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- Author or Editor: H. E. Gerber x
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Abstract
The relative humidity, temperature, droplet size distribution and transmittance of light at 632.8 nm were measured in a radiation fog. A new saturation hygrometer capable of measuring relative humidity between ∼95 and 105%, was used for the first time. Excursions of relative humidity into the supersaturation regime were brief and rapid. Estimated peak values of supersaturation were several tenths of 1%. The mean relative humidity in the fog was <100%. The importance of turbulence in controlling the fog was evident in the rapid fluctuations in all the measured variables, and in the fog's quasi-periodic oscillations which had a mean period of 18 min. Evidence suggested that turbulent mixing of nearly saturated eddies was the cause of fog formation and the broadening of the droplet size distribution.
Abstract
The relative humidity, temperature, droplet size distribution and transmittance of light at 632.8 nm were measured in a radiation fog. A new saturation hygrometer capable of measuring relative humidity between ∼95 and 105%, was used for the first time. Excursions of relative humidity into the supersaturation regime were brief and rapid. Estimated peak values of supersaturation were several tenths of 1%. The mean relative humidity in the fog was <100%. The importance of turbulence in controlling the fog was evident in the rapid fluctuations in all the measured variables, and in the fog's quasi-periodic oscillations which had a mean period of 18 min. Evidence suggested that turbulent mixing of nearly saturated eddies was the cause of fog formation and the broadening of the droplet size distribution.
Abstract
A new saturation hygrometer capable of measuring relative humidity between 95 and 105% is described. The hygrometer uses a thermally thin mirror on which condensation is controlled by limiting the exchange of ambient water vapor to condensation sites consisting of sized submicron salt nuclei. The salt-solution droplets formed on the nuclei are prevented from exceeding a reference size by heating the mirror with infrared diodes which are part of a thermo-optical servo system. The temperature increase of the mirror is directly related to the ambient relative humidity.
The performance of the hygrometer is modeled for the hypothetical case where the instrument is imbedded in an updraft which passes through the base and the lower portion of a typical continental cumulus cloud. The influences of the thermal properties of the mirror, the heat conduction to the mirror support, the radiative energy exchange between the mirror and the environment, the size of the salt nuclei on the mirror, and the interaction with the cloud droplets are evaluated. The hygrometer is tested in the laboratory by measuring temperature increases of the mirror for infrared-heater outputs, and by exposing it to supersaturations produced by a continuous-flow thermal-gradient diffusion chamber.
The theoretical results, the stability of the hygrometer under conditions of supersaturation, its insensitivity to ambient aerosols, and its ease of calibration together indicate that the hygrometer can be developed into a practical sensor for atmospheric use.
Abstract
A new saturation hygrometer capable of measuring relative humidity between 95 and 105% is described. The hygrometer uses a thermally thin mirror on which condensation is controlled by limiting the exchange of ambient water vapor to condensation sites consisting of sized submicron salt nuclei. The salt-solution droplets formed on the nuclei are prevented from exceeding a reference size by heating the mirror with infrared diodes which are part of a thermo-optical servo system. The temperature increase of the mirror is directly related to the ambient relative humidity.
The performance of the hygrometer is modeled for the hypothetical case where the instrument is imbedded in an updraft which passes through the base and the lower portion of a typical continental cumulus cloud. The influences of the thermal properties of the mirror, the heat conduction to the mirror support, the radiative energy exchange between the mirror and the environment, the size of the salt nuclei on the mirror, and the interaction with the cloud droplets are evaluated. The hygrometer is tested in the laboratory by measuring temperature increases of the mirror for infrared-heater outputs, and by exposing it to supersaturations produced by a continuous-flow thermal-gradient diffusion chamber.
The theoretical results, the stability of the hygrometer under conditions of supersaturation, its insensitivity to ambient aerosols, and its ease of calibration together indicate that the hygrometer can be developed into a practical sensor for atmospheric use.
Abstract
The bulk single-scattering albedo of atmospheric particles was determined by simultaneously measuring the optical transmittance and the scattering of light in a multipass cell filled with concentrated ambient aerosol. During a naval research cruise in North Atlantic and the Mediterranean, results were obtained which suggested that the absorption of red light by the maritime aerosol was dominated by particles whose origin was the European continent. Absorption extremes approached 50% of light extinction near industrialized harbor areas. Particle size distributions and albedos measured during the cruise were inverted with Mie theory to give the imaginary part of the particles' refractive index. The mean of the imaginary index for the cruise was 0.0224, which is typical of rural continental areas.
Abstract
The bulk single-scattering albedo of atmospheric particles was determined by simultaneously measuring the optical transmittance and the scattering of light in a multipass cell filled with concentrated ambient aerosol. During a naval research cruise in North Atlantic and the Mediterranean, results were obtained which suggested that the absorption of red light by the maritime aerosol was dominated by particles whose origin was the European continent. Absorption extremes approached 50% of light extinction near industrialized harbor areas. Particle size distributions and albedos measured during the cruise were inverted with Mie theory to give the imaginary part of the particles' refractive index. The mean of the imaginary index for the cruise was 0.0224, which is typical of rural continental areas.
Abstract
Polydisperse NACl and (NH4)2SO4 salt particles were passed through a Goetz centrifuge, which transmitted only those particles smaller than the cutoff size of the centrifuge. In one set of experiments the sample from the centrifuge was passed directly into the thermal gradient cloud chamber and the threshold supersaturation for activating nuclei was recorded. In the second set of experiments the output sample from the centrifuge was passed through a mobility analyzer which transmitted a nearly monodisperse aerosol of a known size to the thermal-gradient cloud chamber. In the first experiment the threshold supersaturation agreed well with the critical supersaturation predicted by theory for particles of radii equal to the cutoff size of the Goetz centrifuge. Likewise, the critical supersaturation found for the nearly monodisperse aerosols transmitted through the mobility analyzer agreed well with that predicted by theory.
Abstract
Polydisperse NACl and (NH4)2SO4 salt particles were passed through a Goetz centrifuge, which transmitted only those particles smaller than the cutoff size of the centrifuge. In one set of experiments the sample from the centrifuge was passed directly into the thermal gradient cloud chamber and the threshold supersaturation for activating nuclei was recorded. In the second set of experiments the output sample from the centrifuge was passed through a mobility analyzer which transmitted a nearly monodisperse aerosol of a known size to the thermal-gradient cloud chamber. In the first experiment the threshold supersaturation agreed well with the critical supersaturation predicted by theory for particles of radii equal to the cutoff size of the Goetz centrifuge. Likewise, the critical supersaturation found for the nearly monodisperse aerosols transmitted through the mobility analyzer agreed well with that predicted by theory.
Abstract
Pairs of buoyant elements, or thermals, are found to merge very rapidly when they are close enough for extensive overlapping of their buoyancy force fields. When this occurs, the merged pair behaves in every respect as a single element with twice the buoyancy of a solitary thermal. In the absence of merging, however, the lateral interaction suppresses the circulation and velocity of rise of both clouds. In addition to these effects, the overlapping buoyancy force field appears to favor coalescence of the clouds, whereas the contacting and interfering fields of toroidal circulation cause a mutual repulsion.
Abstract
Pairs of buoyant elements, or thermals, are found to merge very rapidly when they are close enough for extensive overlapping of their buoyancy force fields. When this occurs, the merged pair behaves in every respect as a single element with twice the buoyancy of a solitary thermal. In the absence of merging, however, the lateral interaction suppresses the circulation and velocity of rise of both clouds. In addition to these effects, the overlapping buoyancy force field appears to favor coalescence of the clouds, whereas the contacting and interfering fields of toroidal circulation cause a mutual repulsion.
Abstract
Infrared digital data from geostationary satellites were used to demonstrate the usefulness of remotely sensed surface temperature data to delineate microscale and mesoscale climates. Nocturnal winter data (December-February) from 1976–77 to 1978–79 over Florida revealed noticeable contrasts in surface temperature patterns. Colder areas were associated with low soil moisture content in the upper layers of excessively-drained and well-drained sandy soils, whereas warmer areas were associated with bodies of water, wetlands, or poorly drained soils. An unexpected surface temperature pattern for one night where the north-central Florida climatic zone was colder than the north Florida climatic zone was found to be caused by differences in antecedent frontal rainfall. Differences in surface radiant energy fluxes over these two areas at 0200 EST 20 December 1977, based on average satellite-sensed surface temperatures, were compared with differences in soil heat fluxes that were computed from 1.5 m climatological temperatures and soil thermal properties by use of a simplified surface energy balance equation. The difference in computed soil heat fluxes was in reasonable agreement with the difference in radiant energy fluxes from the surface of the two areas. It was therefore concluded that this method could be used to compute differences in thermal inertia of the surface layer of different areas based on satellite and climatological temperature data.
Abstract
Infrared digital data from geostationary satellites were used to demonstrate the usefulness of remotely sensed surface temperature data to delineate microscale and mesoscale climates. Nocturnal winter data (December-February) from 1976–77 to 1978–79 over Florida revealed noticeable contrasts in surface temperature patterns. Colder areas were associated with low soil moisture content in the upper layers of excessively-drained and well-drained sandy soils, whereas warmer areas were associated with bodies of water, wetlands, or poorly drained soils. An unexpected surface temperature pattern for one night where the north-central Florida climatic zone was colder than the north Florida climatic zone was found to be caused by differences in antecedent frontal rainfall. Differences in surface radiant energy fluxes over these two areas at 0200 EST 20 December 1977, based on average satellite-sensed surface temperatures, were compared with differences in soil heat fluxes that were computed from 1.5 m climatological temperatures and soil thermal properties by use of a simplified surface energy balance equation. The difference in computed soil heat fluxes was in reasonable agreement with the difference in radiant energy fluxes from the surface of the two areas. It was therefore concluded that this method could be used to compute differences in thermal inertia of the surface layer of different areas based on satellite and climatological temperature data.
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.
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.
Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.
Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.
