Search Results

You are looking at 1 - 10 of 48 items for

  • Author or Editor: J. Baker x
  • Refine by Access: All Content x
Clear All Modify Search
M. B. Baker and J. Latham

Abstract

Calculations have been made of the evolution of droplet spectra within small cumulus clouds which are entraining undersaturated environmental air. The mixing process is assumed to be highly inhomogeneous. In the extreme situation considered, environmental air is entrained in discrete blobs or parcels, causing some droplets of all sizes to be completely removed from the condensate spectrum, while others do not change in size. This model, which is based on laboratory experiments, corresponds to a situation in which the time constant for droplet evaporation is small relative to that for turbulent mixing; in the classical (homogeneous) model, which has been used by other workers, the reverse applies. The calculations produce spectral shapes which agree well with those observed in cumulus by Warner (1969), and they indicate that favored droplets may grow very much faster through the condensate spectrum than is predicted classically.

Full access
J. M. Baker, D. C. Reicosky, and D. G. Baker

Abstract

Many models in a variety of disciplines require air temperature throughout the day as an input, yet often the only data available are daily extrema. Several methods for estimating the diurnal change in temperature from daily extrema have been reported. This paper compares the performance of three such algorithms (a sine wave, a sine-exponential, and a linear model) at all times of the year. Each was used to generate four years of hourly temperatures using as input the daily highs and lows recorded by the National Weather Service at the Minneapolis-St. Paul International Airport for the years 1970, 1971, 1973 and 1974. The output from the models was compared with the actual hourly values recorded at the same site. Residual sums and standard errors computed for all three models, both as functions of day and month of the year, showed that all three were more accurate in summer than in winter.

Differences in the overall standard errors of estimate among the three models were small relative to the standard errors themselves. The sinusoidal model was the most accurate of three in predicting midday summer temperatures, suggesting that it would be the best choice for photosynthesis or transpiration models. There was little difference in overall residual sums, indicating equal suitability for use in models driven by accumulated heat units.

Full access
J. L. Brenguier, D. Baumgardner, and B. Baker

Abstract

The forward-scattering spectrometer probe (FSSP) is an optical particle counter widely used for the measurement of cloud droplet size distributions and concentration. Previous studies have identified operational limitations of these probes and a number of techniques have been developed to minimize the impact of these limitations on the measurements. The majority of effort has been focused on accounting for droplets missed by the FSSP as a result of droplet coincidence and electronic dead time. This note reviews the algorithms that have been developed to account for these losses, describes how and when to apply them to previously acquired measurements, and recommends methods to improve the quality of future measurements.

Full access
W. E. Baker, R. Atlas, M. Halem, and J. Susskind

Abstract

In this study we examine the sensitivity of forecast to individual components of the First GARP (Global Atmospheric Research Programme) Global Experiment database as well as to some modifications in the data analysis techniques. Several short assimilation experiments (0000 GMT 18 January 1979 through 0000 21 January) are performed in order to test the effects of each database or analysis change. Forecasts are then generated from the initial conditions provided by these experiments. The 0000 21 January case is chosen for a detailed investigation because or the poor forecast skill obtained earlier over North America for that particular case. Specifically, we conduct experiments to test the sensitivity of forecast skill to: 1) the addition of individual satellite observing system components; 2) temperature data obtained with different satellite retrieval methods; and 3) the method of vertical interpolation between the mandatory pressure analysis levels and the model sigma levels.

For the single case examined, TIROS-N infrared land retrievals produced operationally are found to degrade the forecast, while the use of TIROS-N retrievals produced with a physical inversion method as part of an analysis/forecast cycle results in an improved forecast. The use of oceanic VTPR (Vertical Temperature Profile Radiometer) satellite retrievals also results in an improved forecast over North America. The forecast is also found to be sensitive to the method of vertical interpolation between the mandatory pressure analysis levels and the model sigma levels.

Full access
Wayman E. Baker, Ernest C. Kung, and Richard C. J. Somerville

Abstract

The energetics in numerical weather forecast experiments with the NCAR general circulation model have been analyzed. The 6-layer, 5-degree, second-generation global model was used to make two 10-day forecasts with the same initial conditions. The two experiments differed primarily in the methods of convective parameterization.

Hemispheric integrals of the model energies and energy transformations are presented in the context of their approach to a quasi-equilibrium climatology. Spectral and spatial analyses of the eddy energies and transformations provide further insight into the model response to the initial conditions. After the initial adjustment, the eddy kinetic energy appears to lag the conversion from eddy available potential energy to eddy kinetic energy by at least 48 h in the long waves (wavenumbers 1–4) and by approximately 24 h in the baroclinic waves (wavenumbers 5–7), whereas little or no time lag is apparent in the short waves (wavenumbers 8–12).

The sensitivity of the forecast energetics to two different convective parameterizations is also examined. There is little appreciable difference between the two experiments in the eddy kinetic energy integrals during the first 36 h of the forecast, but temporal patterns of the eddy transformations are distinctly different after 12 h.

Full access
Wayman E. Baker, Ernest C. Kung, and Richard C. J. Somerville

Abstract

A comprehensive energetics analysis has been performed on the NCAR general circulation model. The analysis involves January and July simulation experiments with the 6-layer, 5-degree, second-generation model with two different convective schemes. Spectral analysis of the energy transformations in the wave-number domain was performed separately on a global and hemispheric basis as well as for the tropics and mid-latitudes. Latitudinal distributions of energy variables were also examined.

A qualitative agreement with observational estimates is generally recognized in the transformations of eddy energies. Quantitatively, however, the eddy energies, conversions and energy transfer between wavenumbers are weaker than observational estimates. It is noteworthy that substantial differences exist in the energetics of the two versions of the model with different convective schemes.

Full access
Donald G. Baker, John W. Enz, and Harold J. Paulus

Abstract

This study is based upon temperatures for the period June 1961 through July 1968, obtained from thermistors installed on a television tower at heights of 70, 170 and 500 ft. The tower was located in the heart of the metropolitan area but well outside of the main business district of either city. Only inversions ≥2 hr in duration were counted.On an annual basis only 2.5 inversions per 100 days occurred in the lower level (70–170ft) compared to 26.3 inversions per 100 days in the upper level (170–500ft). The total frequency, including the two levels and the deeper inversions that extended over both levels, equalled 45.5 per 100 days.The average duration of inversions within both levels was 6.8 hr, the 8.2-hr average in October being the longest of any month. Midnight was the single most common hour for inversion formation. Inversion intensity (defined as the temperature difference between levels) varied directly with duration of the inversions. The average intensity of all inversions within both levels was 3.6F.An inversion rating index was developed that takes into account frequency, duration and intensity of inversions so that either time periods or sites may be quantitatively compared. The index indicated that the major inversion months were October, August and September, the minor months being April, March and May.A decrease in inversion intensity and frequency over the record period was assumed to indicate an increasingly urban influence on the temperature of the region.

Full access
Jonathan A. Baker, Andrew J. Watson, and Geoffrey K. Vallis

Abstract

The variation in the strength and structure of the overturning circulation under varying Southern Ocean buoyancy forcing, corresponding to present day, a cooler (glacial) state, and a possible future warmer state is analyzed in an idealized two-basin general circulation model connected by a southern circumpolar channel. A connection between the North Atlantic Deep Water (NADW) cell in the Atlantic basin and the Pacific Deep Water (PDW) cell in the Pacific basin occurs with a direct flow of NADW into the channel’s lower cell, while PDW upwelled in the Pacific basin can flow directly into the upper wind-driven cell in the channel. The intersection of these cells along with direct zonal flows between the basins completes the interbasin circulation. The present-day Atlantic meridional overturning circulation (AMOC) in the model is upwelled both by wind-driven upwelling in the Southern Ocean and by diffusion in the Pacific and Atlantic. In a cooler climate with enhanced sea ice, the NADW cell shoals, which can then no longer flow directly into the channel’s lower cell, reducing the Pacific pathway of NADW. This leads to a substantial weakening of the AMOC, suggesting buoyancy forcing changes can play a substantial role in the transition of the AMOC to a glacial state. In contrast, in a warmer equilibrium climate state with reduced AABW formation, the NADW cell strengthens and deepens. NADW is increasingly directed along the Pacific pathway, while the direct upwelling in the channel’s wind-driven upper cell plays a smaller role.

Open access
Jonathan A. Baker, Andrew J. Watson, and Geoffrey K. Vallis

Abstract

The response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific Ocean basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to “warm,” “present day,” and “cold” states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively driven upwelling sustain the AMOC in the warmer (present day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

Restricted access
K. N. Bower, T. W. Choularton, J. Latham, J. Nelson, M. B. Baker, and J. Jensen

Abstract

Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

Full access