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M. Steiner
and
A. Waldvogel

Abstract

The multipeak behavior of raindrop size distributions has been studied. Peaks have been found for distinct drop diameters: 0.7, 1.0, 1.9, and possibly 3.2 mm. The probability is about 65% that at least one of these peaks exists in an observed size distribution. Such peaks may represent important clues for investigations into precipitation growth and binary interaction mechanisms of raindrops. The observational finding of the peaks in raindrop spectra is consistent with several theoretical models.

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P. Alpert
,
M. Tsidulko
, and
U. Stein

Abstract

The contribution of a particular process is shown to be strongly dependent upon the other processes under investigation because of synergistic contributions. In general, as the number of relevant factors being investigated increases, the role of any specific factor diminishes because the synergistic interactions with the new factors are extracted. This is illustrated with the variations of the topographic role in the impressive lee cyclone deepening event on 3–5 March 1982 during the Alpine Experiment. When latent heat release, latent heat flux, and sensitive heat flux enter into our comparative study, the topographic contribution to the surface pressure lee cyclone deepening gradually diminishes down to 50% or more.

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Thorwald H. M. Stein
,
Julien Delanoë
, and
Robin J. Hogan

Abstract

The A-Train constellation of satellites provides a new capability to measure vertical cloud profiles that leads to more detailed information on ice-cloud microphysical properties than has been possible up to now. A variational radar–lidar ice-cloud retrieval algorithm (VarCloud) takes advantage of the complementary nature of the CloudSat radar and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar to provide a seamless retrieval of ice water content, effective radius, and extinction coefficient from the thinnest cirrus (seen only by the lidar) to the thickest ice cloud (penetrated only by the radar). In this paper, several versions of the VarCloud retrieval are compared with the CloudSat standard ice-only retrieval of ice water content, two empirical formulas that derive ice water content from radar reflectivity and temperature, and retrievals of vertically integrated properties from the Moderate Resolution Imaging Spectroradiometer (MODIS) radiometer. The retrieved variables typically agree to within a factor of 2, on average, and most of the differences can be explained by the different microphysical assumptions. For example, the ice water content comparison illustrates the sensitivity of the retrievals to assumed ice particle shape. If ice particles are modeled as oblate spheroids rather than spheres for radar scattering then the retrieved ice water content is reduced by on average 50% in clouds with a reflectivity factor larger than 0 dBZ. VarCloud retrieves optical depths that are on average a factor-of-2 lower than those from MODIS, which can be explained by the different assumptions on particle mass and area; if VarCloud mimics the MODIS assumptions then better agreement is found in effective radius and optical depth is overestimated. MODIS predicts the mean vertically integrated ice water content to be around a factor-of-3 lower than that from VarCloud for the same retrievals, however, because the MODIS algorithm assumes that its retrieved effective radius (which is mostly representative of cloud top) is constant throughout the depth of the cloud. These comparisons highlight the need to refine microphysical assumptions in all retrieval algorithms and also for future studies to compare not only the mean values but also the full probability density function.

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Stacey Kawecki
,
Geoffrey M. Henebry
, and
Allison L. Steiner

Abstract

This study examines the effects of urban aerosols on a mesoscale convective system (MCS) in the central Great Plains with the Weather Research and Forecasting Model coupled with chemistry (WRF-Chem). Urban emissions from Kansas City, Missouri, were scaled by factors of 0.5, 1.0, and 2.0 to investigate the impact of urban aerosol load on MCS propagation and strength. The first half of the storm development is characterized by a stationary front to the north of Kansas City (phase I; 1800 UTC 26 May–0600 UTC 27 May), which develops into a squall line south of the urban area (phase II; 0600–1800 UTC 27 May). During phase I, doubling urban emissions shifts the precipitation accumulation, with enhancement downwind of the storm propagation and suppression upwind. During phase II, a squall line develops in the baseline and doubled emissions scenarios but not when emissions are halved. These changes in MCS propagation and strength are a function of cold pool strength, which is determined by microphysical processes and directly influenced by aerosol load. Overall, changes in urban emissions drive changes in cloud microphysics, which trigger large-scale changes in storm morphology and precipitation patterns. These results show that urban emissions can play an important role in mesoscale weather systems.

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David M. Wright
,
Derek J. Posselt
, and
Allison L. Steiner

Abstract

High-resolution Weather Research and Forecasting Model (WRF) simulations are used to explore the sensitivity of Great Lakes lake-effect snowfall (LES) to changes in lake ice cover and surface temperature. A control simulation with observed ice cover is compared with three sensitivity tests: complete ice cover, no lake ice, and warmer lake surface temperatures. The spatial pattern of unfrozen lake surfaces determines the placement of LES, and complete ice cover eliminates it. Removal of ice cover and an increase in lake temperatures result in an expansion of the LES area both along and downwind of the lake shore, as well as an increase in snowfall amount. While lake temperatures and phase determine the amount and spatial coverage of LES, the finescale distribution of LES is strongly affected by the interaction between lake surface fluxes, the large-scale flow, and the local lake shore geography and inland topography. As a consequence, the sensitivity of LES to topography and shore geometry differs for lakes with short versus long overwater fetch. These simulations indicate that coarse-resolution models may be able to realistically reproduce the gross features of LES in future climates, but will miss the important local-scale interactions that determine the location and intensity of LES.

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R. K. Smith
,
R. N. Ridley
,
M. A. Page
,
J. T. Steiner
, and
A. P. Sturman

Abstract

A combined observational and climatological study of orographically influenced cold fronts over New Zealand, known locally as “southerly changes,” is presented.

Four southerly changes that occurred along the east coast of New Zealand during the Southerly Change Experiment (SOUCHEX) in January and February of 1988 are analysed in detail using the higher spatial and temporal density of data established for the experimental period. Three of the southerly changes were associated with fronts originating over the Tasman Sea, while the other was not.

A common feature in all four cases was the shallowness of the southerly flow for sonic hours after the surface wind change. The top of the southerly flow layer was less than the typical height of the Southern Alps (2000). Above this there was usually a maximum of the northerly component of the flow at or just above mountain- top levels and in three casts the prefrontal low-level flow was dominated by a warm northwesterly foehn. In the central South Island the northwards motion of the southerly change line at the surface was more rapid on the coast than inland. In this and other respects, the changes had many of the characteristics of “southerly busters” in southeastern Australia, and it sterns likely that the dynamical mechanisms of both kinds of fronts are similar.

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P. Alpert
,
S. O. Krichak
,
T. N. Krishnamurti
,
U. Stein
, and
M. Tsidulko

Abstract

The contributions of boundary factors, which may be considered to be independent of the physics or the dynamics of the mesoscale model, are explored in a consistent approach for a widely investigated Alpine Experiment (AL-PEX) lee cyclogenesis case. The roles of the lateral boundaries and the initial fields in conjunction with that of the topography, as well as their possible nonlinear interactions in various model settings, are calculated with the aid of the recently developed factor separation method. Focus is given to the influences of the extent of the model domain and of the running period prior to the climax of the lee cyclone development during 3–6 March 1982. It is shown that the initial conditions are dominant in the first 9–15 h, during which time the topography and lateral boundaries play negative roles because of the adjusting processes. The nonlinear interaction BI between lateral boundaries (B) and the initial conditions (I) was found to be the major contributor to the cyclone deepening during the adjustment period. For longer running periods, some equilibrium is reached in which both the BI interaction and the lateral boundary dominate. The topographic contribution to the lee cyclone deepening in this ALPEX case was indeed limited to about 20% only, as already indicated by earlier studies. Testing several distances of the western lateral boundary suggests the existence of an optimal distance for good results. Both too distant and too close lateral boundaries yield worse results. Testing with frozen boundary conditions shows that the update of the lateral boundaries at a specific time of +36 h was crucial to the development. The results are clearly dependent to some extent on the model type and the particular case under investigation, as well as on the boundary conditions, the initialization procedures, and other model characteristics. The current experiments, however, provide a quantitative approach for estimating the relative roles of the aforementioned boundary factors in mesoscale developments with the aid of the Pennsylvania State University-National Center for Atmospheric Research MM4 mesoscale model and The Florida State University regional system.

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Hirohiko Masunaga
,
Christopher E. Holloway
,
Hironari Kanamori
,
Sandrine Bony
, and
Thorwald H. M. Stein

Abstract

Convective self-aggregation is among the most striking features emerging from radiative–convective equilibrium simulations, but its relevance to convective disturbances observed in the real atmosphere remains under debate. This work seeks the observational signals of convective aggregation intrinsic to the life cycle of cloud clusters. To this end, composite time series of the Simple Convective Aggregation Index (SCAI), a metric of aggregation, and other variables from satellite measurements are constructed around the temporal maxima of precipitation. All the parameters analyzed are large-scale means over 10° × 10° domains. The composite evolution for heavy precipitation regimes shows that cloud clusters are gathered into fewer members during a period of ±12 h as precipitation picks up. The high-cloud cover per cluster expands as the number of clusters drops, suggesting a transient occurrence of convective aggregation. The sign of the transient aggregation is less evident or entirely absent in light precipitation regimes. An energy budget analysis is performed in search of the physical processes underlying the transient aggregation. The column moist static energy (MSE) accumulates before the precipitation peak and dissipates after, accounted for primarily by the horizontal MSE advection. The domain-averaged column radiative cooling is greater in a more aggregated composite than in a less aggregated one, although the role of radiative–convective feedback behind this remains unclear.

Open access
T. H. M. Stein
,
C. E. Holloway
,
I. Tobin
, and
S. Bony

Abstract

Using the satellite-infrared-based Simple Convective Aggregation Index (SCAI) to determine the degree of aggregation, 5 years of CloudSat–CALIPSO cloud profiles are composited at a spatial scale of 10 degrees to study the relationship between cloud vertical structure and aggregation. For a given large-scale vertical motion and domain-averaged precipitation rate, there is a large decrease in anvil cloud (and in cloudiness as a whole) and an increase in clear sky and low cloud as aggregation increases. The changes in thick anvil cloud are proportional to the changes in total areal cover of brightness temperatures below 240 K [cold cloud area (CCA)], which is negatively correlated with SCAI. Optically thin anvil cover decreases significantly when aggregation increases, even for a fixed CCA, supporting previous findings of a higher precipitation efficiency for aggregated convection. Cirrus, congestus, and midlevel clouds do not display a consistent relationship with the degree of aggregation. Lidar-observed low-level cloud cover (where the lidar is not attenuated) is presented herein as the best estimate of the true low-level cloud cover, and it is shown that it increases as aggregation increases. Qualitatively, the relationships between cloud distribution and SCAI do not change with sea surface temperature, while cirrus clouds are more abundant and low-level clouds less at higher sea surface temperatures. For the observed regimes, the vertical cloud profile varies more evidently with SCAI than with mean precipitation rate. These results confirm that convective scenes with similar vertical motion and rainfall can be associated with vastly different cloudiness (both high and low cloud) and humidity depending on the degree of convective aggregation.

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Brent M. Lofgren
,
Andrew D. Gronewold
,
Anthony Acciaioli
,
Jessica Cherry
,
Allison Steiner
, and
David Watkins

Abstract

Climate change due to anthropogenic greenhouse gases (GHG) is expected to have important impacts on water resources, with a variety of societal impacts. Recent research has shown that applying different methodologies to assess hydrologic impacts can lead to widely diverging projections of water resources. The authors classify methods of projecting hydrologic impacts of climate change into those that estimate potential evapotranspiration (PET) based on air temperature and those that estimate PET based on components of the surface energy budget. In general, air temperature–based methods more frequently show reductions in measures of water resources (e.g., water yield or soil moisture) and greater sensitivity than those using energy budget–based methods. There are significant trade-offs between these two methods in terms of ease of use, input data required, applicability to specific locales, and adherence to fundamental physical constraints: namely, conservation of energy at the surface. Issues of uncertainty in climate projections, stemming from imperfectly known future atmospheric GHG concentrations and disagreement in projections of the resultant climate, are compounded by questions of methodology and input data availability for models that connect climate change to accompanying changes in hydrology. In the joint atmospheric–hydrologic research community investigating climate change, methods need to be developed in which the energy and moisture budgets remain consistent when considering their interaction with both the atmosphere and water resources. This approach should yield better results for both atmospheric and hydrologic processes.

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