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Hematite occurs in various geologic settings including igneous, metamorphic, and sedimentary rocks as well as in soils. However, it frequently occurs at low concentrations, especially in soils, where it may be <1% by weight. Because hematite has the potential to be an indicator of oxidizing and climatic conditions in soils and paleosols, it is important to understand its limit of detection. In this paper we examine the limits of detection of hematite visually and with diffuse reflectance spectrophotometry (DRS) and X‑ray diffraction (XRD). To accomplish this we used a sample set consisting of “knowns” or calibration samples. These known samples consisted of 15 different matrices of varying mineral composition into which hematite in 7 different concentrations ranging from 0.01 to 4% by weight were mixed. Including the 0% hematite, our calibration data set consisted of 120 samples. Visually, hematite can be detected at a concentration of 0.01% by weight in a light matrix and 0.5% in the darkest of our matrices. However, because of metamerism, visual techniques cannot specifically identify hematite. We find that for both DRS and XRD the limit of detection is also dependent on the matrix. For XRD the limit of detection for hematite in bulk samples is about 1%. For DRS the limit of detection depends on the data reduction technique used. The commonly used Kubelka-Munk remission function and its first and second derivatives can easily identify hematite at the 0.5% level. However, the first derivative of the percent reflectance curve can detect hematite at 0.01% by weight in a light matrix and 0.05% in a dark matrix. We suggest that the first derivative of DRS curves is the best currently available method for qualitatively detecting the mineral hematite at low concentrations found in soils, sediments, and rocks.

In the backdrop of a significant improvement in weather prediction with Numerical Weather Prediction models, quantitative prediction of the intensity of heavy rainfall events and associated disasters has remained a challenge. Encouraged by the recent emergence of compelling observational evidence for a significant electrical influence on cloud/rain microphysical processes (Mudiar, Pawar, Gopalakrishnana et al., 2021, https://doi.org/10.1029/2021gl093577), here we propose a hypothesis that the modification of the raindrop size distribution (RDSD) towards larger drop sizes facilitated by cloud electric fields could be one factor responsible for realistic rainfall intensity in weather/climate models. The robustness of the proposed hypothesis is confirmed through a series of simulations of strongly electrified rain events with the Weather Research and Forecasting model incorporating the electrically modified RDSD parameters in the model physics. Our results indicate a possible roadmap for improving hazard prediction associated with extreme rainfall events in weather prediction models. Lightning and heavy precipitation events in the tropics have significant socio‐economic implications in regard to disaster management. The accurate prediction of these meteorological events has remained a challenge despite considerable improvement in the Numerical Weather Prediction model over the years. With convincing observational evidence of a significant impact of the cloud electric field on the raindrop size distribution (RDSD) in the background, here we have attempted to test the model fidelity to simulate rain events with a strong in‐cloud electrical environment using the Weather Research and Forecasting model. A significant underestimation of the observed rain intensity is noted for the rain events that are associated with lightning. A possible roadmap has been discussed to improve the observed dry bias of rainfall by incorporating the electrically modified raindrop RDSD parameters in the model physics. Substantial improvement in the rain intensity has been observed with the incorporation of electrically modified RDSD parameters. Cloud electric field substantially modifies the raindrop size distribution in strongly electrified cloudsWeather Research and Forecasting model simulations show a significant underestimation of the observed rain intensity for strongly electrified rain eventsThe simulated rain intensity improved significantly with the incorporation of electrically modified raindrop size distribution parameters Cloud electric field substantially modifies the raindrop size distribution in strongly electrified clouds Weather Research and Forecasting model simulations show a significant underestimation of the observed rain intensity for strongly electrified rain events The simulated rain intensity improved significantly with the incorporation of electrically modified raindrop size distribution parameters

Simultaneous measurements of cloud electric field and raindrop size at the Earth's surface have been analyzed to investigate an anticipated relation between the two cloud parameters. A significant positive correlation is observed between the magnitude of the surface electric field and raindrop size supported by a theoretical framework advanced here. It is shown that in the presence of an electric field, raindrops increase their size through electrically induced collision and coalescence processes near the Earth's surface. This study also demonstrates that the presence of an electric field inside the cloud can enhance the rain intensity by influencing the growth rate of raindrops. This new insight may be useful for a proper understanding of the rain microphysics and for developing better numerical weather models for simulation and forecasting of heavy rainfall events associated with stronger in‐cloud electrical environments. The rainfall observed at the Earth's surface is a manifestation of different cloud microphysical processes. The condensation of water vapor on cloud condensation nuclei (CCN) and subsequent collision/coalescence between cloud droplets are the two dominant processes of rain formation in the warm phase of cloud. Many laboratory investigations suggest that both processes are quite sensitive to the electrification of the cloud. But direct evidence of the influence of cloud electric field on the rain formation in the real atmospheric condition is lacking till now. With simultaneous observations of the cloud electric field and the raindrop size distribution below a few strongly electrified clouds, for the first time, it has been shown that larger cloud electric field remains associated with larger raindrops at the Earth's surface. A mathematical framework has also been developed to support the observations. It has also been shown that the cloud electric field can modify the rain rate at the Earth's surface by enhancing the growth of raindrops. Simultaneous measurements of surface electric field, raindrop size and rain rate have been analyzedThe results indicate that the electric field below strongly electrified cloud can enhance the size of raindrops and hence the rain rateThese results may be useful for better understanding and prediction of heavy rainfall events associated with stronger electrification Simultaneous measurements of surface electric field, raindrop size and rain rate have been analyzed The results indicate that the electric field below strongly electrified cloud can enhance the size of raindrops and hence the rain rate These results may be useful for better understanding and prediction of heavy rainfall events associated with stronger electrification

DUNDEE (Down Under Doppler and Electricity Experiment) is described. DUNDEE was carried out in the vicinity of Darwin, Northern Territory, Australia, during the wet seasons of November 1988 through February 1989, and November 1989 through February 1990. The general goal of DUNDEE was to investigate the dynamical and electrical properties of tropical mesoscale convective systems and isolated deep convective storms. Darwin, situated at the southern tip of the “maritime continent,” experiences both monsoon and “break” period conditions during the wet season. We discuss the observational network deployed for DUNDEE and present preliminary scientific results. One particularly interesting observation is a large contrast in the frequency of total lightning between break period convection (high lightning rates) and convection in the monsoon trough (low lightning rates). A relationship between CAPE (convective available potentional energy) and total flash rate is presented and discussed to explain this observation.

AbstractThe ice phase is included in thermodynamic calculations of convective available potential energy (CAPE) for a large number of soundings in the tropical atmosphere, at both land and ocean stations. It is found that the positive-buoyancy contribution to CAPE resulting from the latent heat of fusion more than offsets the negative-buoyancy contribution due to water loading in the reversible thermodynamic process. The departure from moist neutrality in much of the tropical atmosphere exhibits a threshold in boundary-layer wet-bulb potential temperature of 22°–23°C. The corresponding sea surface temperature is approximately 26°C, close to the empirical threshold for hurricane formation, which suggests that conditional instability plays an important role in the latter phenomenon. The simultaneous presence of finite CAPE and infrequent deep convection in the tropics is tentatively attributed to the convective inhibition energy (CINE) and to the mixing process that destroys positive buoyancy in incipient cloud parcels.

Structural features have been identified on positively charged lightning channels that are not present on negatively charged ones. The discovery could explain why these two types of channel have different behaviours. Lightning observed with unprecedented spatial resolution.

An uncommon three‐stroke positive cloud‐to‐ground (+CG) lightning flash was observed by synchronous multifrequency radiation sensors, and the three‐dimensional development of the channel was carefully mapped. Results show that three positive strokes grounded at different points approximately 4–8 km apart and time intervals between neighboring +CG strokes were 85 and 222 ms, respectively. This +CG flash was preceded by an intracloud (IC) discharge. The locations of the termination points on the ground were just below the preestablished horizontal IC discharge channels, and the three strokes were linked with each other through sharing horizontal negative leader channels inside the cloud. There were stepping pulses less than 0.4 ms before the first and second return strokes (RSs), and their location results were distributed nearby the following RS, indicating that the pulses might be associated with the attachment process. The first and second positive strokes occurred when the horizontal in‐cloud negative channel stopped extending, while the third positive stroke occurred below the opposite end of an advancing negative leader in the cloud. The above results suggest that the downward positive leader preceding the strokes possibly developed either from different decayed leader channels or from the opposite end of an advancing leader channel. A high‐speed camera captured a downward positive attempted leader from another +CG flash, revealing how a downward positive leader incepted from a horizontal negative lightning channel. Positive cloud‐to‐ground (+CG) lightning flashes transfer the positive charge from a cloud to the ground and usually have only one return stroke (RS). The development of multistroke positive lightning remains poorly understood. Here we report how a three‐stroke +CG flash occurred based on the 3‐D lightning radiation mapping method, and the results revealed that the +CG flash was preceded by an intracloud (IC) discharge. Three +CG RSs terminated on totally different points, but they were all right below the preestablished horizontal lightning channels inside the cloud, which confirmed that they belong to one lightning flash rather than independent three +CG strokes. In the tens of milliseconds before +RS1 and +RS2, there was no obvious in‐cloud lightning activity (the leaders remained quiet), while developing in‐cloud leader was found preceding the occurrence of +RS3. A high‐speed video of another +CG flash revealed how the downward positive leader could initiate from a horizontal negative leader channel. An uncommon three‐stroke positive cloud‐to‐ground lightning flash was detected with synchronized multifrequency radiation sensorsThree positive strokes terminated at different ground points, but they were associated with channels of an same IC dischargeA downward positive leader possibly developed from either a decayed negative channel or the opposite end of an advancing negative leader