Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: L. León x
  • All content x
Clear All Modify Search
Darryl N. Leon and Michael L. Talbert

Abstract

Modern-day computers have greatly advanced our ability to generate reasonably accurate weather forecast models in a timely manner. High-powered workstations have put the output from these models, as well as a tremendous amount of raw and analyzed data, at forecasters' fingertips. In fact, the sheer volume of available data can be overwhelming, making it difficult for forecasters to conduct detailed analyses of all the products available to them. Instead, they rely heavily on domain experts' distilled analyses of computer models. Though these computer models are the result of a far more extensive analysis of specific weather parameters than a forecaster could perform in real time, they cannot be relied upon to do the work of the forecaster—to produce and disseminate an accurate terminal aerodrome forecast (TAF) every 6 h, then monitor it for correctness and accuracy, and amend as necessary. Given the broad spectrum of variables interacting in the atmosphere, a forecast “solution” to any given meteorological circumstance cannot be algorithmically found. Therefore, though it is advantageous to harness data storage and computational power to generate quality weather products, human reasoning is still a necessary part of the forecasting process. This article discusses the practical application of an object-oriented, knowledge-based “critic” advice system to aid in the generation and monitoring of weather products. This system would provide domain- and situation-specific expert advice, throughout the life cycle of the product, to ensure weather product quality and improve accuracy in the reports meteorologists create.

Full access
Charles L. Hosler, D. C. Jensen, and Leon Goldshlak

Abstract

Manipulation of spheres of ice and observations of ice crystals colliding with a fixed crystal under conditions of controlled temperature and vapor pressure have been employed to determine the limiting conditions for the aggregation of ice crystals to form snow flakes. It is shown that the amount of aggregation is strongly dependent upon environmental vapor pressure and temperature. At ice saturation, no aggregation occurs at temperatures below −25C and aggregation increases and becomes a maximum as 0C is approached. At vapor pressure less-than-ice saturation no aggregation occurs at temperatures below −4C and aggregation increases rapidly as 0C is approached. Under conditions of supersaturation with respect to ice, aggregation occurs at all temperatures. These results are best explained by the existence of a liquid film on the surface of ice at temperatures below 0C where the thickness of the film is a function of temperature and vapor presuure.

Full access
Juan J. González-Alemán, Francisco Valero, Francisco Martín-León, and Jenni L. Evans

Abstract

Since more research is needed on subtropical cyclones (STCs) formed within the North Atlantic eastern basin, this survey analyzes them from a synoptic point of view, on a climatological basis, with the main aims of studying their common features, complementing other studies of these storms in the North Atlantic, and aiding the forecasting community. Fifteen cases of STCs were identified during the period 1979–2011 by applying a set of criteria from two databases. Composite analysis reveals that an extratropical depression acts as a precursor when it is isolated from the westerlies and then suffers a deepening when becoming subtropical instead of decaying through occlusion. This process is accompanied by an atmospheric circulation, within the North Atlantic, whose main feature is characterized by notable departures from the climatological pattern with a statistically significant anomalous high pressure to the north of the STCs. Three conceptual models of synoptic pattern of subtropical cyclogenesis are derived and show that these departures appeared because the westerly circulation moves poleward and/or the flow has a great meridional component, with the possibility of a blocked flow pattern occurring. Moreover, the identified STCs predominantly formed in a highly sheared (>10 m s−1) environment with low sea surface temperature values (<25°C), which differs from the dominant features of STCs in the North Atlantic, especially within its western region. Finally, a recent (2010) STC, identified by the authors, is synoptically discussed in order to achieve a better interpretation of the general results.

Full access
Marius Årthun, Robert C. J. Wills, Helen L. Johnson, Léon Chafik, and Helene R. Langehaug

Abstract

Decadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.

Open access
Zhien Wang, Jeffrey French, Gabor Vali, Perry Wechsler, Samuel Haimov, Alfred Rodi, Min Deng, Dave Leon, Jeff Snider, Liran Peng, and Andrew L. Pazmany

Clouds are a critical component of the Earth's coupled water and energy cycles. Poor understanding of cloud–radiation–dynamics feedbacks results in large uncertainties in forecasting human-induced climate changes. Better understanding of cloud microphysical and dynamical processes is critical to improving cloud parameterizations in climate models as well as in cloud-resolving models. Airborne in situ and remote sensing can make critical contributions to progress. Here, a new integrated cloud observation capability developed for the University of Wyoming King Air is described. The suite of instruments includes the Wyoming Cloud Lidar, a 183- GHz microwave radiometer, the Wyoming Cloud Radar, and in situ probes. Combined use of these remote sensor measurements yields more complete descriptions of the vertical structure of cloud microphysical properties and of cloud-scale dynamics than that attainable through ground-based remote sensing or in situ sampling alone. Together with detailed in situ data on aerosols, hydrometeors, water vapor, thermodynamic, and air motion parameters, an advanced observational capability was created to study cloud-scale processes from a single aircraft. The Wyoming Airborne Integrated Cloud Observation (WAICO) experiment was conducted to demonstrate these new capabilities and examples are presented to illustrate the results obtained.

Full access
Willis L. Webb, James Giraytys, Harold B. Tolefson, R. C. Forsberg, Robert I. Vick, Orville H. Daniel, and Leon R. Tucker

The status of the Meteorological Rocket Network (MRN) is presented at the end of the first six years operation. A total of twenty stations were active at the end of 1965 and almost 6000 observations had been collected. The latest instrumentation systems employed in the MRN are described, and the general results of certain MRN data analyses are presented. The MRN data illustrate very clearly the need for a global MRN to adequately observe the stratospheric circulation.

Full access
N. Hosannah, J. González, R. Rodriguez-Solis, H. Parsiani, F. Moshary, L. Aponte, R. Armstrong, E. Harmsen, P. Ramamurthy, M. Angeles, L. León, N. Ramírez, D. Niyogi, and B. Bornstein

Abstract

Modulated by global-, continental-, regional-, and local-scale processes, convective precipitation in coastal tropical regions is paramount in maintaining the ecological balance and socioeconomic health within them. The western coast of the Caribbean island of Puerto Rico is ideal for observing local convective dynamics as interactions between complex processes involving orography, surface heating, land cover, and sea-breeze–trade wind convergence influence different rainfall climatologies across the island. A multiseason observational effort entitled the Convection, Aerosol, and Synoptic-Effects in the Tropics (CAST) experiment was undertaken using Puerto Rico as a test case, to improve the understanding of island-scale processes and their effects on precipitation. Puerto Rico has a wide network of observational instruments, including ground weather stations, soil moisture sensors, a Next Generation Weather Radar (NEXRAD), twice-daily radiosonde launches, and Aerosol Robotic Network (AERONET) sunphotometers. To achieve the goals of CAST, researchers from multiple institutions supplemented existing observational networks with additional radiosonde launches, three high-resolution radars, continuous ceilometer monitoring, and air sampling in western Puerto Rico to monitor convective precipitation events. Observations during three CAST measurement phases (22 June–10 July 2015, 6–22 February 2016, and 24 April–7 May 2016) captured the most extreme drought in recent history (summer 2015), in addition to anomalously wet early rainfall and dry-season (2016) phases. This short article presents an overview of CAST along with selected campaign data.

Full access
David M. Plummer, Jeffrey R. French, David C. Leon, Alan M. Blyth, Sonia Lasher-Trapp, Lindsay J. Bennett, David R. L. Dufton, Robert C. Jackson, and Ryan R. Neely

Abstract

Analyses of the radar-observed structure and derived rainfall statistics of warm-season convection developing columns of enhanced positive differential reflectivity Z DR over England’s southwest peninsula are presented here. Previous observations of Z DR columns in developing cumulonimbus clouds over England were rare. The observations presented herein suggest otherwise, at least in the southwesterly winds over the peninsula. The results are the most extensive of their kind in the United Kingdom; the data were collected using the National Centre for Atmospheric Science dual-polarization X-band radar (NXPol) during the Convective Precipitation Experiment (COPE). In contrast to recent studies of Z DR columns focused on deep clouds that developed in high-instability environments, the COPE measurements show relatively frequent Z DR columns in shallower clouds, many only 4–5 km deep. The presence of Z DR columns is used to infer that an active warm rain process has contributed to precipitation evolution in convection deep enough for liquid and ice growth to take place. Clouds with Z DR columns were identified objectively in three COPE deployments, with both discrete convection and clouds embedded in larger convective complexes developing columns. Positive Z DR values typically extended to 1–1.25 km above 0°C in the columns, with Z DR ≥ 1 dB sometimes extending nearly 4 km above 0°C. Values above 3 dB typically occurred in the lowest 500 m above 0°C, with coincident airborne measurements confirming the presence of supercooled raindrops. Statistical analyses indicated that the convection that produced Z DR columns was consistently associated with the larger derived rainfall rates when compared with the overall convective population sampled by the NXPol during COPE.

Full access
Leon D. Rotstayn, Emily L. Plymin, Mark A. Collier, Olivier Boucher, Jean-Louis Dufresne, Jing-Jia Luo, Knut von Salzen, Stephen J. Jeffrey, Marie-Alice Foujols, Yi Ming, and Larry W. Horowitz

Abstract

The effects of declining anthropogenic aerosols in representative concentration pathway 4.5 (RCP4.5) are assessed in four models from phase 5 the Coupled Model Intercomparison Project (CMIP5), with a focus on annual, zonal-mean atmospheric temperature structure and zonal winds. For each model, the effect of declining aerosols is diagnosed from the difference between a projection forced by RCP4.5 for 2006–2100 and another that has identical forcing, except that anthropogenic aerosols are fixed at early twenty-first-century levels. The response to declining aerosols is interpreted in terms of the meridional structure of aerosol radiative forcing, which peaks near 40°N and vanishes at the South Pole.

Increasing greenhouse gases cause amplified warming in the tropical upper troposphere and strengthening midlatitude jets in both hemispheres. However, for declining aerosols the vertically averaged tropospheric temperature response peaks near 40°N, rather than in the tropics. This implies that for declining aerosols the tropospheric meridional temperature gradient generally increases in the Southern Hemisphere (SH), but in the Northern Hemisphere (NH) it decreases in the tropics and subtropics. Consistent with thermal wind balance, the NH jet then strengthens on its poleward side and weakens on its equatorward side, whereas the SH jet strengthens more than the NH jet. The asymmetric response of the jets is thus consistent with the meridional structure of aerosol radiative forcing and the associated tropospheric warming: in the NH the latitude of maximum warming is roughly collocated with the jet, whereas in the SH warming is strongest in the tropics and weakest at high latitudes.

Full access