Search Results

You are looking at 1 - 10 of 20 items for

  • Author or Editor: Richard T. McNider x
  • All content x
Clear All Modify Search
Richard T. McNider

Abstract

Abstract not available.

Full access
Richard T. McNider and James J. O'Brien

Abstract

The transient development of wind-driven coastal upwelling in a rotating, stratified ocean is simulated using a four-layer numerical model. All longshore derivatives in the velocity field are neglected as are mixing processes between layers. The neglect of mixing effects necessarily limits the applicability of the model to short time scales of several days. The beta effect and longshore pressure gradients are included. The vertical structure of the nearshore upwelling zone is emphasized and offshore Ekman drift is found to be confined to the surface layer while onshore flow is evenly distributed in the lower three layers. An investigation of the baroclinic coastal current reveals an equatorward surface jet and a weaker poleward undercurrent in the β-plane solutions. The vertical structure of these coastal currents is dependent upon the stratification imposed. A pronounced tilt away from the coast with depth is observed in the coastal jet. A noticeable down-warping of the lowest interface is detected after the poleward undercurrent is established. Stream-function representation of the transverse circulation indicates propagating internal waves due to the impulsively imposed wind stress are trapped within the first 200 km of the coast.

Full access
David E. England and Richard T. McNider

Abstract

Some controversy has developed concerning the results of analytical katabatic-flow models, which appear to show that slope flows become infinite for zero slope angles and adiabatic lapse rates. It is shown that in the limits of zero slope angles and adiabatic conditions an indeterminate form is on hand, and the application of L'Hôpital's rule is in order. Application of the rule results in limiting cases that agree with physical expectations and the original slope-flow model evaluated for a zero slope or adiabatic lapse rate.

Full access
Richard T. McNider and Roger A. Pielke

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, primitive equation model, forced by a surface energy budget-was utilized for the simulations. Both idealized topography and an actual Colorado valley were used in the investigation to emulate the classic mountain-plain configuration discussed above. Special attention was given to the development of the sidewall slope flows which produce the pooling of air in the valley. Results of the simulations indicated shallow slope flows less than 100 m deep started almost immediately after cooling began and reached their maximum velocity after approximately one hour and then slowly decreased through the night. A high level of turbulence was noted near the top of the slope flows. The turbulence was not only due to shear in the slope flow, but to destabilization of the temperature profile because of thermal advection. Definite pooling of cool air occurred in the valley with cooling occurring at all heights to near ridge tops. The secondary flow out of the valley lagged the development of the slope flows by approximately one hour, but was very deep, filling the whole valley to near ridge height.

Full access
Richard T. McNider and Roger A. Pielke

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 the generation of mesoscale pressure gradients in response to the heated and cooled terrain and the development of low-level wind maxima. Model results and an analysis of thermal wind relationships in the transformed equations indicate the development of a mesoscale thermal wind component to the south during the day throughout the convective boundary layer. At night, the thermal wind component is reversed within the nocturnal boundary layer. The mesoscale thermal wind components in conjunction with the frictional stress profile produce low-level jets in the velocity field both day and night which exceed the synoptic geostrophic speeds.

Full access
Richard T. McNider and Fred J. Kopp

Abstract

Boundary layer similarity techniques are employed to specify the scale and intensity of a thermal perturbation used to initialize a cloud in a numerical cloud model. Techniques are outlined to specify the needed similarity variables from external information. Finally, the cloud model response using the similarity scaled thermal is analyzed employing variations in the similarity variables giving an indication of the importance of the correct specification of the initiating thermal.

Full access
John R. Christy and Richard T. McNider

Abstract

Three time series of average summer [June–August (JJA)] daily maximum temperature (TMax) are developed for three interior regions of Alabama from stations with varying periods of record and unknown inhomogeneities. The time frame is 1883–2014. Inhomogeneities for each station’s time series are determined from pairwise comparisons with no use of station metadata other than location. The time series for the three adjoining regions are constructed separately and are then combined as a whole assuming trends over 132 yr will have little spatial variation either intraregionally or interregionally for these spatial scales. Varying the parameters of the construction methodology creates 333 time series with a central trend value based on the largest group of stations of −0.07°C decade−1 with a best-guess estimate of measurement uncertainty from −0.12° to −0.02°C decade−1. This best-guess result is insignificantly different (0.01°C decade−1) from a similar regional calculation using NOAA’s divisional dataset based on daily data from the Global Historical Climatology Network (nClimDiv) beginning in 1895. Summer TMax is a better proxy, when compared with daily minimum temperature and thus daily average temperature, for the deeper tropospheric temperature (where the enhanced greenhouse signal is maximized) as a result of afternoon convective mixing. Thus, TMax more closely represents a critical climate parameter: atmospheric heat content. Comparison between JJA TMax and deep tropospheric temperature anomalies indicates modest agreement (r 2 = 0.51) for interior Alabama while agreement for the conterminous United States as given by TMax from the nClimDiv dataset is much better (r 2 = 0.86). Seventy-seven CMIP5 climate model runs are examined for Alabama and indicate no skill at replicating long-term temperature and precipitation changes since 1895.

Full access
John R. Christy, William B. Norris, and Richard T. McNider

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 (T Max), minimum (T Min), and mean (T Mean) 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 T Max is not significantly different from zero. However, T Min results suggest an accelerating temperature rise.

Uncertainty estimates indicate that the trend of the difference time series (T MaxT Min) is significantly less than zero for 1946–2004, the period with the highest density of observations. This trend difference continues in the most recent period (1979–2004), in contrast with findings in recent periods for global datasets, which generally have sparse coverage of East Africa.

The differences between T Max and T Min trends, especially recently, may reflect a response to complex changes in the boundary layer dynamics; T Max represents the significantly greater daytime vertical connection to the deep atmosphere, whereas T Min often represents only a shallow layer whose temperature is more dependent on the turbulent state than on the temperature aloft.

Because the turbulent state in the stable boundary layer is highly dependent on local land use and perhaps locally produced aerosols, the significant human development of the surface may be responsible for the rising T Min while having little impact on T Max in East Africa. This indicates that time series of T Max and T Min should become separate variables in the study of long-term changes.

Full access
John R. Christy, Roy W. Spencer, and Richard T. McNider

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.

Full access
Andrew T. White, Arastoo Pour-Biazar, Kevin Doty, Bright Dornblaser, and Richard T. McNider

Abstract

Development of clouds in space and time within numerical meteorological models as observed in nature is essential for producing an accurate representation of the physical atmosphere for input into air quality models. In this study, a new technique was developed to assimilate Geostationary Operational Environmental Satellite (GOES)-derived cloud fields into the Weather Research and Forecasting (WRF) meteorological model to improve the placement of clouds in space and time within the model. The simulations were performed on 36-, 12-, and 4-km grid-size domains covering the contiguous United States, the south-southeastern United States, and eastern Texas, respectively. The technique was tested over the month of August 2006. The results indicate that the assimilation technique significantly improves the agreement between the model-predicted and GOES-derived cloud fields. The daily average percentage increase in the cloud agreement was determined to be 14.02%, 11.29%, and 4.96% for the 36-, 12-, and 4-km domains, respectively. This was accomplished without degrading the model performance with respect to surface wind speed, temperature, and mixing ratio, which are important parameters for air quality applications; in some cases these variables were even slightly improved. The assimilation technique also produced improvements in the model-predicted precipitation and predicted downwelling shortwave radiation reaching the surface.

Full access