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Prototype real-time boundary layer prediction in support of the CASES-99 nocturnal boundary layer experiment




Schuster, Douglas C., author

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In September 1999, a modified version of the Regional Atmospheric Modeling System (RAMS), version 4.29, was implemented to provide 48 hour forecasts initialized at 12z in support of the CASES-99 nocturnal boundary layer experiment centered at Leon, KA. The research objective, through use of a forecast model, was to predict the structure of the nocturnal boundary layer, including temperature, relative humidity, and position/direction of the low level jet to provide guidance for field operations, and study the model's effectiveness in predicting such variables. In this effort the RAMS model was run throughout October 1999 over the study area. Additional runs were performed off-line after the CASES-99 project to examine how the model performed under varied configurations. Configuration 1, run at the time of the field project, implemented the Mahrer/Pielke radiation scheme at 35% homogeneous volumetric soil moisture (MP-35). Configuration 2 implemented the Harrington radiation scheme at 35% soil moisture (HAR-35), and configuration 3 implemented Harrington radiation at 25% soil moisture (HAR-25). Through investigation of two representative study days, one in which the model resolved the formation of a low level jet {LLJ) that developed high shear zones near its boundaries, RAMS demonstrated its general functionality as a high resolution forecast tool in all configurations when compared to observations. Specific results from the individual forecast studies displayed a tendency for all model configurations to produce inversion layers 800m to 1000m above ground level (AGL) that did not exist in the observations, or several hundred meters below observed inversion levels at initial nighttime forecast hours. As the model forecast progressed through the night, the fictitious, or lower than observed inversion layer tended to wash out causing the temperature and RH structure to match up well with observations taken around 122. In IOP#9, it appears that the erroneously predicted inversion may coincide with fictitious directional wind shear displayed in model winds near 800m AGL. Near the surface, southerly flow strengthened through the night in association with LLJ development at 80m to 100m AGL. Observations and model predicted winds displayed a similar pattern of directional shear with height strengthening through the nighttime hours. Additionally, near the surface, a warm model temperature bias existed as the near-surface temperature inversion developed through the night. MP-35 and HAR-25 produced the warmest temperature readings, while HAR-35 produced the coolest temperatures and highest RH values in the near surface inversion layer. In all cases, temperatures were 1 to 3 degrees warmer than observed, with RH readings up to 20% RH less than observations within 150m of the surface. In addition to the individual forecast analysis, a comprehensive forecast analysis of several CASES-99 Intensive Operational Periods (IOP's) compares model and observed values at 4 sounding sites and a central fixed 55m tower site. Wind fields displayed little change between configurations, while temperature and relative humidity fields varied with respect to configuration. Wind fields for all configurations were generally within 4 m/s in magnitude of observed values. HAR-35 produced results closest in magnitude to observed values of temperature and RH through the nighttime hours of forecast periods. Additionally, HAR-35 produced the coolest temperatures and highest RH values of all three configurations, especially within 200m of the surface.


February 5, 2002.
Also issued as author's thesis (M.S.) -- Colorado State University, 2001.

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Boundary layer (Meteorology) -- Diurnal variations


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