Search Results

You are looking at 1 - 10 of 36 items for

  • Author or Editor: J. R. Stewart x
  • All content x
Clear All Modify Search
J. Roads, M. Kanamitsu, and R. Stewart

Abstract

During the past several years, the Global Energy and Water Cycle Experiment (GEWEX) continental-scale experiments (CSEs) have started to develop regional hydroclimatological datasets and water and energy budget studies (WEBS). To provide some global background for these regional experiments, the authors describe vertically integrated global and regional water and energy budgets from the National Centers for Environmental Prediction (NCEP)–U.S. Department of Energy (DOE) Reanalysis II (NCEPRII). It is shown that maintaining the NCEPRII close to observations requires some nudging to the short-range model forecast, and this nudging is an important component of analysis budgets. Still, to first order one can discern important hydroclimatological mechanisms in the reanalysis. For example, during summer, atmospheric water vapor, precipitation, evaporation, and surface and atmospheric radiative heating all increase, while the dry static energy convergence decreases almost everywhere over the land regions. One can further distinguish differences between hydrologic cycles in midlatitudes and monsoon regions. The monsoon hydrologic cycle shows increased moisture convergence, soil moisture, and runoff, but decreased sensible heating with increasing surface temperature. The midlatitude hydrologic cycle, on the other hand, shows decreased moisture convergence and surface water, and increased sensible heating.

Full access
J. L. Lumley and R. W. Stewart

Abstract

No abstract available.

Full access
G. B. Raga, R. E. Stewart, and J. W. Strapp

Abstract

The present study discusses the meso- and microscale structures of Precipitation regions within a midlatitude winter storm over the North Atlantic, observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic. Two wide regions of precipitation separated by a narrow band were observed at low levels by airborne radar. These regions were aligned parallel to the cold front and were sampled by aircraft at three different levels. The calculated mesoscale frontogenetical forcing is dominated at low levels by confluence and at mid-levels by the tilting term. The absolute magnitudes are smaller than those reported by Shapiro, and Bond and Fleagle, and are consistent with the broader and less intense front in this study. The frontogenetical forcing due to melting of ice crystals was estimated from observations of precipitation particles. The analysis indicates that the cooling due to melting of ice particles is not a dominant frontogenetical forcing at the observed stage in storm evolution. Precipitation rates larger than those observed (by a factor of 3) behind the cold front are needed before the thermal impact of melting could contribute to frontogenesis as much as confluence at the same level. The region of precipitation ahead of the cold front appears to be linked to convective instability observed in the warm sector. The observed precipitation region to the west of the cold front is consistent with the trajectories of failing particles carried by the relative wind flowing toward the back of the system. The decrease in precipitation rate observed right behind the front can be interpreted as ice particles failing through a deep region in which temperatures are close to 0°C. The presence of such a region leads to a nonuniform precipitation distribution, with areas that would appear as precipitation bands in radar images, and others in which precipitation is reduced.

Full access
R. Paul Lawson, Ronald E. Stewart, and Leigh J. Angus

Abstract

The Canadian Atlantic Storms Program (CASP II) field experiment was conducted near St. John’s, Newfoundland, Canada, during January–March 1992, and it focused on the nature of winter storms. Analyses of CASP II aircraft, surface, satellite, and radar observations collected during an intensive study of the origin and development of 9 mm h−1 precipitation containing 4–5-cm diameter snowflakes are compared in this article with results of the MM5 (mesoscale) and Mitchell (microphysical) models. MM5 simulations of the thermal, kinematic, and bulk microphysical fields were in good agreement with the observations; this comparison provided the basis for extending the spatial and temporal scales of the aircraft observations to a larger-scale domain using the model results. The Mitchell analytical–numerical model was used to improve the understanding of the microphysical processes that led to the development of the very large snowflakes. A synthesis of results using the different techniques leads to the conclusion that the snowflakes originated as 3–5-mm dendritic crystals in an area of weak convective instability at 5 km and were transported downwind in a strongly sheared airflow. The dendrites aggregated, fell into an existing snowzone (supported in some regions by vertical motion with velocities ranging from 0.2–0.6 m s−1), and continued to descend along a deep, downward sloping layer with temperatures near 0°C. Rapid aggregation occurred in the near 0°C region in particular and without appreciable particle breakup. An exponential fit to the particle size distribution in the region of very large snowflakes had a slope parameter on the order of 100 m−1.

Full access
I. H. Bailey, J. R. Hulston, J. R. Stewart, and W. C. Macklin

Abstract

Theoretical considerations show that isotopic fractionation can occur during the freezing of supercooled droplets accreted by a hailstone. This is due to evaporation from the liquid during the freezing process with consequent enrichment of the deuterium and O18 content of the accreted ice. The enrichment is greatest when the temperature of the hailstone is near OC. Calculations, based on the Rayleigh distillation formula and allowing for diffusivity effects, indicate that the maximum increase is about 6 for deuterium and1.5 for O18. Analyses of samples of accreted ice formed in an icing tunnel have confirmed these predictions.The effect is sufficiently large to affect the interpretation of isotopic analyses of hailstones.

Full access
Cynthia M. Lusk, Thomas R. Stewart, Kenneth R. Hammond, and Rodney J. Potts

Abstract

Two studies of microburst forecasting were conducted in order to demonstrate the utility of applying theoretical and methodological concepts from judgment and decision making to meteorology. A hierarchical model of the judgment process is outlined in which a precursor identification phase is separated from the prediction phase. In the first study, forecasters were provided with specific, unambiguous precursor values and were asked to provide judgments regarding the probability of a microburst. Results indicated that the meteorologists' forecast were adequately predicted by a linear model. Modest agreement was observed among the forecasters’ judgments. In the second study, forecasters viewed storms under dynamic conditions representative of their usual operational setting. They made judgments regarding precursor values, as well as of the probability of a microburst occurring. The forecasters’ agreement regarding microburst predictions was found to be lower than in the first study. Surprisingly, agreement regarding the (subjectively) most important precursor value was near zero. These results indicate that there are indeed practical advantages to be gained from a better understanding of the precursor identification and prediction phases of the forecasting process.

Full access
Jasper R. Lewis, James R. Campbell, Ellsworth J. Welton, Sebastian A. Stewart, and Phillip C. Haftings

Abstract

The National Aeronautics and Space Administration Micro Pulse Lidar Network, version 3, cloud detection algorithm is described and differences relative to the previous version are highlighted. Clouds are identified from normalized level 1 signal profiles using two complementary methods. The first method considers vertical signal derivatives for detecting low-level clouds. The second method, which detects high-level clouds like cirrus, is based on signal uncertainties necessitated by the relatively low signal-to-noise ratio exhibited in the upper troposphere by eye-safe network instruments, especially during daytime. Furthermore, a multitemporal averaging scheme is used to improve cloud detection under conditions of a weak signal-to-noise ratio. Diurnal and seasonal cycles of cloud occurrence frequency based on one year of measurements at the Goddard Space Flight Center (Greenbelt, Maryland) site are compared for the new and previous versions. The largest differences, and perceived improvement, in detection occurs for high clouds (above 5 km, above MSL), which increase in occurrence by over 5%. There is also an increase in the detection of multilayered cloud profiles from 9% to 19%. Macrophysical properties and estimates of cloud optical depth are presented for a transparent cirrus dataset. However, the limit to which the cirrus cloud optical depth could be reliably estimated occurs between 0.5 and 0.8. A comparison using collocated CALIPSO measurements at the Goddard Space Flight Center and Singapore Micro Pulse Lidar Network (MPLNET) sites indicates improvements in cloud occurrence frequencies and layer heights.

Full access
J. L. Schols, J. A. Weinman, G. D. Alexander, R. E. Stewart, L. J. Angus, and A. C. L. Lee

Abstract

Microwave brightness temperatures emanating from a North Atlantic cyclone were measured by the Special Sensor Microwave/Imager (SSM/I) on the Defense Meteorological Satellite Program satellite. As other investigators have found before, low 85.5-GHz brightness temperatures (215 ± 20 K) were observed from cumulonimbus clouds along the squall line; however, 85.5-GHz microwave brightness temperatures observed from the nimbostratus clouds north of the low center were significantly higher (255 ± 20 K). In situ measurements from aircraft during the Canadian Atlantic Storm Program II showed that heavy snowfall consisting of large tenuous aggregates existed in the nimbostratus clouds at the time of the SSM/I overpass.

Distributions of snow, rain, liquid cloud water, and cloud ice mass were computed from a modified version of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model. That model employed a mixed-phase ice microphysics (MPIM) scheme that only considered one type of frozen hydrometeor. The frozen hydrometeor size distributions, density, and mass flux were modified to match the in situ observations where they were available and to account for the SSM/I observations using radiative transfer theory. Those revised hydrometeor representations were constrained to preserve the vertical hydrometeor mass flux distributions obtained from the MPIM scheme throughout the analysis.

Frozen dense accreted particles were required near the squall line to account for the microwave scattering effect. Snow aggregates, with density that decreased with increasing size, were needed to reproduce the high brightness temperatures observed from the nimbostratus clouds.

Full access
S. I. Rasool, J. S. Hogan, R. W. Stewart, and L. H. Russell

Abstract

No abstract available.

Full access
K. K. Szeto, R. E. Stewart, M. K. Yau, and J. Gyakum

The Mackenzie Global Energy and Water Cycle Experiment (GEWEX) Study (MAGS) is one of the continental-scale experiments approved specifically by GEWEX to better understand and model water and energy cycling at high latitudes. The project has gone through two phases since its inception in 1994 and conclusion in December 2005. Many scientific results have been achieved through MAGS research to advance our understanding of the Mackenzie River basin climate system. This article is a synthesis of its atmospheric research achievements through an integrative description of the basin's climate system, along with highlights of MAGS research that has advanced our knowledge and understanding of various key aspects of the system. In particular, the significance of MAGS research is discussed in the hancing knowledge of the basin's hydroclimate with focuses on i) the large-scale atmospheric processes that control the transport of water and energy into the basin, and ii) the interactions of the large-scale atmospheric flows with physical features of the basin's environment in affecting the weather and climate of the basin.

Full access