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Zhiyong Meng and Fuqing Zhang

Abstract

Ensemble-based data assimilation is a state estimation technique that uses short-term ensemble forecasts to estimate flow-dependent background error covariance and is best known by varying forms of ensemble Kalman filters (EnKFs). The EnKF has recently emerged as one of the primary alternatives to the variational data assimilation methods widely used in both global and limited-area numerical weather prediction models. In addition to comparing the EnKF with variational methods, this article reviews recent advances and challenges in the development and applications of the EnKF, including its hybrid with variational methods, in limited-area models that resolve weather systems from convective to meso- and regional scales.

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Loïk Berre and Gérald Desroziers

Abstract

The use of local spatial averaging to estimate and validate background error covariances has received increasing attention recently, in particular in the context of variational data assimilation for global numerical weather prediction. First, theoretical and experimental results are presented to examine spatial structures of sampling noise and signal in ensemble-based variance fields in this context. They indicate that sampling noise tends to be relatively small scale, compared to the signal of interest. This difference in spatial structure motivates the use of spatial averaging techniques.

Based on the usual linear estimation theory, it is shown how this information can be taken into account in order to calculate and apply an objective spatial filter. This kind of approach can also be used to compare and validate ensemble-based variances with innovation-based variances. The use of spatial averaging is even more important for innovation-based variances because local innovations correspond to single error realizations.

Similar ideas can be considered for the estimation of correlation functions. The spatial structures of sampling noise and signal in correlation length scale fields suggest that space-averaging techniques could also be applied to correlation functions. The use of wavelets for this purpose is presented in particular. Connections with related approaches in different contexts such as ensemble Kalman filters and probabilistic forecasting are also discussed.

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Clifford Mass and Brigid Dotson

Abstract

The northwest United States is visited frequently by strong midlatitude cyclones that can produce hurricane-force winds and extensive damage. This article reviews these storms, beginning with a survey of the major events of the past century. A climatology of strong windstorms is presented for the area from southern Oregon to northern Washington State and is used to create synoptic composites that show the large-scale evolution associated with such storms. A recent event, the Hanukkah Eve Storm of December 2006, is described in detail, with particular attention given to the impact of the bent-back front/trough and temporal changes in vertical stability and structure. The discussion section examines the general role of the bent-back trough, the interactions of such storms with terrain, and the applicability of the “sting jet” conceptual model. A conceptual model of the evolution of Northwest windstorm events is presented.

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Robert A. Houze Jr.

Abstract

Clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere. Convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts. Within the eyes of mature tropical cyclones, stratus clouds top a boundary layer capped by subsidence. An outward-sloping eyewall cloud is controlled by adjustment of the vortex toward gradient-wind balance, which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state. This balance is intermittently upset by buoyancy arising from high-moist-static-energy air entering the base of the eyewall because of the radial influx of low-level air from the far environment, supergradient wind in the eyewall zone, and/or small-scale intense subvortices. The latter contain strong, erect updrafts. Graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left aloft surround the eyewall, and aggregates are advected radially outward and azimuthally up to 1.5 times around the cyclone before melting and falling as stratiform precipitation. Electrification of the eyewall cloud is controlled by its outward-sloping circulation. Outside the eyewall, a quasi-stationary principal rainband contains convective cells with overturning updrafts and two types of downdrafts, including a deep downdraft on the band’s inner edge. Transient secondary rainbands exhibit propagation characteristics of vortex Rossby waves. Rainbands can coalesce into a secondary eyewall separated from the primary eyewall by a moat that takes on the structure of an eye. Distant rainbands, outside the region dominated by vortex dynamics, consist of cumulonimbus clouds similar to non–tropical storm convection.

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Peter Jan van Leeuwen

Abstract

The application of particle filters in geophysical systems is reviewed. Some background on Bayesian filtering is provided, and the existing methods are discussed. The emphasis is on the methodology, and not so much on the applications themselves. It is shown that direct application of the basic particle filter (i.e., importance sampling using the prior as the importance density) does not work in high-dimensional systems, but several variants are shown to have potential. Approximations to the full problem that try to keep some aspects of the particle filter beyond the Gaussian approximation are also presented and discussed.

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Tammy M. Weckwerth and David B. Parsons

Abstract

The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations.

The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velocity. The corresponding thermodynamic distribution, particularly the moisture field, is not as readily measured. During IHOP_2002, a variety of sensors capable of measuring atmospheric water vapor were brought together in an effort to sample the three-dimensional time-varying moisture field and determine its impact on forecasting convection initiation. The strategy included examining convection initiation with targeted observations aimed at sampling regions forecast to be ripe for initiation, primarily along frontal zones, drylines, and their mergers.

A key aspect of these investigations was the combination of varied moisture measurements with the detailed observations of the wind field, as presented in many of the subsequent papers in this issue. For example, the high-resolution measurements are being used to better understand the role of misocyclones on convection initiation. The analyses are starting to elucidate the value of new datasets, including satellite products and radar refractivity retrievals. Data assimilation studies using some of the state-of-the-art datasets from IHOP_2002 are already proving to be quite promising.

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David M. Schultz

Abstract

The conceptual model of a classical surface-based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface. Many cold fronts, however, do not conform to this model—time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front (hereafter called a prefrontal trough and prefrontal wind shift, respectively). Although many authors have recognized these prefrontal features previously, a review of the responsible mechanisms has not been performed to date. This paper presents such a review. Ten disparate mechanisms with different frontal structures have been identified from the previous literature. These mechanisms include those external to the front (i.e., those not directly associated with the cold front itself): synoptic-scale forcing, interaction with lee troughs/drylines, interaction with fronts in the mid- and upper troposphere, and frontogenesis associated with inhomogeneities in the prefrontal air. Mechanisms internal to the front (i.e., those directly associated with the structure and dynamics of the front) include the following: surface friction, frontogenesis acting on alongfront temperature gradients, moist processes, descent of air, ascent of air at the front, and generation of prefrontal bores/gravity waves. Given the gaps in our knowledge of the structure, evolution, and dynamics of surface cold fronts, this paper closes with an admonition for improving the links between theory, observations, and modeling to advance understanding and develop better conceptual models of cold fronts, with the goal of improving both scientific understanding and operational forecasting.

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Paul M. Markowski

Abstract

Nearly 50 years of observations of hook echoes and their associated rear-flank downdrafts (RFDs) are reviewed. Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis remains poorly understood. Despite many strong similarities between simulated and observed storms, some possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional numerical models.

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David M. Schultz and Philip N. Schumacher

Abstract

A commonly employed explanation for single- and multiple-banded clouds and precipitation in the extratropics is slantwise convection due to the release of moist symmetric instability (MSI), of which one type is conditional symmetric instability (CSI). This article presents a review of CSI with the intent of synthesizing the results from previous observational, theoretical, and modeling studies. This review contends that CSI as a diagnostic tool to assess slantwise convection has been, and continues to be, misused and overused. Drawing parallels to an ingredients-based methodology for forecasting deep, moist convection that requires the simultaneous presence of instability, moisture, and lift, some of the misapplications of CSI can be clarified. Many of these pitfalls have been noted by earlier authors, but are, nevertheless, often understated, misinterpreted, or neglected by later researchers and forecasters. Topics include the evaluation of the potential for slantwise convection, the relationship between frontogenesis and MSI, the coexistence of moist gravitational instability and MSI, the nature of banding associated with slantwise convection, and the diagnosis of slantwise convection using mesoscale numerical models. The review concludes with suggested directions for future observational, theoretical, and diagnostic investigation.

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Dayton G. Vincent

Abstract

The circulation features associated with the South Pacific convergence zone (SPCZ) and its accompanying cloud band are reviewed and discussed. The paper focuses on the following topics: location, structure, and characteristics of the SPCZ; theories and observations concerning its existence; the significance and scope of the SPCZ in global-scale circulation patterns; quasi-periodic changes in its location and strength; and synoptic-scale features within its regional influence (e.g., cyclones, subtropical jets). It concludes with some challenging problems for the future.

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