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An observational study of the energetics and dynamical aspects of GATE cloud clustersWang, Jough-tai 21 November 1986 (has links)
Thermodynamical and dynamical aspects of tropical cloud
clusters are studied using data from the GARP Atlantic Tropical
Experiment (GATE). The data set used in this study is a
three-dimensional gridded set of upper-air analyses constructed by
Ooyama and Chu (Hurricane Research Division, AOML/NOAA and
SSEC-University of Wisconsin) for wind data and Esbensen (Oregon
State University) for thermodynamic data. The energy and momentum
budgets are estimated on the scale of large cloud clusters.
A strong upper-tropospheric heat source and middle-tropospheric
drying are characteristic features of the mature stage
of the observed cloud clusters. The heat source, moisture sink and
the virtual heat flux for cloud clusters are larger than the
corresponding quantities from GATE easterly-wave composites. The
surface precipitation estimates produced from the vertically
integrated moisture budget are consistent with direct observations.
From the momentum budget study, the following conclusions are
drawn concerning the cumulus momentum effects. In the growing
stage, the mesoscale and cumulus scale effect tends to: 1) provide
a vertically integrated net sink for westerly momentum around the
cluster center; 2) induce a convergent circulation in the lower
layer. In the mature stage, the effects are to: 1) induce a
divergent circulation in the upper layer and maintain a vorticity
couplet pattern; 2) maintain a weak convergent circulation in the
lower layer; and 3) cause a relatively weak easterly acceleration
in the upper layer at the center. A hypothesis is postulated to
illustrate the convective dynamical effects.
A simple barotropic non-divergent model was constructed to
investigate the large-scale response to the hypothesized cumulus
momentum forcing similar to that found in the GATE cloud-cluster
momentum budget. The numerical results show that the cumulus
momentum forcing is a plausible kinetic energy source for the
mesoscale wavenumber spectrum. The sporadic nature of the
convective mass flux does not have a significant effect on the
large-scale dynamical response for physically realistic parameters
in a barotropic non-divergent dynamical system. / Graduation date: 1987
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Properties of low-level marine clouds as deduced from advanced very high resolution radiometer satellite observationsChang, Fu-Lung 05 August 1997 (has links)
A radiation model was developed for retrieving cloud visible optical depth,
droplet effective radius, and cloud top emission temperature using AVHRR satellite
observations at 0.63, 3.7, and 11 ��m. The model was used to determine the sensitivity
of the retrieved properties to various approximations often employed in such retrievals.
Droplet effective radius appears to be the most sensitive to the commonly used
approximations. Cloud properties retrieved using a 16-stream scheme were within ��5%
of those retrieved using a 148-stream scheme. Cloud properties retrieved using double
Henyey-Greenstein phase functions were within ��10% of those retrieved using Mie
scattering. The retrieved cloud properties were used to investigate biases that arise when
partly cloudy pixels were assumed to be overcast and biases that arise due to oblique
satellite view angles. On average, cloud visible optical depths retrieved for partly cloudy
pixels were 40-60% of those retrieved for overcast pixels. Likewise, cloud liquid water
paths were 30-50%, droplet effective radii were 1-3 ��m smaller, and cloud top emission
temperatures were 2-4K larger. Cloud visible optical depths retrieved at 60�� satellite
zenith angles were 60-70% of those retrieved at nadir. The retrieved droplet effective
radii and cloud top emission temperatures varied little with changing satellite zenith
angle. For March 1989, cloud optical depths and cloud emission temperatures retrieved
for pixels overcast by single-layer, low-level clouds were negatively correlated. Cloud
optical depth, liquid water path, and droplet effective radius were positively correlated
with the sea surface-cloud top temperature difference.
The retrieved cloud properties were also compared for the spatial coherence,
CLAVR (Clouds from AVHRR), and a threshold method based on International Satellite
Cloud Climatology Project procedures. For regions containing single-layered cloud
systems, fractional cloud cover and cloud brightness temperatures derived by the
ISCCP-like threshold method were systematically larger than those derived by the
spatial coherence method, whereas cloud reflectivities were systematically smaller.
Cloud reflectivities and brightness temperatures derived by CLAVR and the spatial
coherence method were in better agreement. / Graduation date: 1998
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The study of cirrus clouds using airborne and satellite dataMeyer, Kerry Glynne 30 September 2004 (has links)
Cirrus clouds are known to play a key role in the earth's radiation budget, yet are one of the most uncertain components of the earth-atmosphere system. With the development of instruments such as the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and the Moderate-resolution Infrared Spectroradiometer (MODIS), scientists now have an unprecedented ability to study cirrus clouds. To aid in the understanding of such clouds, a significant study of cirrus radiative properties has been undertaken. This research is composed of three parts: 1) the retrieval of tropical cirrus optical thickness using MODIS level-1b calibrated radiance data, 2) a survey of tropical cirrus cloud cover, including seasonal variations, using MODIS level-3 global daily gridded data, and 3) the simultaneous retrieval of cirrus optical thickness and ice crystal effective diameter using AVIRIS reflectance measurements.
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Effects of aerosols on deep convective cumulus cloudsFan, Jiwen 15 May 2009 (has links)
This work investigates the effects of anthropogenic aerosols on deep convective clouds and the associated radiative forcing in the Houston area. The Goddard Cumulus Ensemble model (GCE) coupled with a spectral-bin microphysics is employed to investigate the aerosol effects on clouds and precipitation. First, aerosol indirect effects on clouds are separately investigated under different aerosol compositions, concentrations and size distributions. Then, an updated GCE model coupled with the radiative transfer and land surface processes is employed to investigate the aerosol radiative effects on deep convective clouds. The cloud microphysical and macrophysical properties change considerably with the aerosol properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols decreases gradually, leading to a decrease in the cloud droplet number concentration (CDNC) and an increase in the droplet size. Ice processes are more sensitive to the changes of aerosol chemical properties than the warm rain processes. The most noticeable effect of increasing aerosol number concentrations is an increase of CDNC and cloud water content but a decrease in droplet size. It is indicated that the aerosol indirect effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effects on clouds are strongly dependent on RH: the effect is very significant in humid air. Aerosol radiative effects (ARE) on clouds are very pronounced for mid-visible single-scattering albedo (SSA) of 0.85. Relative to the case without the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. The daytime-mean direct forcing is about 2.2 W m-2 at the TOA and -17.4 W m-2 at the surface. The semi-direct forcing is positive, about 10 and 11.2 W m-2 at the TOA and surface, respectively. Aerosol direct and semi-direct effects are very sensitive to SSA. The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.
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The frequency of tropical precipitating clouds as observed by the TRMM PR and ICESat/GLASCasey, Sean Patrick 02 June 2009 (has links)
Convective clouds in the tropics can be grouped into three categories: shallow clouds with cloud-top heights near 2 km above the surface, mid-level congestus clouds with tops near the 0°C level, and deep convective clouds capped by the tropopause. This trimodal distribution is visible in cloud data from the Geoscience Laser Altimeter System (GLAS), carried aboard the Ice, Cloud, and land Elevation Satellite (ICESat), as well as in precipitation data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Fractional areal coverage (FAC) data is calculated at each of the three levels to describe how often optically thick clouds or precipitation are seen at each level. By dividing the FAC of TRMM PR-observed precipitation by the FAC of thick GLAS/ICESat-observed clouds, the fraction of clouds that are precipitating is derived. The tropical mean precipitating cloud fraction is low: 3.7% for shallow clouds, 6.5% for mid-level clouds, and 24.1% for deep clouds. On a regional basis, the FAC maps created in this study show interesting trends. The presence of nonphysical answers in the PCF graphs, however, suggest that greater study with more precise instruments is needed to properly understand the true precipitating cloud fraction of the tropical atmosphere.
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Freezing and Optical Properties of Model Atmospheric AerosolsEarle, Michael Elliot January 2007 (has links)
The freezing of model atmospheric aerosols – specifically, model cirrus cloud particles – was investigated through laboratory studies of supercooled water aerosols. Water droplets with radii of 1 – 2.7 µm were exposed to well-defined temperature profiles ranging from 240 – 230 K in a cryogenic flow tube apparatus, and observed using infrared extinction spectroscopy. A computational characterization procedure, based on theories of light scattering, was used to determine the size and phase composition of aerosols from extinction spectra. The procedure showed large ice fractions at uncharacteristically warm temperatures, which was attributed to the formation of ordered, “ice-like” clusters of molecules in supercooled water. Temperature-dependent complex indices of refraction were determined from the supercooled water extinction spectra, and showed changes reflecting this ordered formation. Taking the “ice-like” character of clusters into account, the homogeneous nucleation point for micrometre-sized water aerosols was determined to be 236.2 K. A microphysical model was developed to determine temperature-dependent, volume- and surface-based homogeneous nucleation rates from experimental freezing data. The model results indicated that surface nucleation was the dominant process for our range of experimental conditions. This was supported by separate studies of smaller, 0.63 and 0.75 µm radius aerosols, with larger surface-to-volume ratios. An optical microscopy apparatus was placed in the cryogenic flow tube to allow real-time imaging of particles in freezing experiments. The imaging studies demonstrated the utility of the microscopy apparatus for the observation and classification of ice crystal habits. Ray tracing and image processing algorithms were used to analyze particle geometry and size. The latter was used to validate the size retrievals from the aerosol characterization procedure. Additional studies probed the changes in the optical properties of crystalline ammonium sulfate, (NH₄)₂SO₄, due to the paraelectric-to-ferroelectric transition at 223 K. Temperature- dependent refractive indices were determined from crystalline (NH₄)₂SO₄ extinction spectra. Only small changes in these values were observed down to 223 K, below which significant changes were observed, due to the changes in lattice structure accompanying the ferroelectric transition.
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Star Formation in Molecular Clouds Associated with HII RegionsAzimlu Shanjani, Mohaddesseh January 2009 (has links)
We have studied the properties of molecular clouds and the stellar population associated with 10 H II regions. We used the James Clerk Maxwell Telescope (JCMT) to make 12CO(2-1) maps in order to study the structure of the cloud and to identify the dense clumps within the cloud. In half of our sources we found that molecular gas appears to have been pushed and compressed into the shells around the expanding ionized gas and fragmented into clumps. Most of these clumps have higher temperature and density compared to the other clumps within the mapped regions. We made pointed observations in 13CO(2-1) and CS(5-4) at the peak of 12CO(2-1) within each clump to measure and calculate the physical properties of the clumps such as line width, excitation temperature, density and mass. Two gas components were selected in the cloud associated with S175 to investigate the influence of the H II region on the molecular gas: S175A is adjacent to the ionization fronts and probably affected by the expanding H II region while S175B is too distant to be affected. Contrary to our expectation S175B was a turbulent region with broadened line profiles. We made a sub-map in 12CO(3-2) using HARP at the JCMT to search for the source of turbulence and identified a proto-stellar outflow in S175B.
We examined the relationship between gas parameters derived for the clumps within the entire sample. The identified clumps were found to be divided into two categories: “type I” sources in which we can find a relationship between size and line width and “type II” sources where there is no relation. We found
that the power law indices for type I sources are generally larger than the previous studies. Larger line widths and consequently larger indices seems to be an initial environmental characteristic of massive star forming regions
We found that mass and column density increase with line width for both type I and type II sources. We did not find any relation between the size and column density. The influence of the H II region on temperature and line widths was examined and we found that the temperature decreases with distance from the ionized fronts but no change was found for the line width. Although most of the clumps within the compressed shells around the H II region have generally larger line widths, from this test we may conclude that the internal dynamics of the cloud beyond the compressed shells is not much influenced by the expanding H II region.
Finally, our near IR study of the stellar populations using 2MASS data, shows that in half of the regions the exciting star belongs to a cluster. We also found that star formation is consistent with triggering by the expansion of the ionized gas in some of sources in our sample. At least two young embedded clusters have been identified at the same position as the dense clumps within fragmented shells around H II regions. These clumps have high temperature and density and large line widths. We identify some other hot and dense clumps very similar in molecular gas properties as candidates of cluster or massive star formation.
Most of the active star forming regions associated with H II regions have a
population of massive newborn stars compared to a star forming cloud which is distant from the massive star and the ionized gas. We conclude that more massive stars form in the molecular cloud at the peripheries of H II regions but it is not clear f this is a result of the initial conditions that have formed the massive, exciting star of the H II region or a feedback of the massive star itself and the expanding H II region.
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Freezing and Optical Properties of Model Atmospheric AerosolsEarle, Michael Elliot January 2007 (has links)
The freezing of model atmospheric aerosols – specifically, model cirrus cloud particles – was investigated through laboratory studies of supercooled water aerosols. Water droplets with radii of 1 – 2.7 µm were exposed to well-defined temperature profiles ranging from 240 – 230 K in a cryogenic flow tube apparatus, and observed using infrared extinction spectroscopy. A computational characterization procedure, based on theories of light scattering, was used to determine the size and phase composition of aerosols from extinction spectra. The procedure showed large ice fractions at uncharacteristically warm temperatures, which was attributed to the formation of ordered, “ice-like” clusters of molecules in supercooled water. Temperature-dependent complex indices of refraction were determined from the supercooled water extinction spectra, and showed changes reflecting this ordered formation. Taking the “ice-like” character of clusters into account, the homogeneous nucleation point for micrometre-sized water aerosols was determined to be 236.2 K. A microphysical model was developed to determine temperature-dependent, volume- and surface-based homogeneous nucleation rates from experimental freezing data. The model results indicated that surface nucleation was the dominant process for our range of experimental conditions. This was supported by separate studies of smaller, 0.63 and 0.75 µm radius aerosols, with larger surface-to-volume ratios. An optical microscopy apparatus was placed in the cryogenic flow tube to allow real-time imaging of particles in freezing experiments. The imaging studies demonstrated the utility of the microscopy apparatus for the observation and classification of ice crystal habits. Ray tracing and image processing algorithms were used to analyze particle geometry and size. The latter was used to validate the size retrievals from the aerosol characterization procedure. Additional studies probed the changes in the optical properties of crystalline ammonium sulfate, (NH₄)₂SO₄, due to the paraelectric-to-ferroelectric transition at 223 K. Temperature- dependent refractive indices were determined from crystalline (NH₄)₂SO₄ extinction spectra. Only small changes in these values were observed down to 223 K, below which significant changes were observed, due to the changes in lattice structure accompanying the ferroelectric transition.
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Star Formation in Molecular Clouds Associated with HII RegionsAzimlu Shanjani, Mohaddesseh January 2009 (has links)
We have studied the properties of molecular clouds and the stellar population associated with 10 H II regions. We used the James Clerk Maxwell Telescope (JCMT) to make 12CO(2-1) maps in order to study the structure of the cloud and to identify the dense clumps within the cloud. In half of our sources we found that molecular gas appears to have been pushed and compressed into the shells around the expanding ionized gas and fragmented into clumps. Most of these clumps have higher temperature and density compared to the other clumps within the mapped regions. We made pointed observations in 13CO(2-1) and CS(5-4) at the peak of 12CO(2-1) within each clump to measure and calculate the physical properties of the clumps such as line width, excitation temperature, density and mass. Two gas components were selected in the cloud associated with S175 to investigate the influence of the H II region on the molecular gas: S175A is adjacent to the ionization fronts and probably affected by the expanding H II region while S175B is too distant to be affected. Contrary to our expectation S175B was a turbulent region with broadened line profiles. We made a sub-map in 12CO(3-2) using HARP at the JCMT to search for the source of turbulence and identified a proto-stellar outflow in S175B.
We examined the relationship between gas parameters derived for the clumps within the entire sample. The identified clumps were found to be divided into two categories: “type I” sources in which we can find a relationship between size and line width and “type II” sources where there is no relation. We found
that the power law indices for type I sources are generally larger than the previous studies. Larger line widths and consequently larger indices seems to be an initial environmental characteristic of massive star forming regions
We found that mass and column density increase with line width for both type I and type II sources. We did not find any relation between the size and column density. The influence of the H II region on temperature and line widths was examined and we found that the temperature decreases with distance from the ionized fronts but no change was found for the line width. Although most of the clumps within the compressed shells around the H II region have generally larger line widths, from this test we may conclude that the internal dynamics of the cloud beyond the compressed shells is not much influenced by the expanding H II region.
Finally, our near IR study of the stellar populations using 2MASS data, shows that in half of the regions the exciting star belongs to a cluster. We also found that star formation is consistent with triggering by the expansion of the ionized gas in some of sources in our sample. At least two young embedded clusters have been identified at the same position as the dense clumps within fragmented shells around H II regions. These clumps have high temperature and density and large line widths. We identify some other hot and dense clumps very similar in molecular gas properties as candidates of cluster or massive star formation.
Most of the active star forming regions associated with H II regions have a
population of massive newborn stars compared to a star forming cloud which is distant from the massive star and the ionized gas. We conclude that more massive stars form in the molecular cloud at the peripheries of H II regions but it is not clear f this is a result of the initial conditions that have formed the massive, exciting star of the H II region or a feedback of the massive star itself and the expanding H II region.
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Effects of aerosols on deep convective cumulus cloudsFan, Jiwen 15 May 2009 (has links)
This work investigates the effects of anthropogenic aerosols on deep convective clouds and the associated radiative forcing in the Houston area. The Goddard Cumulus Ensemble model (GCE) coupled with a spectral-bin microphysics is employed to investigate the aerosol effects on clouds and precipitation. First, aerosol indirect effects on clouds are separately investigated under different aerosol compositions, concentrations and size distributions. Then, an updated GCE model coupled with the radiative transfer and land surface processes is employed to investigate the aerosol radiative effects on deep convective clouds. The cloud microphysical and macrophysical properties change considerably with the aerosol properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols decreases gradually, leading to a decrease in the cloud droplet number concentration (CDNC) and an increase in the droplet size. Ice processes are more sensitive to the changes of aerosol chemical properties than the warm rain processes. The most noticeable effect of increasing aerosol number concentrations is an increase of CDNC and cloud water content but a decrease in droplet size. It is indicated that the aerosol indirect effect on deep convection is more pronounced in relatively clean air than in heavily polluted air. The aerosol effects on clouds are strongly dependent on RH: the effect is very significant in humid air. Aerosol radiative effects (ARE) on clouds are very pronounced for mid-visible single-scattering albedo (SSA) of 0.85. Relative to the case without the ARE, cloud fraction and optical depth decrease by about 18% and 20%, respectively. The daytime-mean direct forcing is about 2.2 W m-2 at the TOA and -17.4 W m-2 at the surface. The semi-direct forcing is positive, about 10 and 11.2 W m-2 at the TOA and surface, respectively. Aerosol direct and semi-direct effects are very sensitive to SSA. The cloud fraction, optical depth, convective strength, and precipitation decrease with the increase of absorption, resulting from a more stable atmosphere due to enhanced surface cooling and atmospheric heating.
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