Richard T. McNider

Diurnal Boundary-Layer Development over Sloping Terrain

Abstract A one-dimensional prognostic model of the atmospheric boundary layer coupled to a surface energy budget is described which utilizes a profile formulation (non-local) for exchange coefficients in the daytime convective boundary layer and an exchange coefficient scheme based on local Richardson number for the stable nocturnal boundary layer. This combined local and non-local model is used to simulate the full diurnal cycle of the boundary-layer behavior of days 33–34 of the Wangara experiment. Results of the simulation indicate that these simplified exchange coefficient schemes simulate the mean features of the boundary layer as well as the more complex and computationally expensive higher order closure models. This diurnal boundary-layer formulation is incorporated into a two-dimensional mesoscale model employing a terrain-following coordinate system to examine boundary-layer behavior over sloping terrain similar to that of the Great Plains of the United States. Particular emphasis is placed on th...

Impacts of land use/land cover change on climate and future research priorities.

Several recommendations have been proposed for detecting land use and land cover change (LULCC) on the environment from, observed climatic records and to modeling to improve its understanding and its impacts on climate. Researchers need to detect LULCCs accurately at appropriate scales within a specified time period to better understand their impacts on climate and provide improved estimates of future climate. The US Climate Reference Network (USCRN) can be helpful in monitoring impacts of LULCC on near-surface atmospheric conditions, including temperature. The USCRN measures temperature, precipitation, solar radiation, and ground or skin temperature. It is recommended that the National Climatic Data Center (NCDC) and other climate monitoring agencies develop plans and seek funds to address any monitoring biases that are identified and for which detailed analyses have not been completed.

Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode

A 3-day period of strong, synoptic-scale stagnation, in which daytime boundary-layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5-10 m s-1 as a result of nocturnal decoupling from surface friction, which producect inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (03). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled 03 in the boundary layer as the airplane flew along. Vertical cross sections showed that 03 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the 03 hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3-D spatial distribution of 03 and the interaction between urban and rural pollution: (1) daytime buildup of 03 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak 03 concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light-wind conditions were intra-regional differences in daytime mixed-layer depth over distances of 50 km or less, and indications of an urban heat-island circulation.

Influence of diurnal and inertial boundary-layer oscillations on long-range dispersion

Abstract Coupled meteorological and Lagrangian particle models are used to examine horizontal dispersion over one to two diurnal cycles. Seven numerical experiments were run in which different portions of the atmospheric energy spectrum were included or excluded in the simulation of releases of a non-buoyant pollutant from an elevated point source. The results of the experiments indicate that vertical shear in the horizontal wind produced by diurnal and/or inertial oscillations in conjunction with or followed by vertical PBL mixing are capable of maintaining plume growth rates which are nearly linear with diffusion times up to 48 h. The model results agree reasonably well with long-range dispersion observations of an isolated smelter plume made in Australia. Periods of accelerating diffusion found in the simulations have also been indicated in observations. These accelerating diffusion periods in the simulations are due to time-dependent shear in the planetary boundary layer at night. Shear due to synoptic-scale horizontal temperature gradients was also found to have a significant impact on the modeled plume growth rates.

Surface Temperature Variations in East Africa and Possible Causes

Abstract Surface temperatures have been observed in East Africa for more than 100 yr, but heretofore have not been subject to a rigorous climate analysis. To pursue this goal monthly averages of maximum (TMax), minimum (TMin), and mean (TMean) temperatures were obtained for Kenya and Tanzania from several sources. After the data were organized into time series for specific sites (60 in Kenya and 58 in Tanzania), the series were adjusted for break points and merged into individual gridcell squares of 1.25°, 2.5°, and 5.0°. Results for the most data-rich 5° cell, which includes Nairobi, Mount Kilimanjaro, and Mount Kenya, indicate that since 1905, and even recently, the trend of TMax is not significantly different from zero. However, TMin results suggest an accelerating temperature rise. Uncertainty estimates indicate that the trend of the difference time series (TMax − TMin) is significantly less than zero for 1946–2004, the period with the highest density of observations. This trend difference continues i...

Predictability of the Stable Atmospheric Boundary Layer

Abstract The partial differential equation set for the horizontally homogeneous nocturnal boundary layer under first-order closure is discretized and truncated to a two-layer system. This system can be treated as a coupled four-layer ordinary differential equation set Using techniques of nonlinear dynamics, including numerical continuation and nonlinear stability analysis, characteristics of the solutions are developed. The bifurcation diagrams show classic S-shaped behavior so that the equations support multivalued solutions for certain values of external parameters. Both stable and unstable solution regimes exist with multiple, stable limit points. The results have strong implications for the predictability of the stable boundary layer in that even slight changes in initial conditions (or perturbations) would lead to quite different solutions in terms of temperature and wind speed for the regions of multivalued solutions. Practically, this means that predictions of frost or pollution dispersion may not ...

Toward a dynamic-thermodynamic assimilation of satellite surface temperature in numerical atmospheric models

Abstract An assimilation technique is described in which satellite-observed surface skin temperature tendencies are used in a model surface energy budget so that the predicted rate of temperature change in the model more closely agrees with the satellite observations. Both visible and infrared GOES satellite data are used in the assimilation. The technique is based on analytically recovering surface moisture from similarity expressions derived from an evapotranspiration residual obtained as a difference between the unadjusted model evapotranspiration and the satellite-inferred evapotranspiration. The technique has application in regional-scale models where surface parameters such as root zone moisture, soil moisture, etc., are unknown. It is assumed that the largest error in the surface energy budget is in the evapotranspiration term. Two tests are given for the technique, first, a one-dimensional test against FIFE data and, second, a three-dimensional test over Oklahoma. In these cases the technique appe...

Numerical simulation of slope and mountain flows

Abstract Early descriptive models of mountain-valley circulations indicated that the mountain flow (i.e., the along-valley axis component out of the valley) is a true three-dimensional phenomenon. According to these descriptions, at night shallow-down slope flows on the valley sidewalls directly driven by temperature deficits near the surface produce a pooling of cool air in the valley. This deep pool of cool air in the valley compared with a much shallower surface inversion over the plains (to which the valley opens) produces a secondary flow (the mountain flow) out of the valley driven by a deep hydrostatic pressure gradient. It is this deep secondary flow which is most important to pollutant transport in deep valleys and which has not been previously investigated in a numerical model. It is the purpose of this investigation to numerically simulate the above-mentioned secondary circulation using a three-dimensional numerical model. The Colorado State University Hydrostatic Mesoscale Model-a hydrostatic,...

Response and sensitivity of the nocturnal boundary layer over land to added longwave radiative forcing

[1] One of the most significant signals in the thermometer-observed temperature record since 1900 is the decrease in the diurnal temperature range over land, largely due to rising of the minimum temperatures. Generally, climate models have not well replicated this change in diurnal temperature range. Thus, the cause for night-time warming in the observed temperatures has been attributed to a variety of external causes. We take an alternative approach to examine the role that the internal dynamics of the stable nocturnal boundary layer (SNBL) may play in affecting the response and sensitivity of minimum temperatures to added downward longwave forcing. As indicated by previous nonlinear analyses of a truncated two-layer equation system, the SNBL can be very sensitive to changes in greenhouse gas forcing, surface roughness, heat capacity, and wind speed. A new singlecolumn model growing out of these nonlinear studies is used to examine the SNBL. Specifically, budget analyses of the model are provided that evaluate the response of the boundary layer to forcing and sensitivity to mixing formulations. Based on these model analyses, it is likely that part of the observed long-term increase in minimum temperature is reflecting a redistribution of heat by changes in turbulence and not by an accumulation of heat in the boundary layer. Because of the sensitivity of the shelter level temperature to parameters and forcing, especially to uncertain turbulence parameterization in the SNBL, there should be caution about the use of minimum temperatures as a diagnostic global warming metric in either observations or models.

Reducing Noise in the MSU Daily Lower-Tropospheric Global Temperature Dataset

Abstract The daily global-mean values of the lower-tropospheric temperature determined from microwave emissions measured by satellites are examined in terms of their signal, noise, and signal-to-noise ratio. Daily and 30-day average noise estimates are reduced by almost 50% and 35%, respectively, by analysing and adjusting (if necessary) for errors due to 1) missing data, 2) residual harmonics of the annual cycle unique to particular satellites, 3) lack of filtering, and 4) spurious trends. After adjustments, the decadal trend of the lower-tropospheric global temperature from January 1979 through February 1994 becomes −0.058°C, or about 0.03°C per decade cooler than previously calculated.