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Thomas B. McKee

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David C. Bader
and
Thomas B. Mckee

Abstract

A dry, two-dimensional version of the Colorado State University Multi-dimensional Cloud/Mesoscale Model was used to study the cross-valley evolution of the wind and temperature structures in an idealized east-west oriented mountain valley. Two simulations were performed, one in which the valley was heated symmetrically and a second in which a mid-latitude heating distribution was imposed. Both runs were initiated identically with a stable layer filling the valley to ridgetop and a neutral layer above the ridge. A specified sinusoidal surface potential temperature flux function approximating the diurnal cycle forced the model at the lower boundary.

The results of the two simulations were remarkably similar. The model realistically reproduced the gross features found in actual valleys in both structure and timing. The simulated inversions were destroyed three and one-half hours after sunrise as a result of a neutral layer growing up from the surface meeting a descending inversion top. Slope winds with speeds of 3–5 m s−1 developed over both sidewalls two and one-half hours after sunrise. Both cases revealed the development of strongly stable pockets of air over the sidewalls which form when cold air advected upslope loses its buoyancy at higher elevations. These stable pockets temporarily block the slope flow and force transient cross-valley circulations to form which act to destabilize the valley boundary layer. Cross-valley mixing and gravity waves rapidly redistribute heat across the valley to prevent large potential temperature gradients from forming. As a result, oven large differences in heating rates between opposing sidewalls do not result in significant cross-valley potential temperature differences. Organized cross-valley circulations and eddy motions enhance lateral mixing in the stable layer as well.

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Thomas B. McKee
and
Stephen K. Cox

Abstract

A theoretical model of the scattering of shortwave radiation is applied to clouds finite in horizontal extent. The resulting irradiance patterns are then compared with calculations for horizontally semi-infinite clouds. This analysis shows, that the irradiance fields are dramatically dependent upon energy passing through the vertical sides of the finite sized clouds.

Directional reflectance of individual cubic clouds is shown to be approximately 25% less than for semi-infinite clouds of optical depths ranging from 20 to 80. Directional reflectance from the top of cubic clouds for small solar zenith angle continues to increase at large optical depths (∼70) while the infinite cloud becomes nearly asymptotic at this point. It is shown that for a solar zenith angle of 60°, the directional reflectance for a 2/10 sky cover of cubic clouds is 0.29 while for 2/10 coverage of semi-infinite cloud the directional reflectance is 0.185.

Implications of differences between the cubic cloud results and the semi-infinite cloud case are discussed. These implications include: the effect on calculated planetary albedo; a possible explanation for reported correlations of cloud brightness, cloud height and precipitation; and effects on the surface energy budget.

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Thomas B. McKee
and
Robert D. O'Neal

Abstract

Diurnally varying up and down-valley winds are a commonly observed feature of mountain meteorology. These winds are produced through the heating and cooling of the land surface but direct connections from the topography to the winds have been difficult to establish. A concept has been proposed which theoretically relates the energy budget and valley geometry to the rate of atmospheric cooling in the valley. The gradient of the along-valley cooling rate will then lead to an along-valley pressure gradient which provides a topographic control of the wind. The ratio of valley width to cross-section area is shown to be the critical topographic parameter which is proportional to the valley cooling rate. Net radiation and the ground heat flux are also critical to the valley cooling rate. An example is given which illustrates that this new concept can produce pressure gradients about 60% larger than the mountain-plain mechanism. Observations of wind and temperature in three valleys in Colorado which include draining and pooling valleys are consistent with the concept.

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Paul G. Wolyn
and
Thomas B. Mckee

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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C. David Whiteman
and
Thomas B. McKee

Abstract

A thermodynamic model is developed to simulate the evolution of vertical temperature structure during the breakup of nocturnal temperature inversions in mountain valleys. The primary inputs to the model are the valley floor width, sidewall inclination angles, characteristics of the valley inversion at sunrise, and an estimate of sensible heat flux obtained from solar radiation calculations. The outputs, obtained by a numerical integration of the model equations, are the time-dependent height of a convective boundary layer that grows upward from the valley floor after sunrise, the height of the inversion top, and vertical potential temperature profiles of the valley atmosphere. The model can simulate the three patterns of temperature structure evolution observed in deep valleys of western Colorado. The well-known inversion breakup over flat terrain is a special case of the model, for which valley floor width becomes infinite. The characteristics of the model equations are investigated for several limiting conditions using the topography of a reference valley and typical inversion and solar radiation characteristics. The model is applied to simulate observations of inversion breakup taken in Colorado's Eagle and Yampa Valleys in different seasons. Simulations are obtained by fitting two constants in the model, relating to the surface energy budget and energy partitioning, to the data. The model accurately simulates the evolution of vertical potential temperature profiles and predicts the time of inversion destruction.

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Georg J. Mayr
and
Thomas B. McKee

Abstract

The evolution of low-level flow upstream of the Continental Divide (Rocky Mountains) and the Wasatch Range from being unable to surmount the mountain range, to becoming unblocked and blocked again is studied observationally. During two months in the winter of 1991/92, a transect of three wind profilers measured the wind field every few minutes with unprecedented temporal detail.

The average state of that region during winter is blocked. A total of 47 blocked events were observed. A blocked flow event lasted on the average one and a half days, but the duration varied widely from a few hours to eight days controlled by the synoptic situation. The transition between the two states happened rapidly on the order of 1 h with a minimum of 20 min and a maximum of 4 h. The depth of the blocked layer during one blocking episode fluctuated considerably but reached on the average one-half to two-thirds of the barrier depth (depending on the location).

Previous research of idealized equilibrium situations focused on changes of the cross-barrier wind speed and stability as determining variables to build a mesoscale high over the barrier. Since their values were in the blocked range, other mechanisms had to trigger the transitions to an unblocked state.

A conceptual model proposes synoptic and radiative forcing to drive the blocking evolution. When the mountain-induced mesoscale high blocks the low-level flow, an opposing synoptic cross-barrier pressure gradient can negate the mesoscale high. Therefore unblocking happens most frequently when the trough axis of a short wave is immediately upstream of the harder, but synoptic pressure gradients caused by contrasts in vorticity and differential temperature advection are sometimes also strong enough. The flow returns to its blocked state when the ridge behind the trough approaches the barrier so that the synoptic cross-barrier pressure gradient reinforces the mesoscale high.

For a lower barrier or stronger solar insulation, a well-mixed boundary layer can grow almost to the height of the barrier by afternoon and reconnect the blocked layer with the higher cross-barrier winds above the mountain. After sunset the thermal forcing changes sign as the radiative cooling stabilizes the lower atmosphere again and the transition back to the blocked state occurs.

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David C. Bader
and
Thomas B. McKee

Abstract

The development of the nocturnal boundary layer (NBL) over a sloping plateau upwind of a high mountain barrier is studied with a numerical model and field observations. Six numerical simulations and one observed case are used to describe the effects of wind speed, wind direction, and sunset mixed-layer depth on the NBL structure 6 h after sunset. When there is a component of wind into barrier, a two-layer structure develops. A 75-175-m-deep inversion layer that is topped by a 200-300-m-deep, less stable transition layer extends over the length of the plateau. Shear between the 3–4 m s−1 drainage winds in the inversion layer and the large-scale wind mix cold air vertically to build the transition layer. The inversion layer appears to be relatively insensitive to changes in the external parameters, but transition-layer depth is proportional to wind speed.

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David M. Ebel
and
Thomas B. McKee

Abstract

One of the important radiative effects of cloud Shape is to modify diurnal radiance patterns observed from satellites. Theory predicts a diurnal radiance pattern nearly symmetric about local noon for both semi-infinite and finite clouds situated on the equator at the equinox with a satellite directly overhead. For a geostationary satellite (SMS-1) located to the west of the cloud, the semi-infinite cloud still products a pattern nearly symmetric about local noon while finite cubic clouds produce a distinctly different pattern which peaks during the afternoon. Simulated diurnal satellite observations of a finite cubic and semi-infinite cloud were compared with actual diurnal satellite observations of cloud fields with cloud cover varying from less than 30% to greater than 90%. The results for 4 n mi resolution data from the two observed cloud fields demonstrate that the diurnal radiance patterns of both semi-infinite and finite clouds exist in satellite observations. Degrading the resolution to 16 n mi did not significantly alter the semi-infinite or finite diurnal radiance pattern of either observed cloud field. The effects of cloud shape on satellite observations have potential application to problems in data interpretation, cloud cover determination, albedo calculations and identification of cloud fields.

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Thomas B. McKee
and
John T. Klehr

Abstract

Calculations are presented which compare the effects on directional reflectance and relative radiance of changes in microphysical structure and geometric shape for scattered solar radiation in terrestrial water clouds. The effect of changes in microphysical structure was relatively small and was 5.5% in directional reflectance for cloud optical depth of 60 and solar zenith of 23.0°. In contrast, effects of changes in geometric shape from a semi-infinite layer to a cube of optical depth 60 were 35%. Narrow turrets growing above a base cloud are shown to be darker than the base cloud with a reduction in relative radiance of 34.5% for a vertical sun. Directional reflectances for turret clouds are smaller than for clouds with flat tops due to the presence of additional edges. It appears that the interactions between the turret and the base cloud represent a small effect compared to the addition of more edges and surface area of the turret. Cumulus clouds in nature contain many more surface features which, although less sharp than in this model, undoubtedly contribute to a reduced directional reflectance.

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