Repository logo

Browsing by Author "Stephens, Graeme, advisor"

Now showing 1 - 5 of 5
  • Thumbnail Image
    ItemOpen Access
    A tropical radiation and cloud system feedback modulated by sea surface temperature
    (Colorado State University. Libraries, 2011) Igel, Matthew R., author; Stephens, Graeme, advisor; van den Heever, Susan, committee member; Eykholt, Richard, committee member
    A large domain, high resolution cloud system resolving model set up in the tropics over fixed sea surface temperatures (SST) of 298 K and 302 K and run to radiative convective equilibrium has been analyzed with the focus on well equilibrated, domain mean results. The Regional Atmospheric Modeling System (RAMS) is used. The modeled convection organizes into disturbed, convective and undisturbed, subsidence regions. The mean profiles of state variables such as temperature, relative humidity (RH), and convective mass flux are analyzed and found to depend on SST in both predictable and unpredictable ways. The characteristics of rain depend on SST such that higher surface temperatures produce greater variability in intensity and lesser frequency. Next, the large-scale mean state is used to understand the convective system-scale setup. A focus is on the controls in the undisturbed regions of the disturbed region, deep convective anvil detrainment. Upper tropospheric radiation, through diabatic convergence, is used as a paradigm to understand the height at which detrainment occurs. The dependence of upper tropospheric radiation on RH is derived explicitly for the first time. From this new equation, temperature and RH are found to control anvil detrainment. The addition of RH as an anvil detrainment control explains why the model leads to an understanding of cooler anvils with higher SST - a positive climate feedback on the system. Other anvil feedbacks exhibited by the model are similar to those proposed in the Iris and Thermostat hypotheses. The convective system components are shown to enhance one another such that the overall system dependence on SST is nonlinear. To understand the circulation system, a heat engine analogue is made that shows the warmer state is able to more efficiently circulate or move heat. Finally, observational evidence from Cloudsat and CALIPSO shows that some of the modeled results are also apparent in nature.
  • Thumbnail Image
    ItemOpen Access
    Combined multispectral/hyperspectral remote sensing of tropospheric aerosols for quantification of their direct radiative effect
    (Colorado State University. Libraries, 2013) McGarragh, Gregory R., author; Stephens, Graeme, advisor; Kreidenweis, Sonia, committee member; Vonder Haar, Thomas, committee member; Bartels, Randy, committee member
    Scattering and absorption of solar radiation by aerosols in the atmosphere has a direct radiative effect on the climate of the Earth. Unfortunately, according to the IPCC the uncertainties in aerosol properties and their effect on the climate system represent one of the largest uncertainties in climate change research. Related to aerosols, one of the largest uncertainties is the fraction of the incident radiation that is scattered rather than absorbed, or their single scattering albedo. In fact, differences in single scattering albedo have a significant impact on the magnitude of the cooling effect of aerosols (opposite to that of greenhouse gasses) which can even have a warming effect for strongly absorbing aerosols. Satellites provide a unique opportunity to measure aerosol properties on a global scale. Traditional approaches use multispectral measurements of intensity at a single view angle to retrieve at most two aerosol parameters over land but it is being realized that more detail is required for accurate quantification of the direct effect of aerosols, in particular its anthropogenic component, and therefore more measurement information is required. One approach to more advanced measurements is to use not only intensity measurements but also polarimetric measurements and to use multiple view angles. In this work we explore another alternative: the use of hyperspectral measurements in molecular absorption bands. Our study can be divided into three stages the first of which is the development of a fast radiative transfer model for rapid simulation of measurements. Our approach is matrix operator based and uses the PadĆ© approximation for the matrix exponential to evaluate the homogeneous solution. It is shown that the method is two to four times faster than the standard and efficient discrete ordinate technique and is accurate to the 6th decimal place. The second part of our study forms the core and is divided into two chapters the first of which is a rigorous sensitivity and optimal estimation based information content study that explores the use of measurements made by a MODIS type instrument combined with measurements made by an instrument similar to GOSAT TANSO-FTS which supplies hyperspectral measurements of intensity and polarization in the O2 A-band and the 1.61- and 2.06-Ī¼m CO2 bands. It is found that the use of the hyperspectral bands provides a means to separate the effects of the surface and aerosol absorption from effects related to aerosol single scattering parameters. The amount of information increases significantly when the CO2 bands are included rather than just the more traditional O2 A-band, when polarization measurements are included, and when measurements are made at multiple view angles. We then present a retrieval using co-located observations of MODIS and GOSAT TANSO-FTS which are both also co-located with AERONET sites for validation purposes. We introduce an optimal estimation retrieval and perform this retrieval on our co-located observations. We choose a complete state vector to maximize the use of the information in our measurements and use an a priori constraint and regularization to arrive at a stable solution. In addition to the retrieved parameters, we also calculate a self contained estimation of the retrieval error. Validation with AERONET, for retrievals using MODIS plus TANSO-FTS measurements of intensity and polarization in all three bands indicate accuracies within 15% for optical thickness, 10% for fine mode mean radius, 35% for coarse mode mean radius, 15% for the standard deviation of fine mode mean radius, 25% for the standard deviation of the coarse mode mean radius, 0.04 for the real part of the index of refraction, and 0.05 for single scattering albedo. In addition to the retrieved parameters, we also validate the estimated retrieval error and find that the estimations have distributions that are tighter and within the broader distributions of real errors relative to AERONET. The third part of our study uses the retrieval results to calculate radiative fluxes, errors, and sensitivities at solar wavelengths along with aerosol radiative effect and effect efficiency. In addition, we outline how to propagate the errors in the retrieval through the flux calculations to provide an error estimation of the fluxes. These results are then validated against the corresponding AERONET products. It was found that the flux results were most sensitive to single scattering albedo while the size distribution and real part of the index of refraction also have significant effects. Relative to AERONET our fluxes are less accurate than an independent AERONET validation, due to uncertainties in our satellite based retrieval with accuracies within 13 Wm-2 for TOA upward, 9 Wm-2 for BOA upward, and 30 Wm-2 for BOA downward. The estimated errors also contained uncertainties but were in fact more conservative than the actual errors.
  • Thumbnail Image
    ItemOpen Access
    Microphysical and macrophysical responses of marine stratocumulus polluted by underlying ships
    (Colorado State University. Libraries, 2012) Christensen, Matthew Wells, author; Stephens, Graeme, advisor; Kummerow, Christian, committee member; van den Heever, Susan C., committee member; Reising, Steven, committee member
    Multiple sensors flying in the A-train constellation of satellites were used to determine the extent to which aerosol plumes from ships passing below marine stratocumulus alter the microphysical and macrophysical properties of the clouds. Aerosol plumes generated by ships sometimes influence cloud microphysical properties (effective radius) and, to a largely undetermined extent, cloud macrophysical properties (liquid water path, coverage, depth, precipitation, and longevity). Aerosol indirect effects were brought into focus, using observations from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the 94-GHZ radar onboard CloudSat. To assess local cloud scale responses to aerosol, the locations of over one thousand ship tracks coinciding with the radar were meticulously logged by hand from the Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. MODIS imagery was used to distinguish ship tracks that were embedded in closed, open, and unclassifiable mesoscale cellular cloud structures. The impact of aerosol on the microphysical cloud properties in both the closed and open cell regimes were consistent with the changes predicted by the Twomey hypothesis. For the macrophysical changes, differences in the sign and magnitude of these properties were observed between cloud regimes. The results demonstrate that the spatial extent of rainfall (rain cover fraction) and intensity decrease in the clouds contaminated by the ship plume compared to the ambient pristine clouds. Although reductions of precipitation were common amongst the clouds with detectable rainfall (72% of cases), a substantial fraction of ship tracks (28% of cases) exhibited the opposite response. The sign and strength of the response was tied to the type of stratocumulus (e.g., closed vs open cells), depth of the boundary layer, and humidity in the free-troposphere. When closed cellular clouds were identified, liquid water path, drizzle rate, and rain cover fraction (an average relative decrease of 61%) was significantly smaller in the ship-contaminated clouds. Differences in drizzle rate resulted primarily from the reductions in rain cover fraction (i.e., fewer pixels were identified with rain in the clouds polluted by the ship). The opposite occurred in the open cell regime. Ship plumes ingested into this regime resulted in significantly deeper and brighter clouds with higher liquid water amounts and rain rates. Enhanced rain rates (average relative increase of 89%) were primarily due to the changes in intensity (i.e., rain rates on the 1.1 km pixel scale were higher in the ship contaminated clouds) and, to a lesser extent, rain cover fraction. One implication for these differences is that the local aerosol indirect radiative forcing was more than five times larger for ship tracks observed in the open cell regime (-59 W m-2) compared to those identified in the closed cell regime (-12 W m-2). The results presented here underline the need to consider the mesoscale structure of stratocumulus when examining the cloud dynamic response to changes in aerosol concentration. In the final part of the dissertation, the focus shifted to the climate scale to examine the impact of shipping on the Earth's radiation budget. Two studies were employed, in the first; changes to the radiative properties of boundary layer clouds (i.e., cloud top heights less than 3 km) were examined in response to the substantial decreases in ship traffic that resulted from the recent world economic recession in 2008. Differences in the annually averaged droplet effective radius and top of atmosphere outgoing shortwave radiative flux between 2007 and 2009 did not manifest as a clear response in the climate system and, was probably masked either due to competing aerosol cloud feedbacks or by interannual climate variability. In the second study, a method was developed to estimate the radiative forcing from shipping by convolving lanes of densely populated ships onto the global distributions of closed and open cell stratocumulus clouds. Closed cells were observed more than twice as often as open cells. Despite the smaller abundance of open cells, a significant portion of the radiative forcing from shipping was claimed by this regime. On the whole, the global radiative forcing from ship tracks was small (approximately -0.45 mW m-2) compared to the radiative forcing associated with the atmospheric buildup of anthropogenic CO2.
  • Thumbnail Image
    ItemOpen Access
    Properties of the tropical hydrologic cycle as analyzed through 3-dimensional k-means cluster analysis
    (Colorado State University. Libraries, 2008) Rogers, Matthew Alan, author; Stephens, Graeme, advisor
    As the primary locations of deep convective activity and unrestrained tropical wave dynamics, the tropical West Pacific and East Indian oceans are among the most important regions in the tropics. Given that most of the region consists of unpopulated expanses of ocean, observations of tropical atmospheric properties in this important region is exceptionally difficult. Only with the help of satellite observations are we capable of gleaning valuable data from this region, and our utilization of advanced analysis techniques allows us to gain more from these observations then would otherwise be possible. In that vein, this dissertation reports on the use of a unique statistical technique, long known to other fields of research, as applied to a combined-instrument satellite observation dataset over the warm pool region of the tropical West Pacific ocean. The statistical technique, known as k-means cluster analysis, is used to delineate self-similar populations of cloud type, hereafter referred to as cloud regimes, from frequency-distribution histograms of cloud-top height, cloud optical thickness, and rainfall amount. We will show that four primary cloud regimes exist in the tropical region discussed, that the four regimes vary primarily through differences in convective activity, and that these four cloud regimes exist in a coherent temporal structure that explains the long-observed variability in convective activity seen in the tropics. Combining this regime information with satellite observations, along with reanalysis data, we then examine the individual properties of each cloud regime. These observations give us the means to understand the forcings behind cloud regime change in the region. We confirm the structural properties of these regimes using analysis from a cloud-resolving model, and apply our new understanding of the mechanism behind this large-scale forcing to the governance of the tropical hydrologic cycle as a whole. The insights gained from this analysis have benefits to both the fields of atmospheric remote sensing, and of cloud- and climate modeling of the tropical atmosphere. Applications of this technique are of particular interest to researchers developing retrieval algorithms for latent heat profiles using active sensors such as the cloud-profiling radar aboard CloudSat.
  • Thumbnail Image
    ItemOpen Access
    Simulating the effects of coated ice nuclei in the formation of thin ice clouds in the high arctic using RAMS
    (Colorado State University. Libraries, 2010) Seigel, Robert, author; Cotton, William, advisor; Stephens, Graeme, advisor; Fassnacht, Steven, committee member; CarriĆ³, Gustavo, committee member
    The Polar regions are an integral part of Earth's energy budget, however they are poorly understood mainly due to their remoteness and lack of observations. The recent launch of two successful satellites, CloudSat and CALIPSO, into the A-Train constellation are providing excellent insight into wintertime clouds and precipitation at the Poles. One distinguishable characteristic seen from satellite data during Arctic winter and spring is an optically thin cloud containing ice crystals large enough to precipitate out. These "thin ice clouds" (TIC) occur in regions affected by anthropogenic pollution. It is hypothesized that the anthropogenic pollution, likely sulfuric acid, coat the available ice forming nuclei (IN) and render them inactive for forming ice crystals. Therefore, the effective IN concentrations are reduced in these regions and there is less competition for the same available moisture leading to the formation of relatively small concentrations of large ice crystals. The ice crystals grow large enough for sedimentation, which dehydrates the Arctic atmosphere. We use Colorado State University's Regional Atmospheric Modeling System (RAMS) configured as a cloud resolving high-resolution model (CRM) with horizontal grid-spacing of 1 00m to simulate these TI C's. Varying ice nuclei (IN) concentrations from 5 L-1 to 100L-1 are used to simulate the effects of the acidic coating, whereby the low IN concentration represents the IN particles containing the acidic coating. Results show no concrete evidence in support of the hypothesis. Therefore, a sensitivity experiment is conducted to identify the environmental conditions that maximize the production of TIC's. Results indicate that an increase in both the temperature and supersaturation relative to observations provide a better environment for the production of TIC's.