Search Results

You are looking at 1 - 10 of 18 items for

  • Author or Editor: R. David Baker x
  • Refine by Access: All Content x
Clear All Modify Search
Steven R. Hanna
,
Robert Paine
,
David Heinold
,
Elizabeth Kintigh
, and
Dan Baker

Abstract

The uncertainties in simulations of annually averaged concentrations of two air toxics (benzene and 1,3-butadiene) are estimated for two widely used U.S. air quality models, the Industrial Source Complex Short-Term, version 3, (ISCST3) model and the American Meteorological Society–Environmental Protection Agency Model (AERMOD). The effects of uncertainties in emissions input, meteorological input, and dispersion model parameters are investigated using Monte Carlo probabilistic uncertainty methods, which involve simultaneous random and independent perturbations of all inputs. The focus is on a 15 km × 15 km domain in the Houston, Texas, ship channel area. Concentrations are calculated at hypothetical receptors located at the centroids of population census tracts. The model outputs that are analyzed are the maximum annually averaged maximum concentration at any single census tract or monitor as well as the annually averaged concentration averaged over the census tracts. The input emissions uncertainties are estimated to be about a factor of 3 (i.e., covering the 95% range) for each of several major categories. The uncertainties in meteorological inputs (such as wind speed) and dispersion model parameters (such as the vertical dispersion coefficient σz ) also are estimated. The results show that the 95% range in predicted annually averaged concentrations is about a factor of 2–3 for the air toxics, with little variation by model. The input variables whose variations have the strongest effect on the predicted concentrations are on-road mobile sources and some industrial sources (dependent on chemical), as well as wind speed, surface roughness, and σz . In most scenarios, the uncertainties of the emissions input group contribute more to the total uncertainty than do the uncertainties of the meteorological/dispersion input group.

Full access
R. David Baker
,
Gerald Schubert
, and
Philip W. Jones

Abstract

This paper is the first of a two-part study that investigates internal gravity wave generation by convection in the lower atmosphere of Venus. A two-dimensional, nonlinear, fully compressible model of a perfect gas is employed. The calculations consider the lower atmosphere from 12- to 60-km altitude, thereby including two convection regions: the lower atmosphere convection layer from roughly 18- to 30-km altitude and the cloud-level convection layer from roughly 48- to 55-km altitude. The gravity waves of interest are located in the stable layer between these two convection regions. Part I of this study considers gravity wave generation and propagation in the absence of mean wind shear.

In the absence of mean wind shear, internal gravity waves are primarily generated by cloud-level convection. Horizontal wavelengths (∼10–15 km) are similar to dominant horizontal scales in the cloud-level penetrative region, and intrinsic horizontal phase speeds are comparable to cloud-level downdraft velocities. Without mean wind shear, there is no effective coupling between the lower atmosphere below 34-km altitude and the overlying stable layer. Simulated wave amplitudes and vertical wavelengths agree well with spacecraft observations, suggesting that gravity waves generated by cloud-level convection through the “mechanical oscillator” effect may be responsible for observed variations in the stable layer.

Full access
R. David Baker
,
Gerald Schubert
, and
Philip W. Jones

Abstract

This paper is the second of a two-part study that numerically investigates internal gravity wave generation by convection in the lower atmosphere of Venus. Part I of this study considers gravity wave generation and propagation in the absence of mean wind shear. In Part II, the Venus westward superrotation is included, and wave–mean flow interaction is assessed.

Both lower-atmosphere convection and cloud-level convection play active roles in the dynamics of the stable layer from 31- to 47-km altitude when mean wind shear is present. This result contrasts with the simulation without mean wind shear presented in Part I where cloud-level convection was primarily responsible for gravity wave generation in the stable layer. In the presence of mean wind shear, upward entrainment from lower-atmosphere convection and downward penetration from cloud-level convection are comparable in magnitude. Convectively generated internal gravity waves have horizontal wavelengths (∼25–30 km) comparable to horizontal scales in both convection layers. Quasi-stationary gravity waves (with respect to the lower convection layer) occur in the lower part of the stable layer, while both eastward- and westward-propagating waves generated by cloud-level convection exist in the upper part of the stable layer. Simulated wave amplitudes and vertical wavelengths agree well with observations. Eastward-propagating waves generated by cloud-level convection experience critical level absorption in the stable layer and thus decelerate the Venus westward superrotation below the clouds. The deceleration is comparable in magnitude to zonal accelerations above the clouds by thermal tides.

Full access
R. David Baker
,
Gerald Schubert
, and
Philip W. Jones

Abstract

A two-dimensional, nonlinear, fully compressible model of a perfect gas is used to simulate cloud-level penetrative convection in the Venus atmosphere from 40 to 60 km altitude. Three cases with different amounts of solar heating are considered: 60%, 80%, and 100% subsolar heating conditions corresponding to maximum internally heated Rayleigh numbers of 4.0 × 106, 5.4 × 106, and 6.8 × 106, respectively. Cloud-level convection is characterized by cold, narrow downwellings that deeply penetrate (∼5 km) the underlying stable layer. The horizontal spacing of the downwellings is 15–30 km, an order of magnitude smaller than observed cloud-top cells in ultraviolet images. The penetrating head of the downflow is both mechanically forced upward and compressionally heated by the underlying stable layer. The local compressional heating rate induced by penetration is four orders of magnitude larger than the solar heating rate. Although slightly larger in magnitude, the calculated vertical velocities at 54-km altitude are consistent with Vega balloon measurements. The computations show that the Vega balloons drifted in a relatively quiescent part of the convection layer. Vertical velocities are three to five times larger in the lower part of the convection layer than in the upper part of the layer because of the dominance of convection by intense downwellings that acquire their highest speeds as they penetrate the underlying stable region. Mixing length theory underestimates the vertical velocity of convection by a factor of 3 or more because kinetic energy in the convection layer is balanced not only by buoyancy work as assumed by mixing length theory, but also by pressure work and viscous work. A transfer of energy from low-frequency convective modes to higher-frequency “interfacial” penetrative modes occurs in the penetrative region. Internal gravity waves are also generated in the stable layers with horizontal wavelengths of 5–30 km and intrinsic horizontal phase speeds comparable to convective velocities.

Full access
Lucas R. Vargas Zeppetello
,
David S. Battisti
, and
Marcia B. Baker

Abstract

We analyze observations and develop a hierarchy of models to understand heat waves—long-lived, high temperature anomalies—and extremely high daily temperatures during summertime in the continental extratropics. Throughout the extratropics, the number of extremely hot days found in the three hottest months is much greater than expected from a random, single-process model. Furthermore, in many locations the temperature skewness switches from negative on daily time scales to positive on monthly time scales (or shifts from positive on daily time scales to higher positive values on monthly time scales) in ways that cannot be explained by averaging alone. These observations motivate a hierarchy of models of the surface energy and moisture budgets that we use to illuminate the physics responsible for daily and monthly averaged temperature variability. Shortwave radiation fluctuations drive much of the variance and the negative skewness found in daily temperature observations. On longer time scales, precipitation-induced soil moisture anomalies are important for temperature variability and account for the shift toward positive skewness in monthly averaged temperature. Our results demonstrate that long-lived heat waves are due to (i) the residence time of soil moisture anomalies and (ii) a nonlinear feedback between temperature and evapotranspiration via the impact of temperature on vapor pressure deficit. For most climates, these two processes give rise to infrequent, long-lived heat waves in response to randomly distributed precipitation forcing. Combined with our results concerning high-frequency variability, extremely hot days are seen to be state-independent filigree driven by shortwave variability acting on top of longer-lived, moisture-driven heat waves.

Full access
Lucas R. Vargas Zeppetello
,
David S. Battisti
, and
Marcia B. Baker

Abstract

Evaporation plays an extremely important role in determining summertime surface temperature variability over land. Observations show the relationship between evaporation and soil moisture generally conforms to the Budyko “two regime” framework; namely, that evaporation is limited by available soil moisture in dry climates and by radiation in wet climates. This framework has led climate models to different parameterizations of the relationship between evaporation and soil moisture in wet and dry regions. We have developed the Simple Land–Atmosphere Model (SLAM) as a tool for studying land–atmosphere interaction in general, and summertime temperature variability in particular. We use the SLAM to show that a negative feedback between evaporation and surface temperature gives rise to the two apparent evaporation “regimes” and provide analytic solutions for evaporative cooling anomalies that demonstrate the nonlinear impact of soil moisture perturbations. Stemming from the temperature dependence of vapor pressure deficit, the feedback we identify has important implications for how transitions between wet and dry land surfaces may impact temperature variability as the climate warms. We also elucidate the impacts of surface moisture and insolation perturbations on latent and sensible heat fluxes and on surface temperature variability.

Full access
Lucas R. Vargas Zeppetello
,
David S. Battisti
, and
Marcia B. Baker

Abstract

The increasing frequency of very high summertime temperatures has motivated growing interest in the processes determining the probability distribution of surface temperature over land. Here, we show that on monthly time scales, temperature anomalies can be modeled as linear responses to fluctuations in shortwave radiation and precipitation. Our model contains only three adjustable parameters, and, surprisingly, these can be taken as constant across the globe, notwithstanding large spatial variability in topography, vegetation, and hydrological processes. Using observations of shortwave radiation and precipitation from 2000 to 2017, the model accurately reproduces the observed pattern of temperature variance throughout the Northern Hemisphere midlatitudes. In addition, the variance in latent heat flux estimated by the model agrees well with the few long-term records that are available in the central United States. As an application of the model, we investigate the changes in the variance of monthly averaged surface temperature that might be expected due to anthropogenic climate change. We find that a climatic warming of 4°C causes a 10% increase in temperature variance in parts of North America.

Free access
R. David Baker
,
Barry H. Lynn
,
Aaron Boone
,
Wei-Kuo Tao
, and
Joanne Simpson

Abstract

Idealized numerical simulations of Florida convection are performed with a coupled atmosphere–land surface model to identify the roles of initial soil moisture, coastline curvature, and land-breeze circulations on sea-breeze-initiated precipitation. The 3D Goddard Cumulus Ensemble cloud-resolving model is coupled with the Goddard Parameterization for Land–Atmosphere–Cloud Exchange land surface model, thus providing a tool to simulate more realistically land surface–atmosphere interaction and convective initiation. Eight simulations are conducted with either straight or curved coastlines, initially homogeneous soil moisture or initially variable soil moisture, and initially homogeneous horizontal winds or initially variable horizontal winds (land breezes). An additional simulation is performed to assess the role of Lake Okeechobee on convective development.

All model simulations capture the diurnal evolution and general distribution of sea-breeze-initiated precipitation over central Florida. The distribution of initial soil moisture influences the timing and location of subsequent precipitation. Soil moisture acts as a moisture source for the atmosphere, increases the convectively available potential energy, and thus preferentially focuses heavy precipitation over existing wet soil. Soil moisture–induced mesoscale circulations do not produce heavy precipitation. Coastline curvature has a major impact on the timing and location of precipitation. Earlier low-level convergence occurs inland of convex coastlines, and subsequent heavy precipitation occurs earlier in simulations with curved coastlines. Early-morning land breezes influence the timing of precipitation by modifying low-level convergence. Because of nonlinear interaction between coastline curvature and soil moisture, the highest peak accumulated rainfall and highest peak rain rates occur in simulations with both coastline curvature and initial soil moisture variations. Lake Okeechobee influences the timing and location of precipitation because of strong lake-breeze circulations.

Full access
Temple R. Lee
,
Michael Buban
,
David D. Turner
,
Tilden P. Meyers
, and
C. Bruce Baker

Abstract

The High-Resolution Rapid Refresh (HRRR) model became operational at the National Centers for Environmental Prediction (NCEP) in 2014 but the HRRR’s performance over certain regions of the coterminous United States has not been well studied. In the present study, we evaluated how well version 2 of the HRRR, which became operational at NCEP in August 2016, simulates the near-surface meteorological fields and the surface energy balance at two locations in northern Alabama. We evaluated the 1-, 3-, 6-, 12-, and 18-h HRRR forecasts, as well as the HRRR’s initial conditions (i.e., the 0-h initial fields) using meteorological and flux observations obtained from two 10-m micrometeorological towers installed near Belle Mina and Cullman, Alabama. During the 8-month model evaluation period, from 1 September 2016 to 30 April 2017, we found that the HRRR accurately simulated the observations of near-surface air and dewpoint temperature (R 2 > 0.95). When comparing the HRRR output with the observed sensible, latent, and ground heat flux at both sites, we found that the agreement was weaker (R 2 ≈ 0.7), and the root-mean-square errors were much larger than those found for the near-surface meteorological variables. These findings help motivate the need for additional work to improve the representation of surface fluxes and their coupling to the atmosphere in future versions of the HRRR to be more physically realistic.

Full access
Karen I. Mohr
,
R. David Baker
,
Wei-Kuo Tao
, and
James S. Famiglietti

Abstract

This study used a two-dimensional coupled land–atmosphere (cloud resolving) model to investigate the influence of land cover on the water budgets of convective lines in West Africa. Study simulations used the same initial sounding and one of three different land covers: a sparsely vegetated semidesert, a grassy savanna, and a dense evergreen broadleaf forest. All simulations began at midnight and ran for 24 h to capture a full diurnal cycle. During the morning, the forest had the highest latent heat flux, the shallowest, moistest, slowest growing boundary layer, and more convective available potential energy than the savanna and semidesert. Although the savanna and forest environments produced virtually the same total rainfall mass (semidesert 18%), the spatial and temporal patterns of the rainfall were significantly different and can be attributed to the boundary layer evolution. The forest produced numerous convective cells with very high rain rates mainly during the early afternoon. During the morning, the savanna built up less but still significant amounts of convective available potential energy and enough convective inhibition so that the strongest convection in the savanna did not occur until late afternoon. This timing resulted in the largest, most intense convective line of the three land covers.

Full access