The results from 72-hour simulations and forecasts from the Penn State/NCAR limited-area model, in which a number of numerical and physical factors are varied, are analysed to understand the contribution to model error or uncertainty introduced by these factors. The factors include the initial conditions, horizontal resolution (80 km and 160 km) and domain size, lateral boundary conditions, and physical parametrizations. The results are compared with those of a global forecast model (the NCAR Community Climate Model). The simulations and forecasts are verified both for 12 individual cases and for the ensemble average of the 12 cases, using several objective measures of skill. The differences in these skill scores between different models are tested for their statistical significance.



The use of observed lateral boundary conditions (LBC) exerts a strong control on the growth of errors over a domain size 3600 × 4800 km2. Objective measures of error show little growth beyond about 36 hours, so that 72-hour errors are nearly as low as the 36-hour errors. On these time and space scales, the quality of the LBC is more important than any other factor tested in the temporal evolution of the model errors. These results show that the large-scale atmospheric motions have a major effect on the evolution of small-scale features in the model.



Small variations in initial conditions have little effect on the model skill beyond 12–24 hours. Of all the factors examined, small uncertainties or errors in the initial conditions have the least effect on model skill beyond 12 hours.



Latent heating effects associated with condensation and precipitation, and sensible and latent heat fluxes from the surface have a statistically significant effect on model skill. However, relatively simple parametrizations of these effects produce nearly the same skill as do more complex schemes for the cases studied here.



There is a large variation in model skill from case to case; some cases are easier to forecast than others. Also, there is much greater variation in different model performances for a single case than when averaged over all 12 cases.



The concept of climatological use and verification of regional models is introduced. The climatological skill of the control model version which includes all physical processes appears to be quite good. The skill scores of the ensemble average simulation are considerably better than the average scores for individual cases, the model has small bias errors, and the horizontal structure of the model-simulated atmosphere is similar to the observed structure for scales of motion resolved by the upper-air observational network over the United States.

Estimation of skill and uncertainty in regional numerical models

The interaction of transverse vertical circulations associated with two separate jet steak/trough systems is found to be a common feature of cyclogenetic events which produce heavy snow along the East Coast of the United States. The transverse circulations are identified for eight cases that span the period from 1960 to 1987 utilizing an isentropic analysis of the operational radiosonde data. The analyses depict the interaction of 1) a direct circulation located within the confluent entrance region of an upper-level jet streak over the northeastern United States or southeastern Canada with 2) an indirect circulation in the diffluent exit region of a jet streak associated with a trough nearing the East Coast. This interaction contributes to differential moisture and temperature advections and vertical motions necessary to produce heavy snowfall along the coast. It is suggested that the circulation patterns associated with the jet streak establish an environment within which boundary layer processe...

The Interaction of Jet Streak Circulations during Heavy Snow Events along the East Coast of the United States

The study of extratropical cyclones has provided the basis for vigorous scientific debates within the meteorological community for at least the past 150 years In her monograph entitled The Thermal Theory of Cyclones: A History of Meteorological Thought in the Nineteenth Century, Kutzbach (1979) documents the interest of the leading European and American meteorologists of the 19th and early 20th centuries in providing a description of the weather and airflow associated with cyclones and identifying the physical processes that contribute to their development In the 19th century, the emergence of the so-called “thermal theory of cyclones” (see Fig 61) was based, to a large degree, on the work of Espy, who believed that the decrease of surface pressure in storms is related primarily to the release of latent heat in the ascending air near the storm center By the early 20th century, the theoretical work of Margules and V Bjerknes and the observational studies by Dines (which indicated extratropical cyclones were cold core systems) led to a more dynamically based perspective on cyclogenesis The energy conversions and low-level convergence associated with instabilities in regions marked by significant temperature gradients (especially in the lower troposphere) were recognized as important contributing factors in the development of extra tropical storms

Processes Contributing to the Rapid Development of Extratropical Cyclones

The status of satellite-based rainfall estimation over land☆

Heavy rainfall occurred at both the inland frontal zone and coastal warm sector in southern China during 10–11 May 2014, which is a typical pattern in the early-summer rainy season. To clar...

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Heavy Rainfall Associated with Double Low-Level Jets over Southern China. Part I: Ensemble-Based Analysis

A series of numerical simulations of the February 1979 Presidents Day cyclone is presented. The development of the low-level jet (LLJ) associated with the cyclone is described, and the mesoscale numerical model, initial analyses, and experimental design used in the study are discussed. Four numerical simulations are discussed and compared, including an adiabatic simulation that isolates the development of upper-level divergence along the axis of a subtropical jet streak and three other simulations that reveal the contributions of sensible and latent heat release in modifying lower-tropospheric wind fields and reducing the sea-level pressure. The formation of the LLJ is described through an evaluation of trajectories derived from the various model simulations. The effect of the LLJ on secondary cyclogenesis along the East Coast is described.

Synergistic interactions between an upper-level jet streak and diabatic processes that influence the development of a low-level jet and a secondary coastal cyclone

The development of a comprehensive mesoscale atmospheric simulation system (MASS) is described in detail. The modeling system is designed for both research and real-time forecast applications. The 14-level numerical model, which has a 48 km grid mesh, can be run over most of North America and the adjacent oceanic regions. The model employs sixth-order accurate numerics, generalized similarity theory boundary-layer physics, a sophisticated cumulus parameterization scheme, and state of the art analysis and initialization techniques. Examples of model output on the synoptic and subsynoptic scales are presented for the AVE-SESAME I field experiment on 10-11 April 1979. The model output is subjectively compared to the observational analysis and the LFM II output on the synoptic scale. Subsynoptic model output is compared to analyses generated from the AVE-SESAME I data set.

Initial Results from a Mesoscale Atmospheric Simulation System and Comparisons with the AVE-SESAME I Data Set

Numerical experiments are conducted to assess the impact of incorporating temperature data from the VISSR Atmospheric Sounder (VAS) into a regional-scale numerical model using an assimilation technique developed by Gal-Chen. The technique uses a three-dimensional variational approach to combine the VAS observations with model temperature fields during the numerical integration. A nudging technique is also tested, whereby the model temperature field is constrained toward the VAS “updated” values during the assimilation cycle. Results of the experiments indicate that the Gal-Chen assimilation technique successfully combines actual VAS temperature observations with the dynamically balanced model fields without destabilizing the model during the assimilation cycle. Furthermore, increasing the temporal frequency of VAS temperature insertions during the assimilation cycle enhances the impact on the model forecast through successively longer forecast periods. These results support the conclusions of ear...

A VAS-Numerical Model Impact Study Using the Gal-Chen Variational Approach

Abstract Numerical experiments are conducted to assess the impact of incorporating temperature data from the VISSR Atmospheric Sounder (VAS) into a regional-scale numerical model using an assimilation technique developed by Gal-Chen. The technique uses a three-dimensional variational approach to combine the VAS observations with model temperature fields during the numerical integration. A nudging technique is also tested, whereby the model temperature field is constrained toward the VAS “updated” values during the assimilation cycle. Results of the experiments indicate that the Gal-Chen assimilation technique successfully combines actual VAS temperature observations with the dynamically balanced model fields without destabilizing the model during the assimilation cycle. Furthermore, increasing the temporal frequency of VAS temperature insertions during the assimilation cycle enhances the impact on the model forecast through successively longer forecast periods. These results support the conclusions of ear...

James J. Tuccillo

Papers

Synergistic interactions between an upper-level jet streak and diabatic processes that influence the development of a low-level jet and a secondary coastal cyclone

Initial Results from a Mesoscale Atmospheric Simulation System and Comparisons with the AVE-SESAME I Data Set

A VAS-Numerical Model Impact Study Using the Gal-Chen Variational Approach

A VAS-numerical model impact study using the Gal-Chen variational approach. [Visible Infrared Spin-Scan Radiometer Atmospheric Sounder (VAS)]