clouds and precipitation, and processes controlling surface fluxes of energy and mass. Preliminary data from SAIL's first year showcase the rich information content in SAIL's many datastreams and support testing hypotheses that will ultimately improve scientific understanding and predictability of Upper Colorado River hydrology in 2023 and beyond.

A new, advanced radar has been developed at Colorado State University (CSU). The Sea-Going Polarimetric (SEA-POL) radar is a C-band, polarimetric Doppler radar specifically designed to deploy on research ships. SEA-POL is the first such weather radar developed in the United States. Ship-based weather radars have a long history, dating back to GATE in 1974. The GATE radars measured only reflectivity. After GATE, ship radars also provided Doppler measurements. SEA-POL represents the next advancement by adding dual-polarization technology, the ability to transmit and receive both horizontal and vertical polarizations. This configuration provides information about hydrometeor size, shape, and phase. As a result, superior rain-rate estimates are afforded by the dual-polarization technology, along with hydrometeor identification and overall improved data quality. SEA-POL made its first deployment as part of the Salinity Processes in the Upper Ocean Regional Study, second field phase (SPURS-2) fall 2017 cruise to the eastern tropical Pacific, sailing on the R/V Roger Revelle. SPURS-2 was a field project to investigate the fate of freshwater deposited on the ocean's surface. Oceanographers are keenly interested in how fast these freshwater patches mix out by wind and upper-ocean turbulence, as the less dense rainfall sitting atop the salty ocean inhibits mixing through increased stability. To this end, during SPURS-2, SEA-POL produced rain maps identifying the location of freshwater lenses on the ocean's surface thereby providing context for measurements of SST and salinity. Examples of SEA-POL polarization measurements are also discussed to assess microphysical processes within oceanic convection. Future ocean-based field campaigns will now benefit from SEA-POL's advanced dual-polarization technology.

As part of the Salinity Processes in the Upper-ocean Regional Study (SPURS-2) 2017 cruise to the eastern tropical Pacific, the Colorado State University SEA-POL (SEA-going POLarimetric) C-band radar made its first ever ship deployment. Previous ship-based experiments have used Doppler radars to map rainfall and the structure of oceanic convection, but SPURS-2 marked the first time the US research community deployed a dual-polarimetric radar at sea. Dual-polarimetric radar transmits and receives electromagnetic radiation in both horizontal (H) and vertical (V) polarizations simultaneously and thereby makes additional, important measurements of precipitation compared to a single polarization radar, which normally transmits horizontal polarization only. For H-polarization, the electric field vector of the transmit pulse is horizontal to the local Earths surface; for V-polarization, the electric field vector is perpendicular to Earths surface. Polarization measurements provide information about particle size, shape, and phase (water vs. ice). As a result, superior rain rate estimates are afforded by the dual-polarimetric technology. During SPURS-2, SEA-POL produced rain maps in real time to locate freshwater lenses forming on the oceans surface to develop context for oceanographic measurements of surface temperature and salinity.

The CSU-CHILL radar is a dual-wavelength, dual-polarization weather radar system operating at S and X band with coaxial beams. This radar system offers a unique environment to develop and/or validate algorithms that cut across its wavelengths and polarizations. This paper presents a method to retrieve resonance scattering regions from the difference in intrinsic reflectivities after attenuation correction, which is performed using measured reflectivity fields only. The algorithm to retrieve these regions dominated by non-Rayleigh scattering is applied to different storm events, and the obtained data field capturing the difference in S- and X-band reflectivities due to resonance effects (which we will call Mie signal for convenience) is compared to the collocated dual-polarization fields. The obtained Mie signal is also compared to hail reports. In both cases, the retrieved Mie signal is found to be consistent with the rest of the dual-polarization data fields, and in some situations, it is shown to bring information not directly discernible from the usual dual-polarization radar variables.

Sea-Pol is ship- and land-deployable dual-polarization meterological radar system developed at Colorado State University. It is designed for operation aboard Global-class research ships operated by the US oceanographic community. The radar operates at C-band (5.65 GHz) and has a 4.3 meter stabilized antenna system. An inertial navigation unit (INU) is used to measure and compensate for ship platform motion. The radar can operate in simultaneous-transmit, simultaneous-receive (STSR) mode, as well as horizontal-transmit, simultaneous-receive mode. The radar uses a 250 kW coaxial magnetron transmitter capable of a variety of pulse widths and PRFs within a 0.12% duty cycle limit. The radar has a sensitivity of -7 dBZ at 100 km range. The radar is designed to be easily transported by virtue of being packaged in standard ISO-668 1C containers. Special care was taken to make Sea-Pol rapidly deployable and able to withstand the harsh environmental conditions aboard research ships on the open ocean.

Sea-Pol is a marine-deployable meteorological radar developed at Colorado State University. The radar architecture is presented, with a description of the platform motion compensation system and its performance. Initial observations at the Greeley, CO home base as well as during the SPURS-2 field campaign aboard the R/V Revelle are presented.

High resolution, networks of X-band radars can improve severe weather warning operations by observing the lower troposphere at very high spatiotemporal resolution. X-band networks provide unique information on storm features that complement existing radars such as NEXRAD and TDWR. In this paper, we examine the warning benefits of these small radars by looking at the performance of a network of 7 X-band CASA radars in the Dallas Fort Worth Metroplex, linked to real-time product generation and decision-making. By evaluating two severe weather episodes, a squall line and a mesoscale convective system, we begin to identify the strengths and weaknesses of the X-band radar networks, and propose future benefits to warning decision making.

This paper describes the transformation of the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) National Radar Facility from a single-frequency (S band) dual-polarization Doppler weather radar system to a dual-frequency (S and X bands) dual-polarization Doppler system with coaxial beams. A brief history regarding the development of dual-wavelength radars is first presented. In the past, dual-wavelength measurements were used to detect hail using the dual-wavelength ratio defined as the ratio of intrinsic (or attenuation corrected) X-band reflectivity to the S-band reflectivity. Departures of this ratio from unity were taken to indicate the presence of hail, produced by Mie scattering at the shorter wavelength by hail. Most dual-wavelength radars were developed with attempts to match beams for S and X bands, which implies that the sample volumes for the two frequencies were essentially the same. The X-band channel of the CSU-CHILL radar takes a different approach, that of making use of the already existing dual-offset-fed antenna designed to give a 1 degrees beamwidth at S band, resulting in an X-band beamwidth of approximately 0.3 degrees, with very high gain. Thus, the X band provides about a factor of 3 more resolution than the S-band component while maintaining the same sensitivity as the S-band component. Examples of cold season and warm season data from the X-band and S-band radar components are presented, demonstrating the successful transformation of the CSU-CHILL radar into a unique multifrequency, multipolarization system. The new CSU-CHILL dual-wavelength, dual-polarization weather radar will serve as an important asset for the scientific community.

The CSU-CHILL radar is a dual-wavelength, dual-polarization weather radar system operating at S and X bands with coaxial beams. One of the capabilities of this radar system is the possibility of developing and/or validating algorithms across dual wavelengths and dual polarizations. This paper presents one such instance, showing how the rainfall field can be estimated either from the S- and X-band reflectivities or from the differential propagation phase at X band. To do so, the paper first presents a dual-wavelength attenuation correction method that uses the reflectivity measured at S band, as the constraint for the correction of the reflectivity measured at X band, and it describes how Mie scattering regions at X band may be detected from the retrieved path-integrated attenuation field. Then, the paper describes how the resulting specific attenuation field relates to rainfall and specific phase at X band, which can be obtained from dual-polarization data at a single wavelength as well, and shows examples. Finally, the paper looks at the relation between attenuation and the differential phase as a function of elevation angle for a few cases, which may be related to the drop size distribution and mean diameter, as well as temperature.