Search Results
You are looking at 21 - 30 of 71 items for
- Author or Editor: RICHARD A. ANTHES x
- Refine by Access: All Content x
Estimating Error Variances of a Microwave Sensor and Dropsondes aboard the Global Hawk in Hurricanes Using the Three-Cornered Hat MethodAbstract
This study estimates the random error variances and standard deviations (STDs) for four datasets: Global Hawk (GH) dropsondes (DROP), the High-Altitude Monolithic Microwave Integrated Circuit Sounding Radiometer (HAMSR) aboard the GH, the fifth European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5), and the Hurricane Weather Research and Forecasting (HWRF) Model, using the three-cornered hat (3CH) method. These estimates are made during the 2016 Sensing Hazards with Operational Unmanned Technology (SHOUT) season in the environment of four tropical cyclones from August to October. For temperature and specific and relative humidity, the ERA5, HWRF, and DROP datasets all have similar magnitudes of errors, with ERA5 having the smallest. The error STDs of temperature and specific humidity are less than 0.8 K and 1.0 g kg−1 over most of the troposphere, while relative humidity error STDs increase from less than 5% near the surface to between 10% and 20% in the upper troposphere. The HAMSR bias-corrected data have larger errors, with estimated error STDs of temperature and specific humidity in the lower troposphere between 1.5 and 2.0 K and between 1.5 and 2.5 g kg−1. HAMSR’s relative humidity error STD increases from approximately 10% in the lower troposphere to 30% in the upper troposphere. The 3CH method error estimates are generally consistent with prior independent estimates of errors and uncertainties for the HAMSR and dropsonde datasets, although they are somewhat larger, likely due to the inclusion of representativeness errors (differences associated with different spatial and temporal scales represented by the data).
Abstract
This study estimates the random error variances and standard deviations (STDs) for four datasets: Global Hawk (GH) dropsondes (DROP), the High-Altitude Monolithic Microwave Integrated Circuit Sounding Radiometer (HAMSR) aboard the GH, the fifth European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5), and the Hurricane Weather Research and Forecasting (HWRF) Model, using the three-cornered hat (3CH) method. These estimates are made during the 2016 Sensing Hazards with Operational Unmanned Technology (SHOUT) season in the environment of four tropical cyclones from August to October. For temperature and specific and relative humidity, the ERA5, HWRF, and DROP datasets all have similar magnitudes of errors, with ERA5 having the smallest. The error STDs of temperature and specific humidity are less than 0.8 K and 1.0 g kg−1 over most of the troposphere, while relative humidity error STDs increase from less than 5% near the surface to between 10% and 20% in the upper troposphere. The HAMSR bias-corrected data have larger errors, with estimated error STDs of temperature and specific humidity in the lower troposphere between 1.5 and 2.0 K and between 1.5 and 2.5 g kg−1. HAMSR’s relative humidity error STD increases from approximately 10% in the lower troposphere to 30% in the upper troposphere. The 3CH method error estimates are generally consistent with prior independent estimates of errors and uncertainties for the HAMSR and dropsonde datasets, although they are somewhat larger, likely due to the inclusion of representativeness errors (differences associated with different spatial and temporal scales represented by the data).
Abstract
A series of numerical experiments with the Hoskins-Bretherton horizontal shear model of frontogenesis in an, amplifying, two-dimensional baroclinic wave is performed. The analytic solutions from the Boussinesq, semi-geostrophic model provide initial conditions for numerical integrations with a two-dimensional, dry version of the fully compressible, hydrostatic primitive equation (PE) model of Anthes and Warner with 40 km horizontal resolution. The PE model is integrated 1) without planetary boundary layer (PBL) physics; 2) with a one-layer bulk-drag scheme; and 3) with a high-vertical-resolution PBL model. The lower boundary is thermally insulated in order to isolate the effect of the internal mixing of heat in the PBL.
The simulation with the high-resolution PBL physics resolves several realistic features including 1) a narrow updraft at the top of the PBL above the sea-level pressure trough at the warm edge of the frontal zone; 2) a stable layer capping the PBL to the rear of the frontal zone; and 3) slightly unstable or neutral lapse rates in the PBL behind the front and stable lapse rates in the PBL ahead of the front. A diagnostic analysis of the frontogenesis indicates that the fine structure resulting from adding PBL physics can be attributed to the frictionally driven, ageostrophic inflow in the PBL toward the surface pressure trough in which the frontal zone is located. A finding of particular interest is that the stability patterns in the PBL on either side of the front evolve independently of sensible heating at the surface.
Abstract
A series of numerical experiments with the Hoskins-Bretherton horizontal shear model of frontogenesis in an, amplifying, two-dimensional baroclinic wave is performed. The analytic solutions from the Boussinesq, semi-geostrophic model provide initial conditions for numerical integrations with a two-dimensional, dry version of the fully compressible, hydrostatic primitive equation (PE) model of Anthes and Warner with 40 km horizontal resolution. The PE model is integrated 1) without planetary boundary layer (PBL) physics; 2) with a one-layer bulk-drag scheme; and 3) with a high-vertical-resolution PBL model. The lower boundary is thermally insulated in order to isolate the effect of the internal mixing of heat in the PBL.
The simulation with the high-resolution PBL physics resolves several realistic features including 1) a narrow updraft at the top of the PBL above the sea-level pressure trough at the warm edge of the frontal zone; 2) a stable layer capping the PBL to the rear of the frontal zone; and 3) slightly unstable or neutral lapse rates in the PBL behind the front and stable lapse rates in the PBL ahead of the front. A diagnostic analysis of the frontogenesis indicates that the fine structure resulting from adding PBL physics can be attributed to the frictionally driven, ageostrophic inflow in the PBL toward the surface pressure trough in which the frontal zone is located. A finding of particular interest is that the stability patterns in the PBL on either side of the front evolve independently of sensible heating at the surface.
Abstract
The energetics of Eady's (1949) model of baroclinic instability are used to express the wavenumber-dependent disturbance growth rate in terms of upward and northward fluxes of heat and momentum. This formulation leads to simple physical interpretations for the existence of the wavelength of maximum growth rate and the shortwave cutoff.
Abstract
The energetics of Eady's (1949) model of baroclinic instability are used to express the wavenumber-dependent disturbance growth rate in terms of upward and northward fluxes of heat and momentum. This formulation leads to simple physical interpretations for the existence of the wavelength of maximum growth rate and the shortwave cutoff.
Abstract
This paper investigates the generation and propagation of spiral bands on an axisymmetric base-state vortex. A linear model is used to study the formation of bands from internal gravity-inertia waves in a barotropic atmosphere. Spiral bands form random perturbations placed on a vortex with unstable static stability that is equal for ascending and descending motion. This growing mode assumes the characteristics of pseudoadiabatic motion in a conditionally unstable atmosphere due to the coarse vertical resolution of the linear model. Implicit diffusion from the centered finite-difference scheme shifts the preferred growth modes from infinite wavenumbers, characteristic of inviscid analytical solutions, to 4Δλ and 4Δr,ar wavelengths in numerical experiments. Here Δλ and Δr are the angular and radial distances between grid points. Explicit diffusion representing subgrid-scale eddies shifts preferred modes to longer wavelengths. Rotation in the basic state is a necessary condition before the unstable gravity-inertia waves form spiral bands. Rotation also organizes stable perturbations into a banded pattern. Inertial instability and the Coriolis parameter are unimportant for band formation in these linear experiments. The distance between bands increases and the growth rate decreases in experiments in which adiabatic warming occurs with descent and warming due to latent heat release occurs with ascending motion.
Abstract
This paper investigates the generation and propagation of spiral bands on an axisymmetric base-state vortex. A linear model is used to study the formation of bands from internal gravity-inertia waves in a barotropic atmosphere. Spiral bands form random perturbations placed on a vortex with unstable static stability that is equal for ascending and descending motion. This growing mode assumes the characteristics of pseudoadiabatic motion in a conditionally unstable atmosphere due to the coarse vertical resolution of the linear model. Implicit diffusion from the centered finite-difference scheme shifts the preferred growth modes from infinite wavenumbers, characteristic of inviscid analytical solutions, to 4Δλ and 4Δr,ar wavelengths in numerical experiments. Here Δλ and Δr are the angular and radial distances between grid points. Explicit diffusion representing subgrid-scale eddies shifts preferred modes to longer wavelengths. Rotation in the basic state is a necessary condition before the unstable gravity-inertia waves form spiral bands. Rotation also organizes stable perturbations into a banded pattern. Inertial instability and the Coriolis parameter are unimportant for band formation in these linear experiments. The distance between bands increases and the growth rate decreases in experiments in which adiabatic warming occurs with descent and warming due to latent heat release occurs with ascending motion.
Abstract
Results from diagnostic studies of a nonlinear hurricane model support the conclusion that internal gravity-inertia waves are responsible for hurricane rainbands. The mean relative vorticity differed little between the bands and their environment, a characteristic of gravity waves modified slightly by the earth's rotation. Small differences in mean radial and tangential velocity components, divergence, and the radial pressure gradient force were noted between the bands and their environment. The upper layers of the bands were responsible for a small increase in the model storm's kinetic energy due to a net convergence of kinetic energy flux from the environment into the bands. A large net convergence of cyclonic angular momentum flux into the bands occurred in the boundary layer. Conversion of available potential energy to kinetic energy was not significant in the model bands. Finally, latent heating in the bands did not play an important role in the maintenance or propagation of the bands at large radii.
Abstract
Results from diagnostic studies of a nonlinear hurricane model support the conclusion that internal gravity-inertia waves are responsible for hurricane rainbands. The mean relative vorticity differed little between the bands and their environment, a characteristic of gravity waves modified slightly by the earth's rotation. Small differences in mean radial and tangential velocity components, divergence, and the radial pressure gradient force were noted between the bands and their environment. The upper layers of the bands were responsible for a small increase in the model storm's kinetic energy due to a net convergence of kinetic energy flux from the environment into the bands. A large net convergence of cyclonic angular momentum flux into the bands occurred in the boundary layer. Conversion of available potential energy to kinetic energy was not significant in the model bands. Finally, latent heating in the bands did not play an important role in the maintenance or propagation of the bands at large radii.
Abstract
An axisymmetric, multilayer hurricane model is used to investigate the hurricane's response to sudden changes of sea surface temperature (SST). The model contains a parameterization of the planetary boundary layer (PBL) which includes matched formulations for the surface layer and the mixed layer. The heat, moisture and momentum fluxes are mutually dependent through Monin-Obukhov similarity theory.
The height of the model hurricane PEL is 400–500 m, below which the potential temperature and specific humidity are nearly invariant with height. The flow in the hurricane PBL is characterized by subgradient tangential velocities and nearly uniform cross-isobaric flow angles. The sensible heating from the ocean is insignificant, but the evaporation is large. The magnitudes of the equivalent drag coefficients are approximately one-third those of the exchange coefficients for heat and moisture.
As the SST is suddenly decreased (increased), the steady-state model hurricane experiences two stages of modification. The first stage consists of adjustments of the hurricane PBL featuring a weakened (enhanced) dynamic and thermodynamic coupling of the storm with the ocean. No important changes of intensity occur during this stage, which lasts several hours. The decrease (increase) of kinetic energy dissipation offsets part of the decrease (increase) of kinetic energy generation. The second stage is characterized by a steady modification of storm intensity. The fluctuations of intensity in these experiments are less pronounced than those shown by a similar model with a conventional bulk parameterization of the hurricane PBL.
Abstract
An axisymmetric, multilayer hurricane model is used to investigate the hurricane's response to sudden changes of sea surface temperature (SST). The model contains a parameterization of the planetary boundary layer (PBL) which includes matched formulations for the surface layer and the mixed layer. The heat, moisture and momentum fluxes are mutually dependent through Monin-Obukhov similarity theory.
The height of the model hurricane PEL is 400–500 m, below which the potential temperature and specific humidity are nearly invariant with height. The flow in the hurricane PBL is characterized by subgradient tangential velocities and nearly uniform cross-isobaric flow angles. The sensible heating from the ocean is insignificant, but the evaporation is large. The magnitudes of the equivalent drag coefficients are approximately one-third those of the exchange coefficients for heat and moisture.
As the SST is suddenly decreased (increased), the steady-state model hurricane experiences two stages of modification. The first stage consists of adjustments of the hurricane PBL featuring a weakened (enhanced) dynamic and thermodynamic coupling of the storm with the ocean. No important changes of intensity occur during this stage, which lasts several hours. The decrease (increase) of kinetic energy dissipation offsets part of the decrease (increase) of kinetic energy generation. The second stage is characterized by a steady modification of storm intensity. The fluctuations of intensity in these experiments are less pronounced than those shown by a similar model with a conventional bulk parameterization of the hurricane PBL.
Abstract
Observing systems simulation experiments were carried out to estimate the accuracy of temperatures diagnosed from the divergence equation when an army of nearly continuous (in time) wind observations is available. It was found that a useful estimate of temperature can be derived from high-resolution wind observations such as those obtainable from a network of wind profiling systems. Adding the divergence and vertical motion terms to the balance equation to form the complete divergence equation reduces the errors in derived temperatures and geopotential heights. Observations on an irregularly spaced grid lead to greater errors than those on a regularly spaced grid. Moderate errors are also introduced when large-scale errors in geopotential occur in the lateral boundary conditions. This suggests the need for some independent observations of temperature (from rawinsonde or temperature profiler) to prescribe the boundary conditions for the retrieval technique.
In a simulation of a possible operational system in which wind observations with random errors of 1 m s−1 are available on a 350 km grid and boundary values of geopotential height contain errors typical of a 12 h model forecast, the derived temperatures and heights on the interior of the grid contain root-mean-square errors of 1.55°C and 18.8 m, respectively.
Abstract
Observing systems simulation experiments were carried out to estimate the accuracy of temperatures diagnosed from the divergence equation when an army of nearly continuous (in time) wind observations is available. It was found that a useful estimate of temperature can be derived from high-resolution wind observations such as those obtainable from a network of wind profiling systems. Adding the divergence and vertical motion terms to the balance equation to form the complete divergence equation reduces the errors in derived temperatures and geopotential heights. Observations on an irregularly spaced grid lead to greater errors than those on a regularly spaced grid. Moderate errors are also introduced when large-scale errors in geopotential occur in the lateral boundary conditions. This suggests the need for some independent observations of temperature (from rawinsonde or temperature profiler) to prescribe the boundary conditions for the retrieval technique.
In a simulation of a possible operational system in which wind observations with random errors of 1 m s−1 are available on a 350 km grid and boundary values of geopotential height contain errors typical of a 12 h model forecast, the derived temperatures and heights on the interior of the grid contain root-mean-square errors of 1.55°C and 18.8 m, respectively.
In operational numerical weather prediction systems, both observations and numerical models contribute to the skill of the forecast. A simple diagram representing the relative contributions of observations and models to the current level of forecast skill and to the ultimate predictability of atmospheric phenomena is interpreted in this note. The forecast skill of 500 mb heights and an estimate of the ultimate predictability of this variable are used in a quantitative illustration of the diagram.
In operational numerical weather prediction systems, both observations and numerical models contribute to the skill of the forecast. A simple diagram representing the relative contributions of observations and models to the current level of forecast skill and to the ultimate predictability of atmospheric phenomena is interpreted in this note. The forecast skill of 500 mb heights and an estimate of the ultimate predictability of this variable are used in a quantitative illustration of the diagram.
Abstract
A two-dimensional numerical Model with moist physics is used to simulate circulations induced by horizontal variations in surface-moisture availability. The model contains prognostic equations for water vapor, cloud water, and rain water, with a simple parameterization of cloud microphysical processes. Four geometric variations of surface-moisture availability are examined: 1) an edge geometry which includes a land-water contrast (classic sea breeze) and moist land adjacent to dry land (inland sea breeze), 2) a single strip of moist land surrounded by dry land 3) alternating bands of moist and dry land, and 4) a single strip of dry land surrounded by moist land.
For convectively unstable initial conditions with a relative humidity of 50%, lifting associated with the sea-breeze front induces a precipitation system which propagates inland from the coast. The sea-breeze circulation associated with dry land is considerably stronger than that produced by moist land; however, the evaporation over land in the sea-breeze simulation with moist land results in increased rainfall in spite of the weaker circulation. When moist land is located adjacent to dry land, an “inland sea breeze” is generated which is almost as strong as the dry-land sea breeze, and significant precipitation is produced.
In the simulations with a single moist strip surrounded by dry land, two inland sea breezes form and move outward over the dry land. For strips of width 24 and 48 km, the relatively weak circulations fail to produce clouds or precipitation. As the width of the strip increases, however, the increased strength of the inland sea-breeze circulations, together with increased evaporation, results in the formation of precipitation systems, with the amount of precipitation increasing with increasing width of the moist strip.
With alternating bands of dry and moist land, two inland sea-breeze fronts converge toward the center of the dry bands and produce vigorous rainstorms for bandwidths of 96 km and greater. For a given width of moist Land, the bands are more efficient at generating rainfall than a single strip, because of greater evaporation and a constructive interference of the inland see-breeze circulations in the band simulations.
A single strip of dry land of width 144 km and surrounded by moist land produces greater rainfall than either the 144-km moist strip or the 144-km bands, because of the greater total evaporation. The maximum 24-h gridpoint value (6-km average) rainfall in this simulation is 7.93 cm.
The results indicate that inhomogeneities in land moisture on a horizontal scale of 100–200 km can, in a convectively unstable environment with weak environmental flow and sufficient moisture, initiate convective rainfall. They support Anthes' hypothesis that planting bands of vegetation with widths of order 100 km in semiarid regions could under favorable large-scale conditions, produce increases in convective precipitation.
Abstract
A two-dimensional numerical Model with moist physics is used to simulate circulations induced by horizontal variations in surface-moisture availability. The model contains prognostic equations for water vapor, cloud water, and rain water, with a simple parameterization of cloud microphysical processes. Four geometric variations of surface-moisture availability are examined: 1) an edge geometry which includes a land-water contrast (classic sea breeze) and moist land adjacent to dry land (inland sea breeze), 2) a single strip of moist land surrounded by dry land 3) alternating bands of moist and dry land, and 4) a single strip of dry land surrounded by moist land.
For convectively unstable initial conditions with a relative humidity of 50%, lifting associated with the sea-breeze front induces a precipitation system which propagates inland from the coast. The sea-breeze circulation associated with dry land is considerably stronger than that produced by moist land; however, the evaporation over land in the sea-breeze simulation with moist land results in increased rainfall in spite of the weaker circulation. When moist land is located adjacent to dry land, an “inland sea breeze” is generated which is almost as strong as the dry-land sea breeze, and significant precipitation is produced.
In the simulations with a single moist strip surrounded by dry land, two inland sea breezes form and move outward over the dry land. For strips of width 24 and 48 km, the relatively weak circulations fail to produce clouds or precipitation. As the width of the strip increases, however, the increased strength of the inland sea-breeze circulations, together with increased evaporation, results in the formation of precipitation systems, with the amount of precipitation increasing with increasing width of the moist strip.
With alternating bands of dry and moist land, two inland sea-breeze fronts converge toward the center of the dry bands and produce vigorous rainstorms for bandwidths of 96 km and greater. For a given width of moist Land, the bands are more efficient at generating rainfall than a single strip, because of greater evaporation and a constructive interference of the inland see-breeze circulations in the band simulations.
A single strip of dry land of width 144 km and surrounded by moist land produces greater rainfall than either the 144-km moist strip or the 144-km bands, because of the greater total evaporation. The maximum 24-h gridpoint value (6-km average) rainfall in this simulation is 7.93 cm.
The results indicate that inhomogeneities in land moisture on a horizontal scale of 100–200 km can, in a convectively unstable environment with weak environmental flow and sufficient moisture, initiate convective rainfall. They support Anthes' hypothesis that planting bands of vegetation with widths of order 100 km in semiarid regions could under favorable large-scale conditions, produce increases in convective precipitation.
Abstract
Long-term (five-day) integrations of a nonlinear numerical model of the sea breeze at the equator, 20°N, 30°N and 45°N indicate the importance of latitude on the sea breeze circulation. During the hours of strong heating when friction is largest and the static stability is smallest, a local sea-breeze frontal circulation develops in a similar way at all four latitudes. Evaluation of the terms in the circulation theorem indicates the dominance of the solenoid term (horizontal pressure gradient force) associated with the strong temperature contrast during this period. During the rest of the period, however, the pressure gradient and frictional forces weaken, the static stability increases, and the Coriolis force is dominant (except at the equator). Therefore, quite different circulations evolve at the different latitudes. At the equator, the absence of the Coriolis force results in a sea breeze at all times. At the other latitudes, the Coriolis force is responsible for producing the large-scale land breeze. At 20°N, the slower rotation of the horizontal wind after sunset produces a large-scale land breeze that persists until several hours after sunrise. At 30°N, the inertial effects produce a maximum land breeze at about sunrise, and the land breeze is strongest at this latitude. At 45°, the rotational rate of the horizontal wind after sunset is faster, so that the maximum land breeze occurs several hours before sunrise. These results indicate that the Coriolis force may be more important than the reversal of horizontal temperature gradient from day to night in producing large-scale land-scale land breeze away from the equator.
The results pertaining to the large-scale circulation are in general agreement with Rotunno's linear theory, which predicts a fundamentally different behavior of the sea-breeze circulation depending upon whether the Coriolis parameter is greater or less than the frequency of the diurnal heating cycle.
Abstract
Long-term (five-day) integrations of a nonlinear numerical model of the sea breeze at the equator, 20°N, 30°N and 45°N indicate the importance of latitude on the sea breeze circulation. During the hours of strong heating when friction is largest and the static stability is smallest, a local sea-breeze frontal circulation develops in a similar way at all four latitudes. Evaluation of the terms in the circulation theorem indicates the dominance of the solenoid term (horizontal pressure gradient force) associated with the strong temperature contrast during this period. During the rest of the period, however, the pressure gradient and frictional forces weaken, the static stability increases, and the Coriolis force is dominant (except at the equator). Therefore, quite different circulations evolve at the different latitudes. At the equator, the absence of the Coriolis force results in a sea breeze at all times. At the other latitudes, the Coriolis force is responsible for producing the large-scale land breeze. At 20°N, the slower rotation of the horizontal wind after sunset produces a large-scale land breeze that persists until several hours after sunrise. At 30°N, the inertial effects produce a maximum land breeze at about sunrise, and the land breeze is strongest at this latitude. At 45°, the rotational rate of the horizontal wind after sunset is faster, so that the maximum land breeze occurs several hours before sunrise. These results indicate that the Coriolis force may be more important than the reversal of horizontal temperature gradient from day to night in producing large-scale land-scale land breeze away from the equator.
The results pertaining to the large-scale circulation are in general agreement with Rotunno's linear theory, which predicts a fundamentally different behavior of the sea-breeze circulation depending upon whether the Coriolis parameter is greater or less than the frequency of the diurnal heating cycle.
