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Frank P. Colby Jr.

Abstract

Previous investigations of the 8 June 1966 National Severe Storms Laboratory case show that mesoscale lift from an “inland sea breeze” due to a newly formed temperature gradient preceded convection. Using additional surface data, this case was reanalyzed over all of Oklahoma. Cloud work functions were calculated from real and interpolated data to measure convective instability.

The results indicated that the above temperature gradient was the eastern one of two gradients located on either side of a warm tongue. The cells formed over the eastern gradient where the convective instability was highest.

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Frank P. Colby Jr.

Abstract

Since 2012, the National Centers for Environmental Prediction’s Global Ensemble Forecast System (GEFS) has undergone two major upgrades. Version 11 was introduced in December 2015, with a new dynamic scheme, improved physics, increased horizontal and vertical resolution, and a more accurate initialization method. Prior to implementation, retrospective model runs over four years were made, covering multiple hurricane seasons. The second major upgrade was implemented in May 2016, when the data assimilation system for the deterministic Global Forecast System (GFS) was upgraded. Because the GEFS initialization is taken from the deterministic GFS, this upgrade had a direct impact on the GEFS. Unlike the previous upgrade, the model was rerun for only a few tropical cyclones. Hurricane Edouard (2014) was the storm for which the most retrospective runs (4) were made for the new data assimilation system. In this paper, the impact of the GEFS upgrades is examined using seasonal data for the 2014–17 hurricane seasons, and detailed data from the four model runs made for Hurricane Edouard. Both upgrades reduced the spread between ensemble member tracks. The first upgrade reduced the spread but did not reduce the likelihood that the actual track would be included in the family of member tracks. The second upgrade both reduced the spread further and reduced the chance that the real storm track would be within the envelope of member tracks.

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Frank P. Colby Jr.

An affordable rawinsonde system that has proven helpful in teaching undergraduate meteorology is described. Not only have planned and written exercises been designed around data from the rawinsonde launches, but the actual launches themselves have encouraged an active learning process among the students. In addition, the equipment has enabled undergraduate students to participate in a number of research projects.

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Frank P. Colby Jr.

Abstract

The AVE-SESAME 11 data set of 19 April 1979 is examined to determine the thermodynamic conditions prior to the onset of deep convection in western Kansas. The observations indicate that the convective region was characterized locally by substantial potential convective energy and low convective inhibition above the boundary layer.

One-dimensional modeling shows that the convection occurred where the convective inhibition was a minimum, but not where the potential convective energy was a maximum. The model results suggest that the convective inhibition above the boundary layer was almost zero when the first echoes were observed.

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Frank P. Colby Jr.

Abstract

Temperatures taken from model output (FOUS reports) routinely transmitted by the National Centers for Environmental Prediction are tabulated to determine errors during three months in the summer of 1996. These short-term model forecasts are compared to the models’ own initialization over three 1-month periods. The results suggest that the Eta Model consistently forecasted low-level temperatures that were too warm, while the Nested Grid Model consistently forecasted low-level temperatures that were slightly too cold. This paper shows some of these statistics, along with a case example in which the models made very large errors (greater than 10°C). A check of recent model output indicates that changes made recently to the Eta Model have reduced much of the bias found in the summer of 1996. A recent case example shows, however, that large short-term forecast errors still occur, even with the updated Eta Model.

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Frank P. Colby Jr.

Abstract

The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State– NCAR) Mesoscale Model (MM5) was run for seven cases from 2001 in which a sea breeze develops along the eastern New England coast. The results from model runs with 36-, 12-, and 4-km grid spacing are compared with observations as well as output from the National Centers for Environmental Prediction (NCEP) Eta Model on a 40-km grid. As expected, analysis of the individual cases shows that the higher-resolution grids are able to resolve realistic details in the flow, details that the 36-km grids and the 40-km NCEP Eta Model miss entirely. Contrary to expectations, the error statistics show that the ability to forecast specific variables at specific locations does not improve very much when using higher resolution. Even more surprisingly, the 4-km grids produced the biggest errors in forecasting surface dewpoint.

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Frank P. Colby Jr.

Abstract

Operational-model track forecasts for Tropical Storm Debby (June 2012) diverged as much as 180°, making the National Hurricane Center forecast particularly challenging. Forecast tracks from the members of the National Centers for Environmental Prediction Global Ensemble Forecast System (GEFS) were similarly divergent; some forecast Debby to turn westward, some forecast Debby to go northward, and others forecast Debby to go eastward. Part of the divergence in the operational models can be from differences in model code, but the divergence in the GEFS is due only to differences in initial conditions. To understand the meteorological reasons for this model divergence, GEFS forecasts for Debby were examined in detail. Debby formed in the south-central Gulf of Mexico and then became a tropical storm by 1200 UTC 23 June 2012. Careful examination of the GEFS tracks showed that those storms turning east were stronger and deeper than those turning west. The storm strength was measured by the mean sea level pressure at the center of the storms and the maximum 10-m wind speed. The correlation coefficients between these two quantities and the longitude 12 and 24 h later were between 0.60 and 0.90, for all GEFS model runs initialized between 0000 UTC 24 June and 1200 UTC 26 June 2012. Further investigation showed that the initial midlevel moisture (specific humidity at 850 and 700 hPa) determined the subsequent strength and depth of the member storms. This variation in depth changed the effective layer for steering currents, which determined the direction to which the storms moved.

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Keith L. Seitter and Frank P. Colby Jr.

Students in the meteorology program at the University of Lowell provide weather forecasts for the university community and for the residents of the Lowell area. The forecasting service is organized and run entirely by the students through the University of Lowell student chapter of the American Meteorological Society. Students record a forecast and discussion on a phone answering machine and deliver written forecasts to key university offices. The service recently expanded to include periodic discussions by students on the local cable-television station.

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Keith L. Seitter and Frank P. Colby Jr.

Abstract

A mesobeta-scale numerical model is described that is designed specifically for operational use on relatively small computers (supermicro-class computers of the MicroVAX 3000/4000 type). A major aspect of the model leading to improved computational efficiency is that rather than using many model layers near the surface to resolve the growth and decay of the boundary layer explicitly, the model treats the boundary layer as a single model layer of known structure whose depth can evolve during the integration. The model equations are recant in a coordinate system, referred to as boundary-layer coordinates, based on the depth of the evolving boundary layer. The model described here does not include condensation processes, but it does include a radiation parameterization, schemes governing the structure of the stable and unstable boundary layers and the transitions between these regimes, and parameterizations for the fluxes of heat and moisture between the boundary layer and the earth's surface. Simulations have been carried out with a prototype model that has five layers and 20- km grid spacing in the fine grid mesh of its nested domain. Results of these simulations show that the model is capable of reproducing such mesoscale phenomena as mountain lee waves and the Florida sea-breeze circulation fairly well.

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Keith L. Seitter, Frank P. Colby Jr., and Brian Treanor

Students at the University of Lowell provide forecasts for the community in several forms. In addition to a recorded telephone forecast that has been in operation for several years, forecasts are aired on a commercial radio station and a public-access cable television station. These new media forecasts are a result of special cooperative agreements with the stations involved that provide improved educational experiences for the students.

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