510 research outputs found
Production of a pion in association with a high-Q2 dilepton pair in antiproton-proton annihilation at GSI-FAIR
We evaluate the cross section for anti-p p -> l+ l- pi0 in the forward
direction and for large lepton pair invariant mass. In this kinematical region,
the leading-twist amplitude factorises into a short-distance matrix element,
long-distance dominated antiproton Distribution Amplitudes and proton to pion
Transition Distribution Amplitudes (TDA). Using a modelling inspired from the
chiral limit for these TDAs, we obtain a first estimate of this cross section,
thus demonstrating that this process can be measured at GSI-FAIR.Comment: Latex, 5 pages, 3 figure
Where do winds come from? A new theory on how water vapor condensation influences atmospheric pressure and dynamics
Phase transitions of atmospheric water play a ubiquitous role in the Earth's
climate system, but their direct impact on atmospheric dynamics has escaped
wide attention. Here we examine and advance a theory as to how condensation
influences atmospheric pressure through the mass removal of water from the gas
phase with a simultaneous account of the latent heat release. Building from the
fundamental physical principles we show that condensation is associated with a
decline in air pressure in the lower atmosphere. This decline occurs up to a
certain height, which ranges from 3 to 4 km for surface temperatures from 10 to
30 deg C. We then estimate the horizontal pressure differences associated with
water vapor condensation and find that these are comparable in magnitude with
the pressure differences driving observed circulation patterns. The water vapor
delivered to the atmosphere via evaporation represents a store of potential
energy available to accelerate air and thus drive winds. Our estimates suggest
that the global mean power at which this potential energy is released by
condensation is around one per cent of the global solar power -- this is
similar to the known stationary dissipative power of general atmospheric
circulation. We conclude that condensation and evaporation merit attention as
major, if previously overlooked, factors in driving atmospheric dynamics
Heat engines and heat pumps in a hydrostatic atmosphere: How surface pressure and temperature constrain wind power output and circulation cell size
The kinetic energy budget of the atmosphere's meridional circulation cells is
analytically assessed. In the upper atmosphere kinetic energy generation grows
with increasing surface temperature difference \$\Delta T_s\$ between the cold
and warm ends of a circulation cell; in the lower atmosphere it declines. A
requirement that kinetic energy generation is positive in the lower atmosphere
limits the poleward cell extension \$L\$ of Hadley cells via a relationship
between \$\Delta T_s\$ and surface pressure difference \$\Delta p_s\$: an upper
limit exists when \$\Delta p_s\$ does not grow with increasing \$\Delta T_s\$.
This pattern is demonstrated here using monthly data from MERRA re-analysis.
Kinetic energy generation along air streamlines in the boundary layer does not
exceed \$40\$~J~mol\$^{-1}\$; it declines with growing \$L\$ and reaches zero
for the largest observed \$L\$ at 2~km height. The limited meridional cell size
necessitates the appearance of heat pumps -- circulation cells with negative
work output where the low-level air moves towards colder areas. These cells
consume the positive work output of the heat engines -- cells where the
low-level air moves towards the warmer areas -- and can in theory drive the
global efficiency of atmospheric circulation down to zero. Relative
contributions of \$\Delta p_s\$ and \$\Delta T_s\$ to kinetic energy generation
are evaluated: \$\Delta T_s\$ dominates in the upper atmosphere, while \$\Delta
p_s\$ dominates in the lower. Analysis and empirical evidence indicate that the
net kinetic power output on Earth is dominated by surface pressure gradients,
with minor net kinetic energy generation in the upper atmosphere. The role of
condensation in generating surface pressure gradients is discussed.Comment: 26 pages, 9 figures, 2 tables; re-organized presentation, more
discussion and a new figure (Fig. 4) added; in Fig. 3 the previously
invisible dots (observations) can now be see
Comment on "The Tropospheric Land-Sea Warming Contrast as the Driver of Tropical Sea Level Pressure Changes" by Bayr and Dommenget
T Bayr and D Dommenget [J. Climate 26 (2013) 1387] proposed a model of
temperature-driven air redistribution to quantify the ratio between changes of
sea level pressure and mean tropospheric temperature in the
tropics. This model assumes that the height of the tropical troposphere is
isobaric. Here problems with this model are identified. A revised relationship
between and is derived governed by two parameters -- the isobaric
and isothermal heights -- rather than just one. Further insight is provided by
the model of R S Lindzen and S Nigam [J. Atmos. Sci. 44 (1987) 2418], which was
the first to use the concept of isobaric height to relate tropical to air
temperature, and did this by assuming that isobaric height is always around 3
km and isothermal height is likewise near constant. Observational data,
presented here, show that neither of these heights is spatially universal nor
do their mean values match previous assumptions. Analyses show that the ratio
of the long-term changes in and associated with land-sea
temperature contrasts in a warming climate -- the focus of Bayr and Dommenget
[2013] -- is in fact determined by the corresponding ratio of spatial
differences in the annual mean and . The latter ratio, reflecting
lower pressure at higher temperature in the tropics, is dominated by meridional
pressure and temperature differences rather than by land-sea contrasts.
Considerations of isobaric heights are shown to be unable to predict either
spatial or temporal variation in . As noted by Bayr and Dommenget [2013],
the role of moisture dynamics in generating sea level pressure variation
remains in need of further theoretical investigations.Comment: 26 pages, 11 figures. arXiv admin note: text overlap with
arXiv:1404.101
The key physical parameters governing frictional dissipation in a precipitating atmosphere
Precipitation generates small-scale turbulent air flows the energy of which
ultimately dissipates to heat. The power of this process has previously been
estimated to be around 2-4 W m-2 in the tropics: a value comparable in
magnitude to the dynamic power of the global circulation. Here we suggest that
this previous power estimate is approximately double the true figure. Our
result reflects a revised evaluation of the mean precipitation path length Hp.
We investigate the dependence of Hp on surface temperature,relative
humidity,temperature lapse rate and degree of condensation in the ascending
air. We find that the degree of condensation,defined as the relative change of
the saturated water vapor mixing ratio in the region of condensation, is a
major factor determining Hp. We estimate from theory that the mean large-scale
rate of frictional dissipation associated with total precipitation in the
tropics lies between 1 and 2 W m-2 and show that our estimate is supported by
empirical evidence. We show that under terrestrial conditions frictional
dissipation constitutes a minor fraction of the dynamic power of
condensation-induced atmospheric circulation,which is estimated to be at least
2.5 times larger. However,because Hp increases with surface temperature Ts, the
rate of frictional dissipation would exceed that of condensation-induced
dynamics, and thus block major circulation, at Ts >~320 K in a moist adiabatic
atmosphere.Comment: 12 pp, 2 figure
Renormalization Group Theory for a Perturbed KdV Equation
We show that renormalization group(RG) theory can be used to give an analytic
description of the evolution of a perturbed KdV equation. The equations
describing the deformation of its shape as the effect of perturbation are RG
equations. The RG approach may be simpler than inverse scattering theory(IST)
and another approaches, because it dose not rely on any knowledge of IST and it
is very concise and easy to understand. To the best of our knowledge, this is
the first time that RG has been used in this way for the perturbed soliton
dynamics.Comment: 4 pages, no figure, revte
Analysis of the Accuracy of Prediction of the Celestial Pole Motion
VLBI observations carried out by global networks provide the most accurate
values of the precession-nutation angles determining the position of the
celestial pole; as a rule, these results become available two to four weeks
after the observations. Therefore, numerous applications, such as satellite
navigation systems, operational determination of Universal Time, and space
navigation, use predictions of the coordinates of the celestial pole. In
connection with this, the accuracy of predictions of the precession- nutation
angles based on observational data obtained over the last three years is
analyzed for the first time, using three empiric nutation models---namely,
those developed at the US Naval Observatory, the Paris Observatory, and the
Pulkovo Observatory. This analysis shows that the last model has the best of
accuracy in predicting the coordinates of the celestial pole. The rms error for
a one-month prediction proposed by this model is below 100 microarcsecond.Comment: 13 p
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