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Peter Jan van Leeuwen

1970 ). This equation is solved approximately by solving an ensemble of stochastic partial differential equations. The stochastic terms in these equations represent unknown external and internal terms (or factors) in the model. Unknown terms in external forcing and in the model equations are incorporated by adding random numbers, drawn from a known error density, to the model equations. It is also possible to multiply parts of the model equations by unknown factors, sometimes called multiplicative

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Robert A. Houze Jr.

equator because the Coriolis force is too weak for low-level convergence to be able to generate relative vorticity. 2 Gray (1968) found that genesis regions not only are located off the equator, where the Coriolis force is nonzero, but also have higher-than-average relative vorticity of the surface wind field and temperature stratification that is at least moderately conditionally unstable, and weak vertical shear of the horizontal wind. The strong background positive vorticity helps to trap energy

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Roland A. Madden and Paul R. Julian

convection moves from the Indian to the Pacific Ocean. This agrees with our conclusions. However, their work indicated that the singularrole of surface frictional stresses that we proposed isnot correct during northern winter. At that time, theyshowed that surface wind stresses over the central Pacific are nearly out-of-phase with AAM. If they werethe only driving force, the wind stresses would lead theAAM by 0.25 cycles. We computed the coherence andphase between AAM and the torque due to surfacewind

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T. N. Krishnamurti

planetary scaleheat sources in the generation of the eddy availablepotential energy. These in turn were converted to thestrong planetary scale motions via east-west overturnings. A key element in the planetary scale generationTABLE 2. List of useful symbols.xA.B.K,K,,,Kv, K~,,Kxzonal velocitymeridional velocityvertical velocitypotential temperaturespecific humiditystreamfunctionsvelocity potentialenergy exchange from A to Bkinetic energyfrictional force per unit mass of airof eddy available potential

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Robert Wood

subtropics. The hemispheric asymmetry in seasonality is caused by greater orographic forcing from the elevated continent to the east for the southern sheets ( Richter and Mechoso 2004 , 2006 ). Fig . 6. (a) Seasonal amplitude (maximum − minimum coverage), and (b) month of maximum stratocumulus cover. Locations with no reports, or where the seasonal amplitude of stratocumulus cover is less than 2.5% are not shown. Data are from the combined land–ocean cloud atlas database ( Hahn and Warren 2007 ). There

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Volkmar Wirth, Michael Riemer, Edmund K. M. Chang, and Olivia Martius

and E have been created during that time. c. Different types of RWPs and their propagation There are different types of RWPs associated with different temporal and spatial scales, different types of forcing, and specific properties of the background flow. Sometimes an RWP is little more than an individual trough or ridge ( Sanders 1988 ); sometimes an RWP may span a substantial part of the globe ( Branstator 2002 ). Some RWPs propagate along great circles, while others are ducted in the zonal

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Julia H. Keller, Christian M. Grams, Michael Riemer, Heather M. Archambault, Lance Bosart, James D. Doyle, Jenni L. Evans, Thomas J. Galarneau Jr., Kyle Griffin, Patrick A. Harr, Naoko Kitabatake, Ron McTaggart-Cowan, Florian Pantillon, Julian F. Quinting, Carolyn A. Reynolds, Elizabeth A. Ritchie, Ryan D. Torn, and Fuqing Zhang

cyclone constitutes an external forcing with persistent structure to the midlatitude wave. This idea is in marked contrast to traditional initial-value studies of baroclinic development, in which the initial perturbations are embedded in the midlatitude flow and are thus not an external forcing (e.g., Simmons and Hoskins 1979 ; Hakim 2000 ). 2) Evolution of the upstream trough ET may also influence the upstream trough, which may experience modifications of its shape, meridional extension, and

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Bogdan Antonescu, David M. Schultz, Fiona Lomas, and Thilo Kühne

between the occurrence of significant tornadoes and solar heating [Fig. 7 in Dessens and Snow (1989) ], with tornadoes being reported mainly around noon during November–March and between mid and late afternoon during April–October. The tornado database developed by Dessens and Snow (1989) contained 20 significant nighttime tornadoes, which shows that the solar forcing was not directly associated with some of the French tornadoes. The nighttime tornadoes in France may be associated with

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Andrew Staniforth and Jean Côté

time levels (section 2c).b. Forced advection in multidimensions Consider the forced-advection problem dF -~- + G(x, t) = R(x, t), (6)where dF OF - + V(x, t). VF, (7) dt at dx dt V(x, t), (8)Here, x is the position vector (in 1-, 2- or 3D), V isthe gradient operator, and G and R are forcing terms

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P. L. Houtekamer and Fuqing Zhang

) that, without the regular introduction of random forcings, the deterministic filter can develop highly non-Gaussian distributions. In deterministic filters, the regular addition of model error fields, via Eq. (22) [ section 4a(1) ], can serve as a forcing toward Gaussian distributions ( Lawson and Hansen 2004 , their section 5). Whereas, as mentioned, the EAKF and EnSRF algorithms are very similar, actual implementations for complex assimilation systems can be quite different. A deterministic

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