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Abstract
Data on point and areal (100–8100 km2) surface hailfall characteristics are presented for Oklahoma. Data analysis shows Oklahoma experiences a larger mean hailstone per occurrence than South Africa and a larger mean hailfall area than South Africa or Illinois.
Further investigations were made into the parent hailstorms. They were first subdivided using the ordinary/supercell classification system suggested by Browning (1977). The classification system was found to have general utility for central Oklahoma when applied to all observed hailstorms that occurred within an 8100 km2 area over a four-year period. Supercells comprised ∼25% of the storms and had a larger mean observed hailstone diameter (4.4 cm) and a larger mean maximum swath width (18.1 km) than the ordinary cell storms (mean value of 1.4 cm and 8.1 km, respectively). These differences in hail production are significant at the 1% level.
Abstract
Data on point and areal (100–8100 km2) surface hailfall characteristics are presented for Oklahoma. Data analysis shows Oklahoma experiences a larger mean hailstone per occurrence than South Africa and a larger mean hailfall area than South Africa or Illinois.
Further investigations were made into the parent hailstorms. They were first subdivided using the ordinary/supercell classification system suggested by Browning (1977). The classification system was found to have general utility for central Oklahoma when applied to all observed hailstorms that occurred within an 8100 km2 area over a four-year period. Supercells comprised ∼25% of the storms and had a larger mean observed hailstone diameter (4.4 cm) and a larger mean maximum swath width (18.1 km) than the ordinary cell storms (mean value of 1.4 cm and 8.1 km, respectively). These differences in hail production are significant at the 1% level.
Abstract
In this derivation of the equilibrium supersaturation curve for soluble particles, Köhler treated the van't Hoff factor as a constant. McDonald points out that this factor is actually a function of the droplet molality. In this paper, we have rederived equilibrium supersaturation relation following Köhler's approach, except that the van't Hoff factor is treated as a variable in the derivation. The resulting equation is considerably more complicated than the result obtained by Köhler. A simple substitution of a variable van't Hoff factor into Köhler's familiar equation that was derived assuming a constant van't Hoff factor results in a fairly significant error.
Abstract
In this derivation of the equilibrium supersaturation curve for soluble particles, Köhler treated the van't Hoff factor as a constant. McDonald points out that this factor is actually a function of the droplet molality. In this paper, we have rederived equilibrium supersaturation relation following Köhler's approach, except that the van't Hoff factor is treated as a variable in the derivation. The resulting equation is considerably more complicated than the result obtained by Köhler. A simple substitution of a variable van't Hoff factor into Köhler's familiar equation that was derived assuming a constant van't Hoff factor results in a fairly significant error.
Abstract
The Southern Ocean plays a major role in global air–sea carbon fluxes, with some estimates suggesting it contributes to up to 40% of the oceanic anthropogenic carbon dioxide uptake, despite only comprising about 20% of oceanic surface area. Thus, the Southern Ocean overturning, the circulation that transports tracers between the surface and deep ocean interior, is particularly important for climate. Recent studies show that vertical velocities and tracer transport are largest just downstream of bottom topography; these quantities are related to the overturning, but provide incomplete information about the net Lagrangian transport, usually described with the residual-mean theory in a zonally integrated sense. This study uses an idealized Southern Ocean–like channel model with particle tracking to visualize the thickness-weighted velocities that capture the net overturning transport of a parcel, connecting residual-mean overturning theory to the three-dimensional, localized nature of the overturning. From this, we split the flow into three main drivers of transport: a wind-driven Ekman pumping into or out of a density layer, and standing eddies and transient eddies, both of which are localized near the topography. In this framework, the three-dimensional overturning circulation is not a small residual between the eddy and Eulerian-mean transport. The existence of a ridge weakens the response of the overturning to changes in wind, especially in the lower cell. This local understanding of the overturning framework suggests that careful modeling and sampling of specific regions near topography in the Southern Ocean are vital to understand climate sensitivity, transport, carbon export, and connections with the oceans to the north.
Abstract
The Southern Ocean plays a major role in global air–sea carbon fluxes, with some estimates suggesting it contributes to up to 40% of the oceanic anthropogenic carbon dioxide uptake, despite only comprising about 20% of oceanic surface area. Thus, the Southern Ocean overturning, the circulation that transports tracers between the surface and deep ocean interior, is particularly important for climate. Recent studies show that vertical velocities and tracer transport are largest just downstream of bottom topography; these quantities are related to the overturning, but provide incomplete information about the net Lagrangian transport, usually described with the residual-mean theory in a zonally integrated sense. This study uses an idealized Southern Ocean–like channel model with particle tracking to visualize the thickness-weighted velocities that capture the net overturning transport of a parcel, connecting residual-mean overturning theory to the three-dimensional, localized nature of the overturning. From this, we split the flow into three main drivers of transport: a wind-driven Ekman pumping into or out of a density layer, and standing eddies and transient eddies, both of which are localized near the topography. In this framework, the three-dimensional overturning circulation is not a small residual between the eddy and Eulerian-mean transport. The existence of a ridge weakens the response of the overturning to changes in wind, especially in the lower cell. This local understanding of the overturning framework suggests that careful modeling and sampling of specific regions near topography in the Southern Ocean are vital to understand climate sensitivity, transport, carbon export, and connections with the oceans to the north.
Abstract
Equatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.
Abstract
Equatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.
Abstract
Since 1988, what appears to be an abnormal number of maximum temperature records has been set at the National Weather Service Office in Tucson, Arizona (TUS). We present several analyses that indicate that the current measurement system at TUS is indicating daytime temperatures that are 2 to 3 degrees too high. It appears that the instrument is not appropriately aspirated so that, during the day, temperature readings are significantly warmer than ambient air temperatures, while at night they are slightly cooler. The system at TUS is similar to one that has been installed at many National Weather Service sites around the country. We speculate on the impact this system may have on the climate record if the errors noted at Tucson are similar at the other sites.
Abstract
Since 1988, what appears to be an abnormal number of maximum temperature records has been set at the National Weather Service Office in Tucson, Arizona (TUS). We present several analyses that indicate that the current measurement system at TUS is indicating daytime temperatures that are 2 to 3 degrees too high. It appears that the instrument is not appropriately aspirated so that, during the day, temperature readings are significantly warmer than ambient air temperatures, while at night they are slightly cooler. The system at TUS is similar to one that has been installed at many National Weather Service sites around the country. We speculate on the impact this system may have on the climate record if the errors noted at Tucson are similar at the other sites.
Abstract
The response of a wind-sea spectrum to a step function change in wind direction is investigated theoretically for a sequence of direction changes ranging from 30° to 180°, in increments of 30°. Two spectral energy balance models are used: the model EXACT-NL, in which the nonlinear transfer is represented exactly, and the model 3G-WAM, in which the nonlinear transfer is approximated by the discrete interaction parameterization. In both modes the input and dissipation source functions are taken from the energy balance proposed by Komen et al. The operational model 3G-WAM reproduces fairly closely the EXACT-NL results. For wind direction changes less than 60°, the wind-sea direction adjusts smoothly. The high-frequency components relax more rapidly to the new wind direction than the low-frequency components. The computed relaxation rates are generally consistent with the analysis of measured directional spectra by D.E. Hasselman et al. and Allender et al. However, the relaxation rate is found to be a function of wind speed as well as frequency. For wind direction changes greater than 60°, a second, independent wind-sea spectrum is generated in the new wind direction, while the old wind-sea gradually decays as swell.
Abstract
The response of a wind-sea spectrum to a step function change in wind direction is investigated theoretically for a sequence of direction changes ranging from 30° to 180°, in increments of 30°. Two spectral energy balance models are used: the model EXACT-NL, in which the nonlinear transfer is represented exactly, and the model 3G-WAM, in which the nonlinear transfer is approximated by the discrete interaction parameterization. In both modes the input and dissipation source functions are taken from the energy balance proposed by Komen et al. The operational model 3G-WAM reproduces fairly closely the EXACT-NL results. For wind direction changes less than 60°, the wind-sea direction adjusts smoothly. The high-frequency components relax more rapidly to the new wind direction than the low-frequency components. The computed relaxation rates are generally consistent with the analysis of measured directional spectra by D.E. Hasselman et al. and Allender et al. However, the relaxation rate is found to be a function of wind speed as well as frequency. For wind direction changes greater than 60°, a second, independent wind-sea spectrum is generated in the new wind direction, while the old wind-sea gradually decays as swell.
Abstract
A sponge layer is formulated to prevent spurious reflection of vertically propagating quasi-stationary gravity waves at the upper boundary of a two-dimensional numerical anelastic nonhydrostatic model. The sponge layer includes damping of both Newtonian-cooling type and Rayleigh-friction type, whose coefficients are determined in such a way that the reflectivity of wave energy at the bottom of the layer is zero. Unlike the formulations in earlier studies, our formulation includes the effects of vertical discretization, vertical mean density variation, and nonhydrostaticity. This sponge formulation is found effective in suppressing false downward reflection of waves for various types of quasi-stationary forcing.
Abstract
A sponge layer is formulated to prevent spurious reflection of vertically propagating quasi-stationary gravity waves at the upper boundary of a two-dimensional numerical anelastic nonhydrostatic model. The sponge layer includes damping of both Newtonian-cooling type and Rayleigh-friction type, whose coefficients are determined in such a way that the reflectivity of wave energy at the bottom of the layer is zero. Unlike the formulations in earlier studies, our formulation includes the effects of vertical discretization, vertical mean density variation, and nonhydrostaticity. This sponge formulation is found effective in suppressing false downward reflection of waves for various types of quasi-stationary forcing.
Abstract
A mixing-line boundary-layer model is used to retrieve cloud-top height from satellite-derived cloud-top temperature using 700-hPa National Meteorological Center (NMC) analyses and the Comprehensive Ocean and Atmosphere Data Set (COADS) surface data as supporting datasets. Results are compared with the fixed-lapse-rate method of retrieving boundary-layer depth from sea surface temperatures (SST) and cloud-top temperatures. A radiative-convective equilibrium boundary-layer model is used to retrieve boundary-layer structure given SST and surface wind, satellite cloud-top temperatures and cloud fraction, and the 700-hPa NMC thermodynamic analyses. Good agreement is found between the COADS data and the model solutions for low-level temperature and moisture. This suggests that equilibrium boundary-layer models may be of use over remote oceans in the retrieval of boundary-layer structure.
Abstract
A mixing-line boundary-layer model is used to retrieve cloud-top height from satellite-derived cloud-top temperature using 700-hPa National Meteorological Center (NMC) analyses and the Comprehensive Ocean and Atmosphere Data Set (COADS) surface data as supporting datasets. Results are compared with the fixed-lapse-rate method of retrieving boundary-layer depth from sea surface temperatures (SST) and cloud-top temperatures. A radiative-convective equilibrium boundary-layer model is used to retrieve boundary-layer structure given SST and surface wind, satellite cloud-top temperatures and cloud fraction, and the 700-hPa NMC thermodynamic analyses. Good agreement is found between the COADS data and the model solutions for low-level temperature and moisture. This suggests that equilibrium boundary-layer models may be of use over remote oceans in the retrieval of boundary-layer structure.
Abstract
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Abstract
High-rate turbulence data collected by the National Center for Atmospheric Research Electra aircraft on 13 January 1998 over Lake Michigan during the Lake-Induced Convection Experiment are analyzed to explore the turbulence dynamics of the entrainment zone of a rapidly entraining convective boundary layer. Horizontal flight legs combined with the gentle slope of the lake-enhanced boundary layer top yield high-resolution profiles through the entrainment zone.
Data analysis used a new maximally overlapping, subleg, Reynolds decomposition algorithm to compute first-, second-, and third-moment statistics for each flight leg. Then, a compositing algorithm using 25 bins took like statistics from each flight leg and created smoothly varying curves that captured the variation of each statistic across the upper mixed layer and entrainment zone.
The analysis results reveal a conventional convective boundary layer below and a nonturbulent free atmosphere above a well-resolved entrainment zone. In the entrainment zone, temperature and moisture skewness curves show that the last identifiable free atmosphere effects extend down well below the zero crossing of heat flux. Third-moment statistics reveal considerable height dependence of the entrainment zone dynamics.
Abstract
High-rate turbulence data collected by the National Center for Atmospheric Research Electra aircraft on 13 January 1998 over Lake Michigan during the Lake-Induced Convection Experiment are analyzed to explore the turbulence dynamics of the entrainment zone of a rapidly entraining convective boundary layer. Horizontal flight legs combined with the gentle slope of the lake-enhanced boundary layer top yield high-resolution profiles through the entrainment zone.
Data analysis used a new maximally overlapping, subleg, Reynolds decomposition algorithm to compute first-, second-, and third-moment statistics for each flight leg. Then, a compositing algorithm using 25 bins took like statistics from each flight leg and created smoothly varying curves that captured the variation of each statistic across the upper mixed layer and entrainment zone.
The analysis results reveal a conventional convective boundary layer below and a nonturbulent free atmosphere above a well-resolved entrainment zone. In the entrainment zone, temperature and moisture skewness curves show that the last identifiable free atmosphere effects extend down well below the zero crossing of heat flux. Third-moment statistics reveal considerable height dependence of the entrainment zone dynamics.
