Search Results
You are looking at 1 - 8 of 8 items for
- Author or Editor: T. J. Garrett x
- Refine by Access: All Content x
Abstract
This paper presents a simple analytical framework for the dynamic response of cirrus to a local radiative flux convergence, expressible in terms of three independent modes of cloud evolution. Horizontally narrow and tenuous clouds within a stable environment adjust to radiative heating by ascending gradually across isentropes while spreading sufficiently fast that isentropic surfaces stay nearly flat. Alternatively, optically dense clouds experience very concentrated heating, and if they are also very broad, they develop a convecting mixed layer. Along-isentropic spreading still occurs, but in the form of turbulent density currents rather than laminar flows. A third adjustment mode relates to evaporation, which erodes cloudy air as it lofts, regardless of its optical density. The dominant mode is determined from two dimensionless numbers, whose predictive power is shown in comparisons with high-resolution numerical cloud simulations. The power and simplicity of the approach hints that fast, subgrid-scale radiative–dynamic atmospheric interactions might be efficiently parameterized within slower, coarse-grid climate models.
Abstract
This paper presents a simple analytical framework for the dynamic response of cirrus to a local radiative flux convergence, expressible in terms of three independent modes of cloud evolution. Horizontally narrow and tenuous clouds within a stable environment adjust to radiative heating by ascending gradually across isentropes while spreading sufficiently fast that isentropic surfaces stay nearly flat. Alternatively, optically dense clouds experience very concentrated heating, and if they are also very broad, they develop a convecting mixed layer. Along-isentropic spreading still occurs, but in the form of turbulent density currents rather than laminar flows. A third adjustment mode relates to evaporation, which erodes cloudy air as it lofts, regardless of its optical density. The dominant mode is determined from two dimensionless numbers, whose predictive power is shown in comparisons with high-resolution numerical cloud simulations. The power and simplicity of the approach hints that fast, subgrid-scale radiative–dynamic atmospheric interactions might be efficiently parameterized within slower, coarse-grid climate models.
Abstract
The Gerber Scientific, Inc. Particle Volume Monitor (PVM) is widely used to measure the liquid water content (LWC) and droplet effective radius (r eff) of water clouds. The LWC response of the airborne version of this instrument, the PVM-100A, was evaluated in two independent wet wind tunnel experiments under well-controlled conditions. Earlier studies predict that the PVM-100 (the ground-based version of the PVM) theoretical response to monodisperse droplets diminishes for droplet diameters larger than about 40 μm. However, results from the wind tunnel experiments presented in this paper show that the response of the PVM-100A to monodisperse droplets begins to decrease when the droplet diameter is between 20- and 30-μm diameter. For polydisperse droplet populations (such as those found in natural clouds) the efficiency of the airborne PVM-100A for sensing LWC begins to decrease when the median volume diameter (MVD) of the droplet size distribution is above 20 μm, falling to 50% efficiency for an MVD of 50 μm. Therefore, measurements of LWC obtained from the PVM-100A in natural clouds with broad droplet size distributions (and large values of MVD) should be treated with caution.
Abstract
The Gerber Scientific, Inc. Particle Volume Monitor (PVM) is widely used to measure the liquid water content (LWC) and droplet effective radius (r eff) of water clouds. The LWC response of the airborne version of this instrument, the PVM-100A, was evaluated in two independent wet wind tunnel experiments under well-controlled conditions. Earlier studies predict that the PVM-100 (the ground-based version of the PVM) theoretical response to monodisperse droplets diminishes for droplet diameters larger than about 40 μm. However, results from the wind tunnel experiments presented in this paper show that the response of the PVM-100A to monodisperse droplets begins to decrease when the droplet diameter is between 20- and 30-μm diameter. For polydisperse droplet populations (such as those found in natural clouds) the efficiency of the airborne PVM-100A for sensing LWC begins to decrease when the median volume diameter (MVD) of the droplet size distribution is above 20 μm, falling to 50% efficiency for an MVD of 50 μm. Therefore, measurements of LWC obtained from the PVM-100A in natural clouds with broad droplet size distributions (and large values of MVD) should be treated with caution.
Abstract
Mammatus clouds are the pouchlike lobes seen hanging from mid- to high-level clouds. They can look quite dramatic, but they are also interesting because they provide clues to what controls anvil cirrus dynamic evolution. Thus far, the most commonly accepted explanation for observed subsidence of mammatus lobes is that they are driven by evaporative cooling of precipitation, accelerated by mixing with dry subcloud air. Here, an alternative explanation is proposed: radiative temperature contrasts between cloud base and the lower troposphere destabilize cloudy air to create a rapidly deepening mixed layer, which creates positively buoyant intrusions of dry air into the cloud interior; mammatus lobes are just the descending branch of the resulting circulations. In this regard, mammatus cloud fields might be considered an upside-down analog to the radiatively driven formation of “cloud holes” seen at the tops of stratocumulus layers.
Abstract
Mammatus clouds are the pouchlike lobes seen hanging from mid- to high-level clouds. They can look quite dramatic, but they are also interesting because they provide clues to what controls anvil cirrus dynamic evolution. Thus far, the most commonly accepted explanation for observed subsidence of mammatus lobes is that they are driven by evaporative cooling of precipitation, accelerated by mixing with dry subcloud air. Here, an alternative explanation is proposed: radiative temperature contrasts between cloud base and the lower troposphere destabilize cloudy air to create a rapidly deepening mixed layer, which creates positively buoyant intrusions of dry air into the cloud interior; mammatus lobes are just the descending branch of the resulting circulations. In this regard, mammatus cloud fields might be considered an upside-down analog to the radiatively driven formation of “cloud holes” seen at the tops of stratocumulus layers.
Abstract
The purpose of the Tropical Air–Sea Propagation Study (TAPS), which was conducted during November–December 2013, was to gather coordinated atmospheric and radio frequency (RF) data, offshore of northeastern Australia, in order to address the question of how well radio wave propagation can be predicted in a clear-air, tropical, littoral maritime environment. Spatiotemporal variations in vertical gradients of the conserved thermodynamic variables found in surface layers, mixing layers, and entrainment layers have the potential to bend or refract RF energy in directions that can either enhance or limit the intended function of an RF system. TAPS facilitated the collaboration of scientists and technologists from the United Kingdom, the United States, France, New Zealand, and Australia, bringing together expertise in boundary layer meteorology, mesoscale numerical weather prediction (NWP), and RF propagation. The focus of the study was on investigating for the first time in a tropical, littoral environment the i) refractivity structure in the marine and coastal inland boundary layers; ii) the spatial and temporal behavior of momentum, heat, and moisture fluxes; and iii) the ability of propagation models seeded with refractive index functions derived from blended NWP and surface-layer models to predict the propagation of radio wave signals of ultrahigh frequency (UHF; 300 MHz–3 GHz), super-high frequency (SHF; 3–30 GHz), and extremely high frequency (EHF; 30–300 GHz).
Coordinated atmospheric and RF measurements were made using a small research aircraft, slow-ascent radiosondes, lidar, flux towers, a kitesonde, and land-based transmitters. The use of a ship as an RF-receiving platform facilitated variable-range RF links extending to distances of 80 km from the mainland. Four high-resolution NWP forecasting systems were employed to characterize environmental variability. This paper provides an overview of the TAPS experimental design and field campaign, including a description of the unique data that were collected, preliminary findings, and the envisaged interpretation of the results.
Abstract
The purpose of the Tropical Air–Sea Propagation Study (TAPS), which was conducted during November–December 2013, was to gather coordinated atmospheric and radio frequency (RF) data, offshore of northeastern Australia, in order to address the question of how well radio wave propagation can be predicted in a clear-air, tropical, littoral maritime environment. Spatiotemporal variations in vertical gradients of the conserved thermodynamic variables found in surface layers, mixing layers, and entrainment layers have the potential to bend or refract RF energy in directions that can either enhance or limit the intended function of an RF system. TAPS facilitated the collaboration of scientists and technologists from the United Kingdom, the United States, France, New Zealand, and Australia, bringing together expertise in boundary layer meteorology, mesoscale numerical weather prediction (NWP), and RF propagation. The focus of the study was on investigating for the first time in a tropical, littoral environment the i) refractivity structure in the marine and coastal inland boundary layers; ii) the spatial and temporal behavior of momentum, heat, and moisture fluxes; and iii) the ability of propagation models seeded with refractive index functions derived from blended NWP and surface-layer models to predict the propagation of radio wave signals of ultrahigh frequency (UHF; 300 MHz–3 GHz), super-high frequency (SHF; 3–30 GHz), and extremely high frequency (EHF; 30–300 GHz).
Coordinated atmospheric and RF measurements were made using a small research aircraft, slow-ascent radiosondes, lidar, flux towers, a kitesonde, and land-based transmitters. The use of a ship as an RF-receiving platform facilitated variable-range RF links extending to distances of 80 km from the mainland. Four high-resolution NWP forecasting systems were employed to characterize environmental variability. This paper provides an overview of the TAPS experimental design and field campaign, including a description of the unique data that were collected, preliminary findings, and the envisaged interpretation of the results.
Abstract
This paper presents a detailed study of a single thunderstorm anvil cirrus cloud measured on 21 July 2002 near southern Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). NASA WB-57F and University of North Dakota Citation aircraft tracked the microphysical and radiative development of the anvil for 3 h. Measurements showed that the cloud mass that was advected downwind from the thunderstorm was separated vertically into two layers: a cirrus anvil with cloud-top temperatures of −45°C lay below a second, thin tropopause cirrus (TTC) layer with the same horizontal dimensions as the anvil and temperatures near −70°C. In both cloud layers, ice crystals smaller than 50 μm across dominated the size distributions and cloud radiative properties. In the anvil, ice crystals larger than 50 μm aggregated and precipitated while small ice crystals increasingly dominated the size distributions; as a consequence, measured ice water contents and ice crystal effective radii decreased with time. Meanwhile, the anvil thinned vertically and maintained a stratification similar to its environment. Because effective radii were small, radiative heating and cooling were concentrated in layers approximately 100 m thick at the anvil top and base. A simple analysis suggests that the anvil cirrus spread laterally because mixing in these radiatively driven layers created horizontal pressure gradients between the cloud and its stratified environment. The TTC layer also spread but, unlike the anvil, did not dissipate—perhaps because the anvil shielded the TTC from terrestrial infrared heating. Calculations of top-of-troposphere radiative forcing above the anvil and TTC showed strong cooling that tapered as the anvil evolved.
Abstract
This paper presents a detailed study of a single thunderstorm anvil cirrus cloud measured on 21 July 2002 near southern Florida during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). NASA WB-57F and University of North Dakota Citation aircraft tracked the microphysical and radiative development of the anvil for 3 h. Measurements showed that the cloud mass that was advected downwind from the thunderstorm was separated vertically into two layers: a cirrus anvil with cloud-top temperatures of −45°C lay below a second, thin tropopause cirrus (TTC) layer with the same horizontal dimensions as the anvil and temperatures near −70°C. In both cloud layers, ice crystals smaller than 50 μm across dominated the size distributions and cloud radiative properties. In the anvil, ice crystals larger than 50 μm aggregated and precipitated while small ice crystals increasingly dominated the size distributions; as a consequence, measured ice water contents and ice crystal effective radii decreased with time. Meanwhile, the anvil thinned vertically and maintained a stratification similar to its environment. Because effective radii were small, radiative heating and cooling were concentrated in layers approximately 100 m thick at the anvil top and base. A simple analysis suggests that the anvil cirrus spread laterally because mixing in these radiatively driven layers created horizontal pressure gradients between the cloud and its stratified environment. The TTC layer also spread but, unlike the anvil, did not dissipate—perhaps because the anvil shielded the TTC from terrestrial infrared heating. Calculations of top-of-troposphere radiative forcing above the anvil and TTC showed strong cooling that tapered as the anvil evolved.
Abstract
Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks.
This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.
Abstract
Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks.
This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.
An overview is given of the First ISCCP Regional Experiment Arctic Clouds Experiment that was conducted during April–July 1998. The principal goal of the field experiment was to gather the data needed to examine the impact of arctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate model parameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, and understanding of cloud–radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew over surface-based observational sites in the Arctic Ocean and at Barrow, Alaska. This paper describes the programmatic and scientific objectives of the project, the experimental design (including research platforms and instrumentation), the conditions that were encountered during the field experiment, and some highlights of preliminary observations, modeling, and satellite remote sensing studies.
An overview is given of the First ISCCP Regional Experiment Arctic Clouds Experiment that was conducted during April–July 1998. The principal goal of the field experiment was to gather the data needed to examine the impact of arctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate model parameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, and understanding of cloud–radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew over surface-based observational sites in the Arctic Ocean and at Barrow, Alaska. This paper describes the programmatic and scientific objectives of the project, the experimental design (including research platforms and instrumentation), the conditions that were encountered during the field experiment, and some highlights of preliminary observations, modeling, and satellite remote sensing studies.
Abstract
In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
Abstract
In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
