Abstract

Extreme heat is annually the deadliest weather hazard in the United States and is strongly amplified by climate change. In Florida, summer heat waves have increased in frequency and duration, exacerbating negative human health impacts on a state with a substantial older population and industries (e.g., agriculture) that require frequent outdoor work. However, the combined impacts of temperature and humidity (heat stress) have not been previously investigated. For eight Florida cities, this study constructs summer climatologies and trend analyses (1950–2020) of two heat stress metrics: heat index (HI) and wet-bulb globe temperature (WBGT). While both incorporate temperature and humidity, WBGT also includes wind and solar radiation and is a more comprehensive measure of heat stress on the human body. With minor exceptions, results show increases in average summer daily maximum, mean, and minimum HI and WBGT throughout Florida. Daily minimum HI and WBGT exhibit statistically significant increases at all eight stations, emphasizing a hazardous rise in nighttime heat stress. Corresponding to other recent studies, HI and WBGT increases are largest in coastal subtropical locations in central and southern Florida (i.e., Daytona Beach, Tampa, Miami, and Key West) but exhibit no conclusive relationship with urbanization changes. Danger (103°–124°F; 39.4°–51.1°C) HI and high (>88°F; 31.1°C) WBGT summer days exhibit significant frequency increases across the state. Especially at coastal locations in the Florida Peninsula and Keys, danger HI and high WBGT days now account for >20% of total summer days, emphasizing a substantial escalation in heat stress, particularly since 2000.

Abstract

Measurements are presented that were acquired from the National Aeronautics and Space Administration (NASA) DC-8 aircraft during an intensive 3-day study of Tropical Storm/Hurricane Humberto on 22, 23, and 24 September 2001. Particle size distributions, particle image information, vertical velocities, and single- and dual-wavelength Doppler radar observations were obtained during repeated sampling of the eyewall and outer eye regions. Eyewall sampling temperatures ranged from −22° to −57°C and peak updraft velocities from 4 to 15 m s⁻¹.

High concentrations of small ice particles, in the order 50 cm⁻³ and above, were observed within and around the updrafts. Aggregates, some larger than 7 mm, dominated the larger sizes. The slope of the fitted exponential size distributions λ was distinctly different close to the eye than outside of that region. Even at low temperatures, λ was characteristic of warm temperature growth (λ < 30 cm⁻¹) close to the eye and characteristic of low temperature growth outside of it as well (λ > 100 cm⁻¹).

The two modes found for λ are shown to be consistent with observations from nonhurricane ice cloud layers formed through deep convection, but differ markedly from ice cloud layers generated in situ. It is shown that the median, mass-weighted, terminal velocities derived for the Humberto data and from the other datasets are primarily a function of λ.

Microphysical measurements and dual wavelength radar observations are used together to infer and interpret particle growth processes. Rain in the lower portions of the eyewall extended up to the 6- or 7-km level. In the outer eye regions, aggregation progressed downward from between 8.5 and 11.9 km to the melting layer, with some graupel noted in rainbands. Homogeneous ice nucleation is implicated in the high concentrations of small ice particles observed in the vicinity of the updrafts.

Abstract

Airflow patterns and pollution transport in the southern Appalachian Mountains region of the southeastern United States are examined using mesoscale meteorological models and a Lagrangian particle dispersion model (LPDM). The two primary goals of this work are 1) to identify a meteorological modeling methodology that can be used in regional photochemical modeling, and 2) to identify large regional ozone precursor sources that may impact the southern Appalachians during periods having high ozone levels. Four episodes characterized by measured high levels of ozone (1-h average concentrations greater than 90 ppb) at remote monitoring sites are the focus of the modeling efforts. To address the first goal, several methods of airflow modeling involving varying degrees of complexity are examined to find one that reliably simulates the complex wind patterns that occur. A hydrostatic model with homogeneous initialization, a nonhydrostatic model with homogeneous initialization, and a nonhydrostatic model with nonhomogeneous initialization and four-dimensional data assimilation (FDDA) are evaluated against available wind observations. The method using nonhomogeneous initialization and FDDA is found to best reproduce observed wind patterns. Results of a test of model sensitivity to the strength of the FDDA are described.

In addressing the second project goal, a LPDM driven by computed meteorological fields is used to simulate the potential for ozone precursor emissions (in the form of NO _x ) to be transported from nearby major sources toward the mountains. LPDM simulations indicate that one of the urban areas was the most likely source to influence the monitoring sites experiencing high ozone levels during three of the four episodes. However, none of the plumes are computed to be over the monitoring sites for the length of time that the high ozone concentrations were actually observed. Detailed air quality data for one episode suggest the presence of a large urban plume passing over the mountains and originating from outside the modeling domain. This implies that a larger domain is needed for photochemical modeling.

Abstract

While numerous collaborations exist between the atmospheric sciences research community and the U.S. National Weather Service (NWS), collaborative research field studies between undergraduate (UG) students at universities and the NWS are less common. The Summertime Canyon Observations and Research to Characterize Heat Extreme Regimes (SCORCHER) study was an UG student-driven research field campaign conducted in Palo Duro Canyon State Park, Texas, United States, during the summer of 2021. The SCORCHER campaign was mainly aimed at improving our basic scientific understanding of extreme heat, public safety, and forecasting applications, and creating an empowering UG educational field research experience. This “In Box” article highlights the collaborative study design, execution, and lessons learned.

Abstract

This study reports on the evolution of particle size distributions (PSDs) and habits as measured during slow, Lagrangian-type spiral descents through deep subtropical and tropical cloud layers in Florida, Brazil, and Kwajalein, Marshall Islands, most of which were precipitating. The objective of the flight patterns was to learn more about how the PSDs evolved in the vertical and to obtain information of the vertical structure of microphysical properties. New instrumentation yielding better information on the concentrations of particles in the size (D) range between 0.2 and 2 cm, as well as improved particle imagery, produced more comprehensive observations for tropical stratiform precipitation regions and anvils than have been available previously. Collocated radar observations provided additional information on the vertical structure of the cloud layers sampled.

Most of the spirals began at cloud top, with temperatures (T) as low as −50°C, and ended at cloud base or below the melting layer (ML). The PSDs broadened from cloud top toward cloud base, with the largest particles increasing in size from several millimeters at cloud top, to 1 cm or larger toward cloud base. Some continued growth was noted in the upper part of the ML. Concentrations of particles less than 1 mm in size decreased with decreasing height. The result was a consistent change in the PSDs in the vertical. Similarly, systematic changes in the size dependence of the particle cross-sectional area was noted with decreasing height. Aggregation—as ascertained from both the changes in the PSDs and evolution of particle habits as observed in high detail with the cloud particle imager (CPI) probe—was responsible for these trends.

The PSDs were generally well-represented by gamma distributions of the form N = N _0Γ D ^μ e ^{−λ _Γ D} that were fitted to the PSDs over 1-km horizontal intervals throughout the spirals. The intercept (N _0Γ), slope (λ _Γ), and dispersion (μ) values were derived for each PSD. Exponential curves (N = N ₀ e ^−λD ; μ = 0) were also fitted to the distributions. The λ _Γ values for given spirals varied systematically with temperature as did the values of λ (exponential), and the data generally conformed to values found in previous studies involving exponential fits to size distributions in midlatitude frontal and cirrus layers. Considerable variability often noted in the PSD properties during the loops of individual spirals was manifested primarily in large changes in N _0Γ and N ₀, but μ, λ _Γ, and λ remained fairly stable. Temperature is not found to be the sole factor controlling λ _Γ or λ, but is a primary one. Direct relationships were found between λ _Γ and N _0Γ, or λ _Γ and μ, for the gamma distributions, and λ and N ₀ for the exponential. The latter relationship was not found as distinctly in earlier studies; observed PSDs in this study had better fidelity with less scatter. The μ values changed monotonically with T over the range of temperatures and were directly related to N _0Γ or λ _Γ, thereby reducing the number of variables in the PSD functional equation to two. In the upper part of the ML, N ₀, and λ continued to decrease, and in the lower part these values began to increase as the largest particles melted.

General expressions relating various bulk microphysical, radar, and radiative-transfer-related variables to N _0Γ and λ _Γ were developed; they are useful for both tropical and midlatitude clouds. These relationships facilitate the specification of a number of bulk properties in cloud and climate models. The results presented in this paper apply best to temperatures between 0° and −40°C, for which the measured radar reflectivities fall in the range of 0 to 25 dBZ _e .

Abstract

This study explores relationships between lightning, cloud microphysics, and tropical cyclone (TC) storm structure in Hurricane Karl (16 September 2010) using data collected by the NASA DC-8 and Global Hawk (GH) aircraft during NASA’s Genesis and Rapid Intensification Processes (GRIP) experiment. The research capitalizes on the unique opportunity provided by GRIP to synthesize multiple datasets from two aircraft and analyze the microphysical and kinematic properties of an electrified TC. Five coordinated flight legs through Karl by the DC-8 and GH are investigated, focusing on the inner-core region (within 50 km of the storm center) where the lightning was concentrated and the aircraft were well coordinated. GRIP datasets are used to compare properties of electrified and nonelectrified inner-core regions that are related to the noninductive charging mechanism, which is widely accepted to explain the observed electric fields within thunderstorms. Three common characteristics of Karl’s electrified regions are identified: 1) strong updrafts of 10–20 m s⁻¹, 2) deep mixed-phase layers indicated by reflectivities >30 dBZ extending several kilometers above the freezing level, and 3) microphysical environments consisting of graupel, very small ice particles, and the inferred presence of supercooled water. These characteristics describe an environment favorable for in situ noninductive charging and, hence, TC electrification. The electrified regions in Karl’s inner core are attributable to a microphysical environment that was conducive to electrification because of occasional, strong convective updrafts in the eyewall.

Abstract

This article illustrates how multifrequency radar observations can refine the mass–size parameterization of frozen hydrometeors in scattering models and improve the correlation between the radar observations and in situ measurements of microphysical properties of ice and snow. The data presented in this article were collected during the GPM Cold Season Precipitation Experiment (GCPEx) (2012) and Olympic Mountain Experiment (OLYMPEx) (2015) field campaigns, where the true mass–size relationship was not measured. Starting from size and shape distributions of ice particles measured in situ, scattering models are used to simulate an ensemble of reflectivity factors for various assumed mass–size parameterizations (MSP) of the power-law type. This ensemble is then collocated to airborne and ground-based radar observations, and the MSPs are refined by retaining only those that reproduce the radar observations to a prescribed level of accuracy. A versatile “retrieval dashboard” is built to jointly analyze the optimal MSPs and associated retrievals. The analysis shows that the optimality of an MSP depends on the physical assumptions made in the scattering simulators. This work confirms also the existence of a relationship between parameters of the optimal MSPs. Through the MSP optimization, the retrievals of ice water content M and mean diameter D _m seem robust to the change in meteorological regime (between GCPEx and OLYMPEx); whereas the retrieval of the diameter spread S _m seems more campaign dependent.

Abstract

The Cyclone Global Navigation Satellite System (CYGNSS) is a new NASA earth science mission scheduled to be launched in 2016 that focuses on tropical cyclones (TCs) and tropical convection. The mission’s two primary objectives are the measurement of ocean surface wind speed with sufficient temporal resolution to resolve short-time-scale processes such as the rapid intensification phase of TC development and the ability of the surface measurements to penetrate through the extremely high precipitation rates typically encountered in the TC inner core. The mission’s goal is to support significant improvements in our ability to forecast TC track, intensity, and storm surge through better observations and, ultimately, better understanding of inner-core processes. CYGNSS meets its temporal sampling objective by deploying a constellation of eight satellites. Its ability to see through heavy precipitation is enabled by its operation as a bistatic radar using low-frequency GPS signals. The mission will deploy an eight-spacecraft constellation in a low-inclination (35°) circular orbit to maximize coverage and sampling in the tropics. Each CYGNSS spacecraft carries a four-channel radar receiver that measures GPS navigation signals scattered by the ocean surface. The mission will measure inner-core surface winds with high temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of TC wind speeds.