Search Results

You are looking at 1 - 10 of 33 items for

  • Author or Editor: Robert G. Fovell x
  • All content x
Clear All Modify Search
Robert G. Fovell

Abstract

Two-dimensional model simulations were made to gage the effect of the Coriolis force on model squall lines. The case chosen for intensive study had low-to-moderate wind shear confined to low levels. With this wind shear, two Coriolis simulations were made, with and without a geostrophically balanced along-line temperature gradient. Additional simulations were made with other wind shear intensifies to test the sensitivity to low-level shear.

Unlike their nonrotational counterparts, none of the Coriolis model storms were able to attain or maintain a “quasi-equilibrium” state. Quasi-equilibrium storms possess mature phases characterized by essentially statistically steady behavior with respect to storm strength, propagation speed, etc. Instead, the Coriolis storms possessed mature phases marked by gradual but definite decay. These are the first model storms created with the present model sounding and wind profiles that have terminal mature phases due to physically realistic forcings. However, the time scale of the decay, at least in these cases, makes it unlikely that Coriolis forcing is the primary mechanism behind the demise of real long-lived, mature squall line thunderstorms.

In each rotational case, the decay phase was marked by two major temporal trends absent in the mature phase of the nonrotational simulations: the continued contamination of the forward environment with storm-induced subsidence warming and the decline in intensity of the rear inflow current. The subsidence warming was slowly eradicating the convective instability of the air flowing into the storm, and the dissipating inflow current appeared to be at least partially responsible for the progressive collapse of the storm's subcloud cold pool. The accumulation of subsidence warming was clearly injuring the model storm. The role that the declining rear inflow played in the decay phase is less clear and requires additional study.

It was found that the inclusion of the geostrophically balanced along-line temperature gradient had small but measurable consequences in this situation. Warm advection at low levels ahead of the storm worked to negate the effect of warm advection aloft on the convective instability, and cold advection into the cold pool opposed the general decline in pool intensity. The net effect was that the Coriolis-associated mature-phase decaying tendency was stowed somewhat, but not arrested.

Full access
Robert G. Fovell

Abstract

A “consensus clustering” strategy is applied to long-term temperature and precipitation time series data for the purpose of delineating climate zones of the conterminous United States in a “data-driven” (as opposed to “rule-driven”) fashion. Cluster analysis simplifies a dataset by arranging “objects” (here, climate divisions or stations) into a smaller number of relatively homogeneous groups or clusters on the basis of interobject dissimilarities computed using the identified “attributes” (here, temperature and precipitation measurements recorded for the objects). The results demonstrate the spatial scales associated with climatic variability and may suggest climatically justified ways in which the number of objects in a dataset may be reduced. Implicit in this work is the arguable contention that temperature and precipitation data are both necessary and sufficient for the delineation of climatic zones.

In prior work, the temperature and precipitation data were mixed during the computation of the interobject dissimilarities. This allowed the clusters to jointly reflect temperature and precipitation distinctions, but also had inherent problems relating to arbitrary attribute scaling and information redundancy that proved difficult to resolve. In the present approach, the temperature and precipitation data are clustered separately and then categorically intersected to forge consensus clusters. The consensus outcome may be viewed as having identified the temperature subzones of precipitation clusters (or vice versa) or as representing distinct groupings that are relatively homogeneous with respect to both attribute types simultaneously.

The dissimilarity measure employed herein is the Euclidean distance. As it employs only continuous time series data representing a single information type (temperature or precipitation), the consensus approach has the advantage of allowing an attractively simple interpretation of the total Euclidean distance between object pairs. The total squared distance may be subdivided into three components representing object dissimilarity with respect to temporal mean (level), seasonality (variability), and coseasonality (relative temporal phasing). Therefore, concerns about redundancy or arbitrary scaling problems are neutralized. This is seen as the chief advantage of consensus clustering.

The consensus strategy has several disadvantages. It is possible for two (or more) relatively general, undetailed clusterings to produce a very complex and fragmented clustering following categorical intersection. Further, the fact that the analyst chooses the clustering levels of the separate, contributing clusterings means that he or she has considerable freedom in fashioning the consensus outcome, which makes it difficult (if not impossible) to argue that true, “natural” clusters have been identified. The latter often applies to cluster analysis in general, however. It is believed that the consensus approach merits consideration owing to its advantages.

Two consensus outcomes are presented: a lower-order solution with 14 clusters and a higher-order solution with 26 clusters. The sensitivity of these clusterings to perturbations in the input data is assessed. The regionalizations are compared with those presented in prior work.

Full access
Robert G. Fovell

Abstract

In earlier work, a three-dimensional cloud model was used to simulate the interaction between the sea-breeze front (SBF) and front-parallel horizontal convective rolls (HCRs), resulting in the SBF systematically encountering roll updrafts and downdrafts as it progressed inland. Interestingly, deep convection was spawned above an HCR updraft ahead of the SBF as the front approached, well before the inevitable front–roll merger. Ostensibly, both the sea-breeze and roll circulations were required for deep convection to be present in this case at all because convection was entirely absent when either phenomenon was removed.

Further analysis reveals why both circulations were necessary yet not sufficient for the excitation of deep convection in this case. The sea-breeze circulation (SBC) made its upstream (inland) environment more favorable for convection by bringing about persistent if gentle lifting over an extended region stretching well ahead of the SBF. This persistent ascent established a moist and cool tongue of air, manifested by a visible and/or subvisible cloud feature termed the cloud shelf emanating ahead of the front. Though this lifting moistened and destabilized the environment, the roll’s direct and indirect effects on this moist tongue were also required. The former consisted of a moisture plume lofted by the roll updraft, and the latter consisted of obstacle effect gravity waves generated as the roll drafts penetrated through the top of the boundary layer, into the SBC-associated offshore flow farther aloft. These provided the missing spark, which led to rapid growth of cumulus above the roll updraft, drawing first from air located above the boundary layer.

Once established, deep convection above the roll updraft modulated cloudiness above the approaching SBF, at first suppressing it but subsequently assuring its reestablishment and eventual growth into deep convection, again prior to the front–roll merger. This resulted from the influence of gravity waves excited owing to heating and cooling within the roll cloud.

Full access
Robert G. Fovell and Mei-Ying C. Fovell

Abstract

A regionalization of the conterminous United States is accomplished using hierarchical cluster analysis on temperature and precipitation data. The “best” combination of clustering method and data preprocessing strategy yields a set of candidate clustering levels, from which the 14-, 25-, and 8-duster solutions are chosen. Collectively, these are termed the “reference clusterings.” At the 14-cluster level, the bulk of the nation is partitioned into four principal climate zones: the Southeast, East Central, Northeastern Tier, and Interior West clusters. Many small clusters are concentrated in the Pacific Northwest. The 25-cluster solution can be used to identify the subzones within the 14 clusters. At that more detailed level, many of the areally more extensive clusters are partitioned into smaller, more internally cohesive subgroups.

The “best” clustering approach is the one that minimizes the influences of three forms of bias-methodological, latent, and information-for the dataset at hand. Sources of, and remedies for, these biases are discussed. Sensitivity tests indicate that some of the clusters in the reference clusterings lack robustness, especially those in the Northeast quadrant of the United States. Some of the tests involve small and large alterations to the data preprocessing strategy.

The major shortcomings of the analysis procedure are that the clusters are unnaturally constrained to he nonoverlapping and also that potentially important data from points outside of the political boundaries of the conterminous United States and over water are not included. Also, other variables that could be important or useful in characterizing climate type could be added to, or used in place of, the temperature and precipitation variables used herein. Further work on data preprocessing techniques is also required. Remedies for these and other shortcomings are proposed.

Full access
Robert G. Fovell and Yoshi Ogura

Abstract

A two-dimensional, anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line. Initial conditions were adapted from observations of the 22 May 1976 Oklahoma line. Model simulations were made with and without considering the ice phase of water. These model storms have, within the constraints of the model's geometry, replicated the basic multicellular character and general line-normal airflow typical of these lines. In addition, the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable.

Further, it was found that the initial conditions chosen resulted in model storms which were not only long-lasting but also essentially repetitive, indicating that a state of quasi-equilibrium had been attained. The storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations.

The inclusion of ice phase processes resulted in the production of more realistic appearing features in the trailing portion of the storm as well as more widespread precipitation. These were for the most part due to the enhanced rearward transport of precipitation particles from the convective cells which resulted from including ice, particularly low density snow. The underlying structures of these model storms were investigated by averaging model fields across time, smoothing out the transient components. These analyses indicated that the addition of ice had its greatest impact on the scale of the storm's circulation features.

Full access
Robert G. Fovell and Alex Gallagher

Abstract

While numerical weather prediction models have made considerable progress regarding forecast skill, less attention has been paid to the planetary boundary layer. This study leverages High-Resolution Rapid Refresh (HRRR) forecasts on native levels, 1-s radiosonde data, and (primarily airport) surface observations across the conterminous United States. We construct temporally and spatially averaged composites of wind speed and potential temperature in the lowest 1 km for selected months to identify systematic errors in both forecasts and observations in this critical layer. We find near-surface temperature and wind speed predictions to be skillful, although wind biases were negatively correlated with observed speed and temperature biases revealed a robust relationship with station elevation. Above ≈250 m above ground level, below which radiosonde wind data were apparently contaminated by processing, biases were small for wind speed and potential temperature at the analysis time (which incorporates sonde data) but became substantial by the 24-h forecast. Wind biases were positive through the layer for both 0000 and 1200 UTC, and morning potential temperature profiles were marked by excessively steep lapse rates that persisted across seasons and (again) exaggerated at higher elevation sites. While the source or cause of these systematic errors are not fully understood, this analysis highlights areas for potential model improvement and the need for a continued and accessible archive of the data that make analyses like this possible.

Restricted access
Yang Cao and Robert G. Fovell

Abstract

The “Santa Ana” wind is an offshore flow that affects Southern California periodically during the winter half of the year, typically between September and May. The winds can be locally gusty, particularly in the complex terrain of San Diego County, where the winds have characteristics of downslope windstorms. These winds can cause and/or rapidly spread wildfires, the threat of which is particularly acute during the autumn season before the onset of winter rains. San Diego’s largest fires, including the Cedar fire of 2003 and Witch Creek fire of 2007, occurred during Santa Ana wind events.

A case study of downslope flow during a moderately intense Santa Ana event during mid-February 2013 is presented. Motivated by the need to forecast winds impinging on electrical lines, the authors make use of an exceptionally dense network of near-surface observations in San Diego County to calibrate and verify simulations made utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model, which in turn is employed to augment the observations. Results demonstrate that this particular Santa Ana episode consists of two pulses separated by a protracted lull. During the first pulse, the downslope flow is characterized by a prominent hydraulic jumplike feature, while during the second one the flow possesses a clear temporal progression of winds downslope. WRF has skill in capturing the evolution and magnitude of the event at most locations, although most model configurations overpredict the observed sustained wind and the forecast bias is itself biased.

Full access
Robert G. Fovell and Yoshi Ogura

Abstract

A strictly two-dimensional cloud model was used to gauge the effect of vertical wind shear on the mature phase behavior of model-simulated multicellular storms, extending the previous work of the authors. We specifically examined the propagation speed, quasi-equilibrium behavior, storm scale and updraft orientation of the model storms as a function of shear intensity. We also considered the precipitation efficiencies of our. model storms and applied density current and Rotunno–Klemp–Weisman theories to our results.

Our previous work revealed that model storms could achieve a mature phase consisting of repetitive multicellular development when certain numerical obstacles were overcome. This was referred to as a “quasi-equilibrium state.” We found herein that this state was also reached by model storms even when subjected to a very wide range of low-level wind shear intensities, although the temporal behavior during this stage was clearly dependent on the shear. We also found a very systematic relationship between the storm speed and the shear strength. Therefore, small shear values produced slowly moving storms which generally exhibited simple oscillations with time, fitting the classic multicell model. Larger shears resulted in complex oscillations similar to what has been termed “weak evolution,” culminating in a nearly unicellular storm in the most extreme case.

The transition between the strongly and weakly evolving modes was abrupt in the wind shear spectrum, and the temporal behavior of the precipitation production was quite different between the two regimes. Yet, we also found that the precipitation efficiencies of these model storms were roughly constant among the simulations, irrespective of the low-level shear. The larger shear storms typically produced more precipitation, because they were processing water vapor at faster rates due to their more rapid propagation speeds, but were neither identifiably more nor less efficient in doing so. The rear inflow current feature, present in each case, appeared to play a major role in creating the colder subcloud cold pools which helped the storms formed in larger shear to move faster.

An important result is that none of the model storms suffered a terminal decaying phase, certainly not within a reasonable period of time. This suggests that the storm itself does not sow the seeds of its own demise, at least for the favorable, homogeneous environmental conditions considered and the simple, strictly two-dimensional framework adopted for this study.

Full access
Yang Cao and Robert G. Fovell

Abstract

The “Santa Ana” winds of Southern California represent a high-impact weather event because their dry, fast winds can significantly elevate the wildfire threat. This high-resolution numerical study of six events of moderate or greater strength employs physics parameterization and stochastic perturbation ensembles to determine the optimal model configuration for predicting winds in San Diego County, with verification performed against observations from the San Diego Gas and Electric (SDG&E) mesonet. Results demonstrate model physics can have a material effect on the strength, location, and timing of the winds, with the land surface model playing an outsized role via its specification of surface roughness lengths. Even when bias in the network-averaged sustained wind forecasts is minimized, systematic biases remain in that many stations are consistently over- or underforecasted. The argument is made that this is an “unavoidable” error that represents localized anemometer exposure issues revealed through the station gust factor. A very simple gust parameterization is proposed for the mesonet based on the discovery that the network-averaged gust factor is independent of weather conditions and results in unbiased forecasts of gusts at individual stations and the mesonet as a whole. Combined with atmospheric humidity and fuel moisture information, gust forecasts can help in the assessment of wildfire risks.

Full access
Robert G. Fovell and Peter S. Dailey

Abstract

A three-dimensional, high-resolution model is employed to examine the interaction between the sea-breeze front (SBF) and horizontal convective rolls (HCRs) aligned parallel to the front. This study extends the perpendicular case that was the focus of Part I. In this situation, the SBF systematically encounters roll downdrafts and updrafts as it propagates inland.

The sea-breeze circulation is found to significantly influence HCR strength and development. In turn, the rolls are found to dramatically modulate the overall convective activity, alternately suppressing and enhancing SBF-associated convection. Suppression occurs as the SBF merges with a roll downdraft. This is in part due to the downdraft's introduction of dry air into the mixed layer that becomes part of the SBF cloud's inflow.

Following suppression, the SBF accelerates as convective heating above the frontal head diminishes. This leads to reinvigorated convection above the front prior to its contact with the next roll updraft, which itself sports a strong, deep cloud of its own by this time. This brings about two strong updrafts obscured by a single, merged cloud shield. During this time, a strong yet brief midlevel downdraft occurs in between the two updrafts; forcing mechanisms for this feature are discussed. The SBF propagation speed also declines significantly during this period; the near-surface portion of the front actually becoming retrograde for a period of a few minutes. Two other, less dramatic roll encounters are also examined.

Full access