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J. Michael Fritsch

Abstract

Modification of mesoscale convective weather systems through ice-phase seeding is briefly reviewed. a simple mathematical framework for estimating the likely mesoscale response to convective cloud modification is presented, and previous mesoscale modification hypotheses are discussed in the context of this mathematical framework. Some basic differences between cloud-scale and mesoscale modification hypotheses are also discussed. Numerical model experiments to test the mesoscale sensitivity of convective weather systems are reviewed, and several focal points for identifying mesoscale modification potential are presented.

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J. MICHAEL FRITSCH

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J. MICHAEL FRITSCH

Abstract

A procedure to use the spline interpolation technique on an arbitrarily prescribed two-dimensional data field is described. For using this technique, one must obtain an initial approximation to the data at the grid points. This is achieved by fitting spherical surfaces to the data. Bidirectional spline interpolation is then applied repeatedly on the grid point estimates of the data to produce convergence to the true surface.

The spline interpolation technique and another objective analysis technique developed by Cressman are tested against an exact solution, and the resulting analyses are compared. Real temperature, geopotential height, and wind data for various pressure surfaces are analyzed by the spline method; and the results are compared to subjective analyses of the same data.

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David J. Stensrud
and
J. Michael Fritsch

Abstract

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of five mesoscale convective systems (MCSs) developed within a weakly forced large-scale environment. Analyses indicate that the large-scale flow created a broad region of potential buoyant energy, but owing to a restraining inversion and weak large-scale upward motion, convection was initiated only where lifting associated with mesoscale features was able to eliminate the inversion. The series of MCSs developed sequentially and moved eastward across the moist axis. Two of these systems had a large component of motion against the mean tropospheric flow and propagated in a direction nearly opposite to that of the traveling upper-level disturbances. Each system produced an outflow of cold downdraft air that spread progressively farther south than that from the preceding system. This description of the development and evolution of convection is very different from traditional ones wherein convection develops and moves more or less in phase with traveling upper-level disturbances.

Simple analytic models are used to determine the likely mechanisms of upstream propagation. These results suggest that the combined effects of both density currents and internal gravity waves produce the upstream propagation of the region of convection. Density currents dominate the propagation of convection once it forms, while internal gravity waves may help initiate new convection upstream of the region of existing convection, thereby producing a jump in the region of convective activity.

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David J. Stensrud
and
J. Michael Fritsch

Abstract

A series of five mesoscale convective systems (MCSs) developed within a weakly forced large-scale environment on 11 and 12 May 1982. Two of these systems had a large component of motion against the midtroposphoric flow and propagated in a direction nearly opposite to that of the traveling upper-level disturbances. This description of the evolution of convection is very different from traditional ones in which convection develops and moves more or less in phase with traveling upper-level disturbances. Observations indicate that the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conventional observing network, making the structure of the model initial condition a potentially crucial factor in the success or failure of any subsequent numerical simulation.

It is found that the initial conditions created using the conventional initialization procedure of The Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model do not include several of the mesoscale-sized features observed at 1200 UTC 11 May 1982, 9 h before the development of the first MCS. This is attributed to the lack of observed data with mesoscale resolution, and, therefore, likely is a deficiency in most initialization procedures in use today. Although it is true that new operational observing systems, such as the WSR-88D radar and the 404-MHz radar wind profilers, provide more detailed information, the data density on the mesoscale remains subcritical. A methodology to include mesoscale features, based upon using subjective interpretations of all the available observations, is developed. 11 is found that the mesoscale initial condition created using this subjective approach produces a more reasonable representation of the observed mesoscale features in comparison with the conventionally produced initial condition.

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David J. Stensrud
and
J. Michael Fritsch

Abstract

During a 24-h period, beginning 1200 UTC 11 May 1982, a series of mesoscale convective systems developed within a weakly forced large-scale environment. Two of these systems had a large component of motion against the midtropospheric flow and propagated in a direction nearly opposite to that of the travelling upper-level disturbances. This evolution of convection is very different from traditional ones in which convection develops and moves more or less in phase with traveling upper-level disturbances. It presents a tremendous challenge for three-dimensional numerical models, since the initiation and evolution of convection are tied to mesoscale features that are not well observed by the conventional upper-air network and may not be well approximated in the model parameterization schemes.

Mesoscale model simulations are conducted to evaluate the ability of the model to reproduce this complex event and to examine the model sensitivities to differences in the convective trigger function and model initial condition. Results suggest deal mesoscale models may be capable of producing useful simulations of convective events associated with weak, large-scale forcing, including quantitative precipitation forecasts with the correct magnitude and approximate location of heavy rainfall. If the important mesoscale circulations are incorporated into the model initial condition and a sufficiently realistic trigger function is used. However, model sensitivities to both the initial condition and the convective trigger function are large. Results indicate that the effects of boundary layer forcing must be included in the trigger function in order to initiate convection at the proper time and location. Timing errors in the initial development of convection of greater than 4 h occur if an unpresentative trigger function is used. Mesoscale features in the model initial condition also play an important role in the development and evolution of convection. The locations of heavy rainfall are shifted by greater than 100 km, or disappear altogether, if particular mesoscale features are not included subjectively in the initial condition. These sensitivities suggest that using an ensemble forecast approach to mesoscale model output needs to be considered seriously as mesoscale models move into the operational forecasting environment.

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Joseph J. Charney
and
J. Michael Fritsch

Abstract

Surface discrete frontal propagation in a wintertime, nonconvective environment is documented using conventional surface and upper-air data and simulated using the PSU–NCAR mesoscale model.

Synoptic and mesoscale surface analyses show a cold front associated with a synoptic-scale low-pressure system propagating from northwest to southeast across the central United States. Apparently discrete frontal propagation occurs when the surface front dissipates and a new front forms approximately 500 km ahead of the original front, with no compelling evidence of frontal passage in the intervening space. Upper-air analyses indicate the infusion of three different airstreams into the frontal region, resulting in the formation of a ribbon of low static stability air parallel to and several hundred kilometers in advance of the original front. This static stability structure appears to be involved in the observed evolution of the front. The development of precipitation over the intervening zone between the old and new frontal positions suggests that precipitation-induced diabatic processes also played a role in the discrete frontal propagation.

The numerical simulation captures the essential surface, upper-air, and precipitation features associated with the discrete propagation. Cross-section analyses of the simulated atmospheric fields indicate that the front propagated discretely only at the surface and in the lowest 200 hPa of the atmosphere, while the midtropospheric trough associated with the surface front propagated continuously though the region. The cross sections also indicate that the vertical winds associated with the frontal system adjust very quickly to the new frontal location while the horizontal winds and mass fields adjust more slowly. Analysis of frontogenetical forcing verifies that the new surface front forms at the expense of the original front. A careful examination of the temperature budgets within the simulation shows that the mass field redistribution associated with the discrete frontal propagation occurred as a result of the lifting of a strong temperature inversion in the prefrontal environment combined with precipitation induced diabatic cooling.

Based on the results of the model simulation, a conceptual model of discrete frontal propagation is presented that incorporates the observed and simulated sequence of events.

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Peter J. Sousounis
and
J. Michael Fritsch

A high-resolution numerical model is employed to examine effects of the Great Lakes aggregate, defined to be the five major Great Lakes, on regional and synoptic-scale weather. Simulations wherein the effects of the lakes are included and then excluded are performed on a selected cold air outbreak episode during late autumn when the lakes are still ice-free.

Examination of the differences between the model simulations reveals that several dynamical effects result from heating and moistening by the lake aggregate. These effects are manifested primarily in the form of a 4-km-deep, 2000-km-wide, lake-aggregate mesoscaie disturbance (circulation) that develops slowly over the region. The simulated lake-aggregate circulation splits a synoptic-scale high into two distinct centers and redirects and intensifies a weak synoptic-scale low, as verified by existing observations. These modifications of the synoptic-scale environment result in additional precipitation over, downstream, and upwind from the lakes.

The model simulations also reveal that the developing lake-aggregate circulation influences significantly the lake shore surface winds. In some locations, the surface winds switch from onshore to offshore or vice versa. Because it is well known from observations that the location and orientation of lake-induced snow bands are very sensitive to the low-level wind direction over the lakes, it is concluded that the exact locations of heavy snowfall are the result of a complex multiscale interaction among circulations on three different scales: synoptic, individual lake, and lake aggregate.

In addition to the developing primary lake-aggregate circulation, a secondary dynamic response appears at a distant location, adjacent to the eastern seaboard. The organization of this secondary circulation suggests that the lakes may play a direct role in some cases of East Coast cyclogenesis.

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Da-Lin Zhang
and
J. Michael Fritsch

Abstract

A 36-h nested-grid numerical simulation of the life cycle of a convectively generated, inertially stable, warm-core mesovortex is presented. The vortex evolved from a mesoscale convective complex that developed from a squall line over Oklahoma during 7–8 July 1982. A modified version of the Pennsylvania State University /National Center for Atmospheric Research mesoscale hydrostatic model with a fine-mesh grid resolution of 25 km is utilized for this study. The model simultaneously incorporates parameterized convection and a grid-resolved convective scheme containing the effects of hydrostatic water loading, condensation (evaporation), freezing (melting) and sublimation.

Genesis, intensification and maintenance of a low- to midtropospheric closed meso-β scale cyclone as well as the associated surface pressure perturbations, the evolution of moist convection, and the distribution and magnitude of total rainfall are simulated by the model. Similarly, the observed amplification of a 700-mb meso-α scale short-wave trough, the development of a midlevel warm-core structure and an upper-level mesoanticyclone during the mature stage, the quasi-stationary nature of the vortex circulation, and the vertical distribution of horizontal wind and relative vorticity in the vicinity of the rotating mesoscale convective system (MCS) are all reasonably well simulated up to 36 h. During the mature stage of the rotating MCS, both the observed and simulated vertical structure are characterized by a low-level mesohigh in association with a cool pool and sinking motion, a midtropospheric warm-core structure, and an upper-level cold dome with an associated anticyclonic circulation. The horizontal momentum and equivalent potential temperature are uniformly distributed in the vortex layer with the vorticity maximum located between 600 and 700 mb.

The model simulation shows that the upward motion and cyclonic vorticity associated with the front and vortex system are out of phase. The phase difference appears to be a propagation mechanism of the rotating MCS and the low-level front. Another important finding is that most of the vortex properties tilt downstream with height during the decay period. Such a vertical distribution helps explain why a well-defined and long-lived hydrostatic surface mesolow did not form in either the observations or the simulation. It also helps explain why other midlatitude rotating MCSs often exhibit weak surface pressure predictions.

It is found that a propagating mesoscale vorticity disturbance, preexisting low-level frontal forcing and a convectively favorable environment ahead of the front help generate an organized area of upward motion wherein the vortex develops. However, it is the resolvable-scale latent heat release that appears to be directly responsible for producing the rotating MCS. The quasi-stationary nature of the rotating MCS is related to the fact that the vortex develops within a slow-moving, low-level horizontal deformation field. The vortex is well maintained because of the weak horizontal and vertical shear in the deformation zone and the generated 1arge inertial stability of the vortex. The results indicate that in some situations, numerical forecasts of the genesis, evolution and rainfall of rotating MCSs are possible up to 36 h using the currently available observations if a high-grid resolution model can be utilized.

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Da-Lin Zhang
and
J. Michael Fritsch

Abstract

A 12 h nested-grid numerical simulation of a warm-season mesoscale convective weather system (Zhang and Fritsch, 1986) is utilized as a control run in order to 1) test the sensitivity of the numerical simulation to different types of initial conditions; 2) examine the need for an observing system that would resolve mesoscale features; and 3) determine which meteorological variables need to be most carefully considered in observing system design and preprocessing analysis.

It is found that improved observational capabilities are likely to have an important impact on the successful prediction of the timing and location of summertime mesoscale convective weather systems if mesoscale features can be resolved. In particular, the resolution of the moisture field significantly affects the prediction of the evolution of the convective weather systems. Correspondingly, the mesoscale distribution of precipitation is substantially affected, especially the location of the areas of heavy rain. It is also found that procedures to account for the effects of convective systems that are in progress at the time of initialization can make significant contributions to the prediction of the evolution of the meteorological events and to the improvement of the quantitative precipitation forecasts. In particular, in weak-gradient summertime situations, mesoscale convective systems can severely alter their near environment within a short time period by producing strong mesoscale circulations, thermal boundaries, moist adiabatic stratification etc.

For summertime situations where the large-scale gradients are weak, detailed temperature and moisture fields appear to be more important than the detailed wind fields in determining the development and evolution of deep convection. However, poor resolution of the wind field such that wind speed magnitudes and gradients are underestimated tends to reduce the degree of mesoscale organization. It also alters the magnitude and distribution of low-level convergence, and this affects the evolution of the thermodynamic fields and the deep convection.

Incorporation of dense surface observations into the initial conditions can be very important in improving forecasts of meso-β-scale structures such as moist (dry) tongues, thermal boundaries, and, in particular, pressure distribution. Most significantly, the large (meso-α)-scale environment appears to contain some type of signal such that the general evolution of events is similar, even when the initial mesoscale structure and the simulated meso-β-scale evolution of events are significantly different. On the other hand, poor resolution of meso-α-scale gradients can substantially alter the predicted evolution of meso-β-scale features and the location of heavy rain.

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