Paul H. Ruscher

Numerical Models of Boundary Layer Processes over and around the Gulf of Mexico during a Return-Flow Event

Abstract The return-flow of low-level air from the Gulf of Mexico over the southeast United States during the cool season is studied using numerical models. The key models are a newly developed airmass transformation (AMT) model and a one-dimensional planetary boundary layer (PBL) model. Both are employed to examine the thermodynamic structure over and to the north of the Gulf. Model errors for predicting minimum, maximum, and dewpoint temperatures at the surface during both offshore and onshore phases of the return-flow cycle are analyzed. PBL model forecasts indicate soil moisture values obtained from the Eta Model improve accuracy. It is shown that forecasts of maximum temperature for coastal locations are sensitive to the soil moisture used in the PBL model. The AMT model performs well in determining boundary layer parameters since it includes horizontal advective processes. The AMT model is also able to predict the regional differences caused by different surface forcing while passing over land or se...

Tallahassee, Florida, Minimum Temperature Anomaly: Description and Speculations

The Tallahassee Regional Airport, on the citys southwest side, has a minimum temperature that is colder than minima of neighboring towns and other parts of Tallahassee when clear and calm conditions prevail over the area. The anomaly is described in detail with a series of comparison studies using daily minimum temperature data from nearby locations. This cold anomaly is statistically significant and is most common during the cold season. Its cause appears to be anomalous radiational cooling resulting from enhanced exposure of the ground to open sky, and local cooling rates are probably intensified by sandy soil conditions. Cold-air drainage does not appear to be an important factor.

Physical Mechanism for the Tallahassee, Florida, Minimum Temperature Anomaly

Abstract Nighttime minimum temperatures at the Tallahassee Regional Airport (TLH) are colder in comparison with surrounding locations and other parts of the city, especially during the cool season (TLH minimum temperature anomaly). These cold events are examined using the one-dimensional Oregon State University atmospheric boundary layer (ABL) model including a two-layer model of soil hydrology. The model is used for 12-h forecasts of the ABL parameters, such as surface fluxes, surface inversion height, and minimum temperature when clear, calm synoptic conditions existed over the region at night. The minimum temperature forecasts are performed at TLH and a nearby location. Cooling in the surface inversion layer is examined in terms of turbulence and clear-air radiative effects, and it is confirmed that the lower temperatures at TLH are related to the clear-air radiative cooling even in the lower part of the inversion layer but not to cold-air drainage. Stability, ABL height, and surface inversion height a...

Forecast errors in the Florida State University atmospheric boundary layer model (FSU1DPBL). Part I: Thermal advection effects

Short range weather forecasting and atmospheric boundary layer (ABL) processes are studied by using a one-dimensional ABL model where the horizontal advection terms are neglected. The ability of the model, the Florida State University atmospheric boundary layer (FSU1DPBL) model, to forecast minimum temperature is investigated in terms of thermal advection effects. The model is used for 12-, 24-, and 36-h forecasts of minimum temperatures. The minimum temperatures obtained from the model forecasts are compared with observations. The investigation is confined to days meeting criteria representative of clear and calm synoptic conditions. Errors between observations and model results are determined using thermal advection analysis in the boundary layer since horizontal thermal advection affects cooling rates at night. The errors are found to be strongly correlated with thermal advection in the lowest part of the ABL. By carrying out some case studies, it is seen that the model is more prone to errors in dry conditions. It is also confirmed that the strong thermal advection aloft getting closer and closer to the ground largely explains the difference in minimum temperatures between the model and observations. The results show that the model is also a useful tool for the short-range weather forecasting since it is not computationally expensive. The model is able to simulate well the nighttime minimum temperatures, and can be used operationally.

Forecast errors in the Florida State University atmospheric boundary layer model (FSU1DPBL). Part II: Statistical investigation

The performance of Florida State University atmospheric boundary layer model (FSU1DPBL) in predicting the daily maximum temperature drop is investigated with respect to sensible and latent heat fluxes. Turbulent exchanges in the surface boundary layer are taken into account in the model. Nocturnal atmospheric boundary layer heights obtained from the FSU1DPBL model are compared with those obtained from other model formulations, and stability conditions of the ABL are discussed to derive information concerning the prediction of minimum temperature. It is explained that a growth in the ABL at night can occur under cloudy and/or windy conditions in the model, causing a bias as high as 5°C in predicting the daily minimum temperature. The usual similarity theory for the stable boundary layer leads to a significant overestimation of surface cooling in the model. From the model perspective, the success of the 12-h model forecast in predicting minimum temperature compared to that of the 24-h and 36-h forecasts is explained. The results show the prediction of nocturnal ABL height under the extremely stable and moderately stable cases by the model yields acceptable values in comparison with other model formulations. The ABL height from the model can be replaced by an interpolation formula to yield more realistic values under very stable conditions at night.