53 research outputs found
Convective Self-Aggregation as a Cold-Pool Driven Critical Phenomenon
Convective self-aggregation is when thunderstorm clouds cluster over a
constant temperature surface in radiative convective equilibrium.
Self-aggregation was implicated in the Madden-Julian Oscillation and
hurricanes. Yet, numerical simulations succeed or fail at producing
self-aggregation, depending on modeling choices. Common explanations for
self-aggregation invoke radiative effects, acting to concentrate moisture in a
sub-domain. Interaction between cold pools, caused by rain evaporation, drives
reorganization of boundary layer moisture and triggers new updrafts. We propose
a simple model for aggregation by cold pool interaction, assuming a local
number density of precipitation cells, and that interaction
scales quadratically with . Our model mimics global energy
constraints by limiting further cell production when many cells are present.
The phase diagram shows a continuous phase transition between a continuum and
an aggregated state. Strong cold pool-cold pool interaction gives a uniform
convective phase, while weak interaction yields few and independent cells.
Segregation results for intermediate interaction strength
Communication dynamics in finite capacity social networks
In communication networks structure and dynamics are tightly coupled. The
structure controls the flow of information and is itself shaped by the
dynamical process of information exchanged between nodes. In order to reconcile
structure and dynamics, a generic model, based on the local interaction between
nodes, is considered for the communication in large social networks. In
agreement with data from a large human organization, we show that the flow is
non-Markovian and controlled by the temporal limitations of individuals. We
confirm the versatility of our model by predicting simultaneously the
degree-dependent node activity, the balance between information input and
output of nodes and the degree distribution. Finally, we quantify the
limitations to network analysis when it is based on data sampled over a finite
period of time.Comment: Physical Review Letter, accepted (5 pages, 4 figures
A statistical model for isolated convective precipitation events
To study the diurnal evolution of the convective cloud field, we develop a
precipitation cell tracking algorithm which records the merging and
fragmentation of convective cells during their life cycles, and apply it on
large eddy simulation (LES) data. Conditioning on the area covered by each
cell, our algorithm is capable of analyzing an arbitrary number of auxiliary
fields, such as the anomalies of temperature and moisture, convective available
potential energy (CAPE) and convective inhibition (CIN). For tracks that do not
merge or split (termed "solitary"), many of these quantities show generic,
often nearly linear relations that hardly depend on the forcing conditions of
the simulations, such as surface temperature. This finding allows us to propose
a highly idealized model of rain events, where the surface precipitation area
is circular and a cell's precipitation intensity falls off linearly with the
distance from the respective cell center. The drop-off gradient is nearly
independent of track duration and cell size, which allows for a generic
description of such solitary tracks, with the only remaining parameter the peak
intensity. In contrast to the simple and robust behavior of solitary tracks,
tracks that result from merging of two or more cells show a much more
complicated behavior. The most intense, long lasting and largest tracks indeed
stem from multi-mergers - tracks involved in repeated merging. Another
interesting finding is that the precipitation intensity of tracks does not
strongly depend on the absolute amount of local initial CAPE, which is only
partially consumed by most rain events. Rather, our results speak to boundary
layer cooling, induced by rain re-evaporation, as the cause for CAPE reduction,
CIN increase and shutdown of precipitation cells.Comment: Manuscript under review in Journal of Advances in Modeling Earth
System
Strong Correlations Produce the Curie-Weiss Phase of NaCoO
Within the t-J model we study several experimentally accessible properties of
the 2D-triangular lattice system NaCoO, using a numerically exact
canonical ensemble study of 12 to 18 site triangular toroidal clusters as well
as the icosahedron. Focusing on the doping regime of , we study the
temperature dependent chemical potential, specific heat, magnetic
susceptibility and the dynamic Hall coefficient as well as the
magnetic field dependent thermopower. We find a crossover between two phases
near in susceptibility and field suppression of the thermopower
arising from strong correlations. An interesting connection is found between
the temperature dependence of the diamagnetic susceptibility and the
Hall-coefficient. We predict a large thermopower enhancement, arising from {\em
transport corrections} to the Heikes-Mott formula, in a model situation where
the sign of hopping is reversed from that applicable to NaCoO.Comment: 5 pages, 4 figure
Cold Pools as Conveyor Belts of Moisture
Observations and simulations have found convective cold pools to trigger and organize subsequent updrafts by modifying boundary layer temperature and moisture as well as by lifting air parcels at the outflow boundaries. We study the causality between cold pools and subsequent deep convection in idealized large‐eddy simulations by tracking colliding outflow boundaries preceding hundreds of deep convection events. When outflow boundaries collide, their common front position remains immobile, whereas the internal cold pool dynamics continues for hours. We analyze how this dynamics “funnels” moisture from a relatively large volume into a narrow convergence zone. We quantify moisture convergence and separate the contribution from surface fluxes, which we find to play a secondary role. Our results highlight that dynamical effects are crucial in triggering convection, even in radiative‐convective equilibrium. However, it is the low‐level convergence resulting from this dynamics that removes inhibition, moistens the atmosphere aloft, and ultimately permits deep convection
The Hall Number, Optical Sum Rule and Carrier Density for the -- model
We revisit the relationship between three classical measures of particle
number, namely the chemical doping , the Hall number and the
particle number inferred from the optical sum rule . We study the
-- model of correlations on a square lattice, as a minimal model for
High systems, using numerical methods to evaluate the low temperature
Kubo conductivites. These measures disagree significantly in this type of
system, owing to Mott Hubbard correlations. The Hall constant has a complex
behavior with several changes of sign as a function of filling , depending
upon the model parameters. Thus depends sensitively on and ,
due to a kind of quantum interference.Comment: Typos removed,9 Figures, (Revised Figure.3 contains comparison with
experiments
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