1,721,134 research outputs found
Scale of plume clustering in large-Prandtl-number convection
Full text linkClustering of plumes in turbulent Rayleigh-Bénard convection has been numerically observed in low-Prandtl-number fluids. In this framework, turbulent plumes undergo a phase-separation process leading to large-scale clusters and circulations, sometimes called plume superstructures and reminiscent of solar granulation and supergranulation. On the other hand, the possible presence of large-scale plume aggregates has not been explored in the case of large values of the Prandtl number, Pr, relevant to geological settings such as convection in planetary interiors. Here we address this problem and numerically explore the behavior of plume ensembles in turbulent convection at very high Prandtl number values, including the case Pr→∞. The results indicate the presence of plume clustering, albeit at smaller scale, also for large Pr number fluids, suggesting interesting consequences for mantle convection processes
Precipitation in the Hindu-Kush Karakoram Himalaya: Observations and future scenarios
Full text linkWe study the properties of precipitation in the Hindu-Kush Karakoram
Himalaya (HKKH) region using currently available data sets. We consider
satellite rainfall estimates (Tropical Rainfall Measuring Mission),
reanalyses (ERA-Interim), gridded in situ rain gauge data (Asian
Precipitation Highly Resolved Observational Data Integration Towards
Evaluation of Water Resources, Climate Research Unit, and Global
Precipitation Climatology Centre), and a merged satellite and rain gauge
climatology (Global Precipitation Climatology Project). The data are
compared with simulation results from the global climate model EC-Earth.
All data sets, despite having different resolutions, coherently
reproduce the mean annual cycle of precipitation in the western and
eastern stretches of the HKKH. While for the Himalaya only a strong
summer precipitation signal is present, associated with the monsoon, the
data indicate that the Hindu-Kush Karakoram, which is exposed to
midlatitude ``western weather patterns{''}, receives water inputs in
winter. Time series of seasonal precipitation confirm that the various
data sets provide a consistent measurement of interannual variability
for the HKKH. The longest observational data sets indicate a
statistically significant decreasing trend in Himalaya during summer.
None of the data sets gives statistically significant precipitation
trends in Hindu-Kush Karakoram during winter. Precipitation data from
EC-Earth are in good agreement with the climatology of the observations
(rainfall distribution and seasonality). The evolution of precipitation
under two different future scenarios (RCP 4.5 and RCP 8.5) reveals an
increasing trend over the Himalaya during summer, associated with an
increase in wet extremes and daily intensity and a decrease in the
number of rainy days. Unlike the observations, the model shows an
increasing precipitation trend also in the period 1950-2009, possibly as
a result of the poor representation of aerosols in this type of GCMs.
Citation: Palazzi, E., J. von Hardenberg, and A. Provenzale (2013),
Precipitation in the Hindu-Kush Karakoram Himalaya: Observations and
future scenarios, J. Geophys. Res. Atmos., 118, 85-100, doi:
10.1029/2012JD018697
The shape of convective rain cells
No full textThe shape of rainfall cells is a crucial ingredient of stochastic
point-process models that describe intense precipitation events. Here we
study the shape of the individual rain cells measured during the
TOGA/COARE and GATE radar experiments. We find that, on average,
rainfall intensity decreases exponentially from the cell's center. An
exponential profile provides a good estimate of the precipitation
intensity in the bulk of the cell. By contrast, the standard assumption
of a Gaussian shape leads to significant underestimation of the
precipitation intensity in the central portion of the cell and of the
associated hydrologic response
Large scale dissipation and filament instability in two-dimensional turbulence
Full text linkCoherent vortices in two-dimensional turbulence induce far-field effects
that stabilize vorticity filaments and inhibit the generation of new
vortices. We show that the large-scale energy sink often included in
numerical simulations of statistically stationary two-dimensional
turbulence reduces the stabilizing role of the vortices, leading to
filament instability and to continuous formation of new coherent
vortices. This counterintuitive effect sheds new light on the mechanisms
responsible for vortex formation in forced-dissipated two-dimensional
turbulence, and it has significant impact on the temporal evolution of
the vortex population in freely decaying turbulence. The time dependence
of vortex statistics in the presence of a large-scale energy sink can be
approximately described by a modified version of the scaling theory
developed for small-scale dissipation
The Shape of Convective Rain Cells
No full textThe shape of rainfall cells is a crucial ingredient of stochastic point-process models that describe intense precipitation events. Here we study the shape of the individual rain cells measured during the TOGA/COARE and GATE radar experiments. We find that, on average, rainfall intensity decreases exponentially from the cell's center. An exponential profile provides a good estimate of the precipitation intensity in the bulk of the cell. By contrast, the standard assumption of a Gaussian shape leads to significant underestimation of the precipitation intensity in the central portion of the cell and of the associated hydrologic response
Modelling the survival of bacteria in drylands: the advantage of being dormant
No full textWe introduce a simple mathematical model for the description of
`dormancy', a survival strategy used by some bacterial populations that
are intermittently exposed to external stress. We focus on the case of
the cyanobacterial crust in drylands, exposed to severe water shortage,
and compare the fate of ideal populations that are, respectively,
capable or incapable of becoming dormant. The results of the simple
model introduced here indicate that under a constant, even though low,
supply of water the dormant strategy does not provide any benefit and it
can, instead, decrease the chances of survival of the population. The
situation is reversed for highly intermittent external stress, due to
the presence of prolonged periods of dry conditions intermingled with
short periods of intense precipitation. In this case, dormancy allows
for the survival of the population during the dry periods. In contrast,
bacteria that are incapable of turning into a dormant state cannot
overcome the difficult times. The model also rationalizes why dormant
bacteria, such as those composing the cyanobacterial crust in the
desert, are extremely sensitive to other disturbances, such as trampling
cattle
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