Thomas B. McKee

Scattering of Visible Radiation by Finite Clouds

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-...

Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997

Abstract On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU—CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses. Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; w...

Deep Stable Layers in the Intermountain Western United States

Abstract A deep stable layer (DSL) is a layer much deeper than a typical nocturnal inversion with stabilities not frequently found over a sizable portion of the lowest 1.5 km. They have traits that can cause the stagnation of cold air in basins, i.e., light winds at the surface even if moderately strong winds aloft are present, and the restriction of the growth of daytime convective boundary layers. The objective definition used in this study is that, if 65% of the lowest 1.5 km of the 1200 UTC [0500 mountain standard time (MST)] sounding has a lapse rate of 2.5°C km−1 or less, then the day is under the influence of a DSL. A climatology of days under the influence of a DSL was performed at four sites in the intermountain western United States: Grand Junction, Colorado; Salt Lake City, Utah; Winnemucca, Nevada; and Boise, Idaho. The DSL is a wintertime phenomenon with 10% to 20% of the days in December and January at the four stations being under the influence of a DSL. Successive days with a DSL present l...

Annual Snowpack Patterns across the Rockies: Long-Term Trends and Associated 500-mb Synoptic Patterns

Abstract Winter snowpack was investigated to determine spatial and temporal climate variability in a five-state region (Colorado, Idaho, Montana, Utah, and Wyoming) in the northern Rocky Mountains, covering the period 1951–85. Annual 1 April snowpack (SN) measurements were selected for analyses. Three basic and persistent patterns of annual SN values surfaced: years with a consistent anomaly over the entire region (wet or dry); years with a distinct north-to-south gradient; and average years. Nearly 90% of the nonaverage annual SN patterns were explained by the frequency of seven 500-mb winter synoptic patterns. The wet-north-dry-south gradient SN patterns occurred only before 1974, and the dry-north-wet-south gradient SN patterns did not occur before 1973. The long-term wet and dry periods experienced in the northern and southern areas of the five-state region are due to periods when one of the two north-to-south gradient SN patterns occurred and are explained by the changes in the frequency of synoptic ...

The Role of Valley Geometry and Energy Budget in the Formation of Nocturnal Valley Winds

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 incl...

Effects of Shear, Stability and Valley Characteristics on the Destruction of Temperature Inversions

Abstract A dry two-dimensional version of the Colorado State Cloud/Mesoscale Model was used to study the morning, inversion destruction cycle in a variety of deep mountain valley configurations. Eleven simulations were run to examine the effects of valley width, surface heating rate, wind shear above the valley, valley orientation, sidewall slope, initial stability and variable surface albedo on the evolution of the daytime boundary layer in the valley. Each was initiated with a stable layer filling the valley to ridgetop with a neutral layer above the ridge. The model was driven at the lower surface by a sinusoidally varying potential temperature flux which approximates the diurnal heating cycle. All simulations show that the initial inversion layer is destroyed by a combination of three processes; a growing surface based neutral layer over the valley floor, the destabilization of the stable air mass by the recirculation of air warmed over the slopes and the descent of the inversion top by the transport ...

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

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 nig...

Dynamical Model Simulation of the Morning Boundary Layer Development in Deep Mountain Valleys

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 w...

Simulated Radiance Patterns for Finite Cubic Clouds

Abstract Calculated distributions of scattered shortwave radiance are presented for simulated cumulus clouds using a cubic shape. Comparison with similar clouds of semi-infinite horizontal extent is included. For an incident solar zenith angle of 0° the angular distribution of the radiance exiting the cloud top is similar for the cube and the semi-infinite layer, but the radiance from the cube is much smaller for optical depths between 9.8 and 73.5. At an optical depth of 73.5 the vertical radiance from the cube is only 58% of the radiance from the semi-infinite layer cloud. For an incident solar zenith angle of 60°, the angular distribution and the magnitudes of the scattered radiances are similar for the cube top and the semi-infinite layer. A comparison of the total radiance from the cube top and side in the solar plane shows a dramatic change in angular distribution compared with the semi-infinite cloud. Radiances exiting the antisolar side of the cube illustrate the strong forward scattering for shor...

The Interaction of Katabatic Flow and Mountain Waves. Part II: Case Study Analysis and Conceptual Model

Via numerical analysis of detailed simulations of an early September 1993 case night, the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves (MKI). A companion paper (Part I) describes the synoptic and mesoscale observations of the case night from the Atmospheric Studies in Complex Terrain (ASCOT) experiment and idealized numerical simulations that manifest components of the conceptual model of MKI presented herein. The reader is also referred to Part I for detailed scientific background and motivation. The interaction of these phenomena is complicated and nonlinear since the amplitude, wavelength, and vertical structure of the mountain-wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop. Simultaneously, katabatic flows are impacted by the topographically induced gravity wave evolution, which may include significantly changing wavelength, amplitude, flow magnitude, and wave breaking behavior. In addition to effects caused by turbulence (including scouring), perturbations to the leeside gravity wave structure at altitudes physically distant from the surface-based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column. The simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow. Variability is found to occur on two scales, on the meso- due to evolution of the mountain-wave system on the order of one hour, and on the microscale due to rapid wave evolution (short wavelength) and wave breaking–induced fluctuations. It is proposed that the MKI mechanism explains a portion of the variability in observational records of katabatic flow.