Search Results

You are looking at 1 - 10 of 45 items for

  • Author or Editor: William Collins x
  • Refine by Access: All Content x
Clear All Modify Search
William G. Collins

Abstract

The adjustment properties of numerical models are important both from the Point of view of the handling of noise and the ability to produce proper quasi-geostrophic states from physically derived imbalances between mass and motion fields. This paper considers the adjustment properties of simplified barotropic one-dimensional numerical models with explicit and semi-implicit time differencing.

The classical Rossby adjustment problem provides the framework for comparison with the numerical results. In addition, linear analysis of the models provides useful insight. Primary emphasis is on the time required for adequate adjustment and the accuracy of the adjusted state. The adjustment time increases with the scale of the initial disturbance, and does not differ significantly between the explicit and semi-implicit models. The grid increment is one of the dominant factors in determining the adjustment time. Both models accurately calculated the final adjusted state.

Full access
William D. Collins

Abstract

The effects of enhanced shortwave absorption on coupled simulations of the tropical climate have been tested using the National Center for Atmospheric Research Climate System Model. The enhancement in cloudy-sky shortwave absorption is consistent with several recent intercomparisons of models and observations. The primary reasons for introducing enhanced absorption are to improve the fidelity of the modeled shortwave fluxes in comparison with observations, to improve the simulation of sea surface temperature in the tropical Pacific, and to reduce or eliminate transient behavior in the model associated with compensating errors between latent heat fluxes and surface insolation. The simulations incorporate a new method to increase shortwave absorption in cloudy atmospheres. In previous studies, adjustments to cloud optical properties or modifications to the shortwave radiative transfer were used to increase shortwave cloud absorption. In the new “generic” implementation, the in-cloud shortwave flux divergences have been adjusted to yield the observed global surface insolation while leaving the top-of-atmosphere fluxes fixed. The simulations with enhanced absorption show that several aspects of the atmospheric state and ocean–atmosphere fluxes are much closer to observational estimates. All the terms in the surface and top-of-atmosphere heat budget of the western Pacific warm pool are within 4 W m−2 of the measured values. The latent heat fluxes are within 10–15 W m−2 of estimates from buoy data over most of the tropical Pacific. Without absorption, the model consistently overestimates the latent heat fluxes. The differences between the observed and simulated sea surface temperatures in the equatorial Pacific are reduced from 2 K to less than 1 K with the introduction of enhanced absorption.

Full access
William G. Collins

Abstract

The method of complex quality control of radiosonde heights and temperatures (CQCHT) has been under continuous development and improvement at the National Centers for Environmental Prediction since 1988. Part I of this paper gives the background for the method and details for the currently operational version of the code, which contains significant improvements over previous versions. Part II shows a number of interesting examples of operation of the algorithm and gives statistics on its performance during the first year of operation, September 1997 through August 1998. In a few examples, it is seen how even complicated errors may be corrected. The statistics show that of the 5700 hydrostatically detected errors each month, 77% were corrected. There is a great variation in the geographical distribution of errors, but it is found that a majority of all stations have at least one hydrostatically suspected error during a month’s time. In addition to hydrostatically detected errors, the CQCHT detects almost 16 000 so-called observation errors in height and temperature each month.

Full access
William G. Collins

Abstract

The quality control of meteorological data has always been an important, if not always fully appreciated, step in the use of the data for analysis and forecasting. In most quality-control approaches, erroneous data are treated as nonrandom “outliers” to the data distribution, which must be eliminated. The elimination of such data traditionally proceeds from coarse to finer filters. More recent methods use the fit (or lack of fit) of such data to an analysis, excluding the data, to determine whether data are acceptable. The complex quality-control (CQC) approach, on the other hand, recognizes that most rough errors are caused by human error and can likely be corrected. In the CQC approach, several independent checks are made that provide numerical measures of any error magnitude. It is only after all check magnitudes, called residuals, are calculated that data quality is determined and errors are corrected when possible. The data-quality assessment and correction is made by the sophisticated logic of the decision-making algorithm (DMA). The principles and development of the method of CQC for radiosonde data were given by Gandin. The development of CQC at the National Centers for Environmental Protection (NCEP) for the detection and correction of errors in radiosonde heights and temperatures, called the complex quality control for heights and temperatures (CQCHT), has progressed from the use of a complex of hydrostatic checks only to the use of statistical and other checks as well, thereby becoming progressively sophisticated. This paper describes a major restructuring in the use of the radiosonde data and in the logical basis of the DMA in the operational CQCHT algorithm at NCEP so that, unlike the previous implementations, all data levels are treated together, thus potentially allowing the correction at any level to influence subsequent correction at adjacent levels, whether they are mandatory or significant. At each level, treated one by one from the surface upward, all available checks are used to make the appropriate decisions. Several vertical passes may be made through the data until no more corrections are possible. Final passes look for “observation” errors. The methods of error determination are outlined, and the effect of errors on the residuals is illustrated. The calculation of residuals is described, their availability for each type of data surface (e.g., earth’s surface, mandatory level, significant level) is given, and their use by the DMA is presented. The limitations of the use of various checks are discussed.

Full access
William G. Collins

Abstract

Rawinsonde heights and temperatures have been quality controlled using complex quality control at the National Centers for Atmospheric Prediction since December 1988 when an algorithm using only hydrostatic checking was introduced for the checking of mandatory level heights and temperatures. The quality control of significant level temperatures was added to the hydrostatic code in April 1990. In November 1991, the mandatory level checking was greatly expanded and improved by the inclusion of additional checks: increment (observation minus 6-h forecast), horizontal, and vertical. This paper describes a major improvement to the significant level quality control, introduced in May 1994, using complex quality control techniques. The philosophy of the method and the various checks are described. The principles of the decision-making algorithm are stated, examples are shown, and some statistics of the use of the significant level checking are presented.

Full access
William D. Collins

Abstract

New radiative parameterizations have been developed for the National Center for Atmospheric Research (NCAR) Community Atmospheric Model (CAM). The CAM is the next version of the NCAR Community Climate Model (CCM). This paper describes the generalized treatment of vertical cloud overlap in the radiative calculations. The new parameterizations compute the shortwave and longwave fluxes and heating rates for random overlap, maximum overlap, or an arbitrary combination of maximum and random overlap. The specification of the type of overlap is identical for the two bands, and it is completely separated from the radiative parameterizations. In the prototype of CAM (CAM 0.1), adjacent cloud layers are maximally overlapped and groups of clouds separated by cloud-free layers are randomly overlapped. The introduction of the generalized overlap assumptions permits more realistic treatments of cloud–radiative interactions. The parameterizations are based upon representations of the radiative transfer equations that are more accurate than previous approximations. These techniques increase the computational cost of the radiative calculations by approximately 30%. The methodology has been designed and validated against calculations based upon the independent pixel approximation (IPA). The solution techniques and validation procedure are described in detail. The hourly radiative fluxes and heating rates from the parameterizations and IPA have been compared for a 1-yr integration of CAM. The mean and rms errors in the hourly longwave top of the atmosphere (TOA) fluxes are −0.006 ± 0.066 W m−2, and the corresponding errors in the shortwave TOA fluxes are −0.20 ± 1.58 W m−2. Heating rate errors are O(10−3) K day−1. In switching from random to maximum/random overlap, the largest changes in TOA shortwave fluxes occur over tropical continental areas, and the largest changes in TOA longwave fluxes occur in tropical convective regions. The effects on global climate are determined largely by the instantaneous changes in the fluxes rather than feedbacks related to cloud overlap.

Full access
William G. Collins and Lev S. Gandin

Abstract

The Comprehensive Hydrostatic Quality Control (CHQC) of rawinsonde data on height and temperature at mandatory isobaric surfaces designed and implemented at the National Meteorological Center in Washington is described in detail. Main principles of the quality control design are discussed, followed by a brief description of the CHQC design and implementation at NMC. The CHQC algorithm is presented with particular emphasis on the Decision Making Algorithm. Numerous examples taken from the operational CHQC outputs illustrate the CHQC performance in general as well as its reaction to errors of various types and to their combinations.

Full access
LLOYD W. VANDERMAN and WILLIAM G. COLLINS

Abstract

A primitive equation barotropic forecast model is employed independently at 700 mb. and 300 mb. to produce a wind forecast to 36 hr. once daily (from 1200 gmt real data analyses) for the entire tropical belt between 48°N. and 48°S. Experiments have been made in calculating with tropical barotropic forecast models taking values along the northern boundary from a previously calculated Northern Hemisphere forecast. Descriptions of the forecast models and examples and verifications of the forecasts are presented.

Full access
Kyle G. Pressel and William D. Collins

Abstract

The power-law scale dependence, or scaling, of first-order structure functions of the tropospheric water vapor field between 58°S and 58°N is investigated using observations from the Atmospheric Infrared Sounder (AIRS). Power-law scale dependence of the first-order structure function would indicate that the water vapor field exhibits statistical scale invariance. Directional and directionally independent first-order structure functions are computed to assess the directional dependence of derived first-order structure function scaling exponents (H) for a range of scales from 50 to 500 km. In comparison to other methods of assessing statistical scale invariance, the methodology used here requires minimal assumptions regarding the homogeneity of the spatial distribution of data within regions of analysis. Additionally, the methodology facilitates the evaluation of anisotropy and quantifies the extent to which the structure functions exhibit scale invariance.

The spatial and seasonal dependence of the computed scaling exponents are explored. Minimum scaling exponents at all levels are shown to occur proximate to the equator, while the global maximum is shown to occur in the middle troposphere near the tropical–subtropical margin of the winter hemisphere.

From a detailed analysis of AIRS maritime scaling exponents, it is concluded that the AIRS observations suggest the existence of two scaling regimes in the extratropics. One of these regimes characterizes the statistical scale invariance the free troposphere with H approximately = 0.55 and a second that characterizes the statistical scale invariance of the boundary layer with H approximately = ⅓.

Full access
Lisa N. Murphy, William J. Riley, and William D. Collins

Abstract

Many efforts have been taken to find energy alternatives to reduce anthropogenic influences on climate. Recent studies have shown that using land for bioenergy plantations may be more cost effective and provide a greater potential for CO2 abatement than using land for carbon sequestration. Native southern U.S. pines (i.e., loblolly) have excellent potential as bioenergy feedstocks. However, the land-cover change due to expansion of biofuels may impact climate through biophysical feedbacks. Here, the authors access the local and remote consequences of greater forest management and biofuel feedstock expansion on climate and hydrology using a global climate model, the NCAR Community Climate System Model, version 4 (CCSM4).

The authors examine a plausible U.S. Department of Energy (DOE) biofuel feedstock goal by afforesting 50 million acres of C4 grasslands in the southeastern United States with an optimized loblolly plant functional type. Changes in sensible and latent heat fluxes are related to increased surface roughness, reduced bare-ground evaporation, and changes in stomatal conductance. In the coupled simulations, these mechanisms lead to a 1°C cooling, higher atmospheric stability, and a more shallow planetary boundary layer over the southeastern United States during the summer; in winter, a cooling of up to 0.25°C between 40° and 60°N, a weakened Aleutian low, and a wetter Australia occurs. A weakened Aleutian low shifts the North Pacific storm track poleward in the future loblolly scenarios. These local and global impacts suggest that biophysical feedbacks need to be considered when evaluating the benefits of bioenergy feedstock production.

Full access