The Mountain-Plains Circulation East of a 2-km-High North–South Barrier

Paul G. Wolyn Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Thomas B. Mckee Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

The daytime mountain-plains circulation east of a 2-km-high and 60-km-wide barrier is examined for conditions of clear skies, light ambient winds with a westerly component around 5 m s−1, and little spatial and temporal change to the synoptic-scale thermal fields and wind fields. Fourteen nonhydrostatic, two-dimensional, horizontally homogeneously initialized simulations, employing the Colorado State University Regional Atmospheric Modeling System, are used to study the important physical processes in the daytime evolution. A synthesis of simulations with various initial conditions and boundary conditions are used to derive a conceptual model of the daytime evolution. The simulations am run for different times of the year, different patterns of soil moisture (which affects the surface sensible heat flux), different ambient winds, different thermal structures, half-barrier height, and absence of a nighttime phase. Except for the simulation without a nighttime phase, the simulations have a full nighttime phase before the daytime evolution is studied. Observations, consisting of frequent (every 2–3 h) airsonde launches from sunrise until the afternoon in the vicinity of Fort Collins, Colorado, are used to gauge how well the simulations match the daytime evolution. The simulations and observations qualitatively agree well, showing that the simulations satisfactorily re-create the daytime evolution.

The variety of simulations and observations show a complex sunrise state that is not close to horizontally homogeneous. The sunrise state has a complex interaction between the thermally driven nocturnal flows and the ambient flow. Three distinct phases appear in the daytime evolution. Phase 1 results from the weakening nocturnal flows interacting with the daytime heating, and it lasts until 3–4 h after sunrise. Phase 2 is characterized by a developing solenoid. The solenoid is not horizontally or vertically symmetric, and it has two stages of development. Phase 2 lasts until at least 7 h after sunrise, and it can exist until sunset. The main feature in phase 3 is a migrating solenoid moving beneath the leading edge of the cold core. This phase exists from the end of phase 2 until near sunset, and this phase does not exist on all days. The migrating solenoid is a disturbance (which can significantly influence the atmosphere east of the barter) in the main daytime circulation.

The simulations generally show that for phases 2 and 3 the circulation is weaker and shallower for moister soil on the eastern plains (less surface sensible heat flux), moister soil west of the barrier crest, days closer to the winter solstice, stronger ambient winds, and lower convective boundary layer (CBL) the previous day. The circulation is generally deeper and stronger for less stability (after 5 h after sunrise) and for times closer to the solstice, especially by 5 h after sunrise. The CBL on the eastern plains is shallower for moister soil on the eastern plains, days closer to the winter solstice, stronger ambient winds, and lower CBL the previous day (after the solenoid passes). The proper boundary and initial conditions are needed to accurately simulate the daytime evolution. Inclusion of the nighttime phase is important to properly replicate the daytime evolution, especially the sunrise state and phase 1. The evolution east of a 1-km-high barrier is different from an evolution east of a 2-km-high barrier. In a simulation without ambient winds, the sunrise state is significantly different from the simulations with ambient westerly flow with phases 1 and 3 being absent.

Abstract

The daytime mountain-plains circulation east of a 2-km-high and 60-km-wide barrier is examined for conditions of clear skies, light ambient winds with a westerly component around 5 m s−1, and little spatial and temporal change to the synoptic-scale thermal fields and wind fields. Fourteen nonhydrostatic, two-dimensional, horizontally homogeneously initialized simulations, employing the Colorado State University Regional Atmospheric Modeling System, are used to study the important physical processes in the daytime evolution. A synthesis of simulations with various initial conditions and boundary conditions are used to derive a conceptual model of the daytime evolution. The simulations am run for different times of the year, different patterns of soil moisture (which affects the surface sensible heat flux), different ambient winds, different thermal structures, half-barrier height, and absence of a nighttime phase. Except for the simulation without a nighttime phase, the simulations have a full nighttime phase before the daytime evolution is studied. Observations, consisting of frequent (every 2–3 h) airsonde launches from sunrise until the afternoon in the vicinity of Fort Collins, Colorado, are used to gauge how well the simulations match the daytime evolution. The simulations and observations qualitatively agree well, showing that the simulations satisfactorily re-create the daytime evolution.

The variety of simulations and observations show a complex sunrise state that is not close to horizontally homogeneous. The sunrise state has a complex interaction between the thermally driven nocturnal flows and the ambient flow. Three distinct phases appear in the daytime evolution. Phase 1 results from the weakening nocturnal flows interacting with the daytime heating, and it lasts until 3–4 h after sunrise. Phase 2 is characterized by a developing solenoid. The solenoid is not horizontally or vertically symmetric, and it has two stages of development. Phase 2 lasts until at least 7 h after sunrise, and it can exist until sunset. The main feature in phase 3 is a migrating solenoid moving beneath the leading edge of the cold core. This phase exists from the end of phase 2 until near sunset, and this phase does not exist on all days. The migrating solenoid is a disturbance (which can significantly influence the atmosphere east of the barter) in the main daytime circulation.

The simulations generally show that for phases 2 and 3 the circulation is weaker and shallower for moister soil on the eastern plains (less surface sensible heat flux), moister soil west of the barrier crest, days closer to the winter solstice, stronger ambient winds, and lower convective boundary layer (CBL) the previous day. The circulation is generally deeper and stronger for less stability (after 5 h after sunrise) and for times closer to the solstice, especially by 5 h after sunrise. The CBL on the eastern plains is shallower for moister soil on the eastern plains, days closer to the winter solstice, stronger ambient winds, and lower CBL the previous day (after the solenoid passes). The proper boundary and initial conditions are needed to accurately simulate the daytime evolution. Inclusion of the nighttime phase is important to properly replicate the daytime evolution, especially the sunrise state and phase 1. The evolution east of a 1-km-high barrier is different from an evolution east of a 2-km-high barrier. In a simulation without ambient winds, the sunrise state is significantly different from the simulations with ambient westerly flow with phases 1 and 3 being absent.

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