In 1978 we began a coordinated effort to study the electrical behavior of large and severe thunderstorms that form over the Great Plains of the central United States. Methods of approach include the study of characteristics of individual phenomena and storm case studies. Our goal is to understand the interrelationships between electrical phenomena and the dynamics and precipitation of storms. Evidence that interrelationships do exist can be seen in the results to date. In one squall-line storm we have studied, 44% of all observed lightning flashes were cloud-to-ground (CG); the total flashing rate averaged 12 min⁻¹ and coarsely followed the changes in Doppler-derived maximum updraft speed. Most of the intracloud (IC) discharge processes in a supercell severe storm were located predominately around the region of the intense updraft of the mesocyclone and near large gradients in reflectivity and horizontal velocity.

Both 10 cm and 23 cm wavelength radars have been used to detect lightning radar echoes. The lightning echoes from the 10 cm radar generally had peak signals 10–25 dB greater than the largest precipitation echo in the storm, and they usually were observed where precipitation reflectivities were less than maximum. Comparison of lightning echoes and electric field changes shows that abrupt increases in radar reflectivity often are associated with return strokes and K-type field changes.

CG flashes that lower positive charge to earth have been observed to emanate from the wall cloud, high on the main storm tower, and well out in the downwind anvil of severe storms. The percentage of CG flashes that lower positive charge is apparently small.

Abstract

Wildfires are often governed by rapid changes in seasonal rainfall. Therefore, measuring seasonal rainfall on a temporally finescale should facilitate the prediction of wildfire regimes. To explore this hypothesis, daily rainfall data over a 58-yr period (1950–2007) in south-central Florida were transformed into cumulative rainfall anomalies (CRAs). This transformation allowed precise estimation of onset dates and durations of the dry and wet seasons, as well as a number of other variables characterizing seasonal rainfall. These variables were compared with parameters that describe ENSO and a wildfire regime in the region (at the Avon Park Air Force Range). Onset dates and durations were found to be highly variable among years, with standard deviations ranging from 27 to 41 days. Rainfall during the two seasons was distinctive, with the dry season having half as much as the wet season despite being nearly 2 times as long. The precise quantification of seasonal rainfall led to strong statistical models describing linkages between climate and wildfires: a multiple-regression technique relating the area burned with the seasonal rainfall characteristics had an of 0.61, and a similar analysis examining the number of wildfires had an of 0.56. Moreover, the CRA approach was effective in outlining how seasonal rainfall was associated with ENSO, particularly during the strongest and most unusual events (e.g., El Niño of 1997/98). Overall, the results presented here show that using CRAs helped to define the linkages among seasonality, ENSO, and wildfires in south-central Florida, and they suggest that this approach can be used in other fire-prone ecosystems.

Abstract

On 22 May 1981, we acquired lightning and Doppler radar data on two tornadic storms in Oklahoma. Cloud-to-ground lightning flash rates were measured with a magnetic direction-finder network, and total flash rates in the vicinity of the mesocyclone were measured with an L-band radar. In both storms, there was no clear relationship between tornado occurrence and ground flash rates of the storm as a whole, but the stroke rate of each storm was highest after it stopped producing tornadoes. For the second storm, we examined both intracloud and cloud-t-ground lightning rates relative to mesocyclone evolution, analyzing the region within 10 km of the mesocyclone core. Our analysis began during initial stages of the mesocyclone core associated with the fourth and strongest of five tornadoes in the storm and continued until all mesocyclone cores in the storm dissipated. During this period, intracloud lightning flash rates reached a peak of almost 14 min⁻¹ approximately 10 min after the peak in cyclonic shear at the 6 km level and at the same time as the peak in cyclonic shear at the 1.5 km level. The peak in intracloud rates also occurred 5–10 min after the peak in the area within 40 and 45 dBZ contours at the 8 km level and at about the same time as the peak in the area within 50 dBZ contours at 8 km and within 40 dBZ at 6 km. However, ground flash rates in the mesocyclone region were usually less than 1 min⁻¹ during periods when intracloud rates were high and were negatively correlated with cyclonic shear at both 1.5 and 6 km. The ground flash rate was the last parameter to peak, approximately 15 min after intracloud lightning and a few minutes after the latest reflectivity area (the area having >55 dBZ at the 1 km level).

We suggest that intracloud rates were governed, in part, by particle interactions during the growth in reflectivity at 7–9 km and, in part, by some process associated with the evolution of cyclonic shear at low altitudes. Earlier studies of tornado storms indicate that the evolution of updrafts and downdrafts affects the evolution of both reflectivity and low-altitude cyclonic shear and so, as in previous storm studies, updraft evolution will affect intracloud rates. We suggest that the peaks in ground flash rates resulted from increasing the distance between the main positive and negative charge centers, from the sedimentation of negative charge to lower altitudes, or from the generation or advection of positive charge below the main negative charge. Although these data are from only a single day, consideration of sferics data from previous studies suggests that 1) most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado, and 2) the increase in total flash rates is often dominated by intracloud flashes.

Abstract

Observations of robust scaling behavior in clouds and precipitation are used to derive constraints on how partitioning of precipitation should change with model resolution. Analysis indicates that 90%–99% of stratiform precipitation should occur in clouds that are resolvable by contemporary climate models (e.g., with 200-km or finer grid spacing). Furthermore, this resolved fraction of stratiform precipitation should increase sharply with resolution, such that effectively all stratiform precipitation should be resolvable above scales of ~50 km. It is shown that the Community Atmosphere Model (CAM) and the Weather Research and Forecasting model (WRF) also exhibit the robust cloud and precipitation scaling behavior that is present in observations, yet the resolved fraction of stratiform precipitation actually decreases with increasing model resolution. A suite of experiments with multiple dynamical cores provides strong evidence that this “scale-incognizant” behavior originates in one of the CAM4 parameterizations. An additional set of sensitivity experiments rules out both convection parameterizations, and by a process of elimination these results implicate the stratiform cloud and precipitation parameterization. Tests with the CAM5 physics package show improvements in the resolution dependence of resolved cloud fraction and resolved stratiform precipitation fraction.

Abstract

Satellite observations from Clouds and the Earth’s Radiant Energy System (CERES) radiometers have produced over two decades of world-class data documenting time–space variations in Earth’s top-of-atmosphere (TOA) radiation budget. In addition to energy exchanges among Earth and space, climate studies require accurate information on radiant energy exchanges at the surface and within the atmosphere. The CERES Cloud Radiative Swath (CRS) data product extends the standard Single Scanner Footprint (SSF) data product by calculating a suite of radiative fluxes from the surface to TOA at the instantaneous CERES footprint scale using the NASA Langley Fu–Liou radiative transfer model. Here, we describe the CRS flux algorithm and evaluate its performance against a network of ground-based measurements and CERES TOA observations. CRS all-sky downwelling broadband fluxes show significant improvements in surface validation statistics relative to the parameterized fluxes on the SSF product, including a ∼30%–40% (∼20%) reduction in SW↓ (LW↓) root-mean-square error (RMSΔ), improved correlation coefficients, and the lowest SW↓ bias over most surface types. RMSΔ and correlation statistics improve over five different surface types under both overcast and clear-sky conditions. The global mean computed TOA outgoing LW radiation (OLR) remains within <1% (2–3 W m⁻²) of CERES observations, while the global mean reflected SW radiation (RSW) is excessive by ∼3.5% (∼9 W m⁻²) owing to cloudy-sky computation errors. As we highlight using data from two remote field campaigns, the CRS data product provides many benefits for studies requiring advanced surface radiative fluxes.

Abstract

Thirteen regional climate model (RCM) simulations of June–July 1993 were compared with each other and observations. Water vapor conservation and precipitation characteristics in each RCM were examined for a 10° × 10° subregion of the upper Mississippi River basin, containing the region of maximum 60-day accumulated precipitation in all RCMs and station reports.

All RCMs produced positive precipitation minus evapotranspiration (P − E > 0), though most RCMs produced P − E below the observed range. RCM recycling ratios were within the range estimated from observations. No evidence of common errors of E was found. In contrast, common dry bias of P was found in the simulations.

Daily cycles of terms in the water vapor conservation equation were qualitatively similar in most RCMs. Nocturnal maximums of P and C (convergence) occurred in 9 of 13 RCMs, consistent with observations. Three of the four driest simulations failed to couple P and C overnight, producing afternoon maximum P. Further, dry simulations tended to produce a larger fraction of their 60-day accumulated precipitation from low 3-h totals.

In station reports, accumulation from high (low) 3-h totals had a nocturnal (early morning) maximum. This time lag occurred, in part, because many mesoscale convective systems had reached peak intensity overnight and had declined in intensity by early morning. None of the RCMs contained such a time lag. It is recommended that short-period experiments be performed to examine the ability of RCMs to simulate mesoscale convective systems prior to generating long-period simulations for hydroclimatology.

Abstract

The National Aeronautics and Space Administration (NASA)’s Arctic Radiation-IceBridge Sea and Ice Experiment (ARISE) acquired unique aircraft data on atmospheric radiation and sea ice properties during the critical late summer to autumn sea ice minimum and commencement of refreezing. The C-130 aircraft flew 15 missions over the Beaufort Sea between 4 and 24 September 2014. ARISE deployed a shortwave and longwave broadband radiometer (BBR) system from the Naval Research Laboratory; a Solar Spectral Flux Radiometer (SSFR) from the University of Colorado Boulder; the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) from the NASA Ames Research Center; cloud microprobes from the NASA Langley Research Center; and the Land, Vegetation and Ice Sensor (LVIS) laser altimeter system from the NASA Goddard Space Flight Center. These instruments sampled the radiant energy exchange between clouds and a variety of sea ice scenarios, including prior to and after refreezing began. The most critical and unique aspect of ARISE mission planning was to coordinate the flight tracks with NASA Cloud and the Earth’s Radiant Energy System (CERES) satellite sensor observations in such a way that satellite sensor angular dependence models and derived top-of-atmosphere fluxes could be validated against the aircraft data over large gridbox domains of order 100–200 km. This was accomplished over open ocean, over the marginal ice zone (MIZ), and over a region of heavy sea ice concentration, in cloudy and clear skies. ARISE data will be valuable to the community for providing better interpretation of satellite energy budget measurements in the Arctic and for process studies involving ice–cloud–atmosphere energy exchange during the sea ice transition period.