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Browsing by Author "Seaman, Curtis J., author"

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    Assimilation of geostationary, infrared satellite data to improve forecasting of mid-level, mixed-phase clouds
    (Colorado State University. Libraries, 2009) Seaman, Curtis J., author; Vonder Haar, Thomas H., advisor
    Mid-level, mixed-phase clouds (altocumulus and altostratus) are difficult to forecast due to the fact that they are generally thin and form in areas of weak vertical velocity where operational models typically have poor vertical resolution and poor moisture initialization. This study presents experiments designed to test the utility of assimilating infrared window and water vapor channels from the Geostationary Operational Environmental Satellite (GOES) instruments, Imager and Sounder, into a mesoscale cloud-resolving model to improve model forecasts of mid-level clouds. The Regional Atmospheric Modeling Data Assimilation System (RAMDAS) is a four-dimensional variational (4-DVAR) assimilation system used to test the viability of assimilating cloudy scene radiances into a cloud-free initial model state for one case of a long-lived, isolated altocumulus cloud over the Great Plains of the United States. Observations from one observation time are assimilated and significant innovations are achieved. Three experiments are performed: (1) assimilation of the 6.7 μm (water vapor) and 10.7 μm (window) channels from GOES Imager, (2) assimilation of the 7.02μm (water vapor) and 12.02 μm (window) channels from GOES Sounder, and (3) assimilation of the 6.7 μm channel from GOES Imager and the 7.02 μm channel from GOES Sounder. It is shown that the GOES Sounder channels provide more useful information than the GOES Imager channels due to increased sensitivity to the mid-troposphere. The decorrelation lengths and variance used in the background error covariance are varied and the impact on the results of the experiments is discussed. The effect of constraining the surface temperatures during assimilation of the window channels is also discussed. It is found that, in a cloud-free initial model state, the adjoint sensitivities are calculated on the assumption that there is no cloud, even with cloud in the satellite observations. This has a significant impact on the success of other 4-DVAR satellite data assimilation experiments.
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    Observed and calculated properties of mid-level, mixed-phase clouds
    (Colorado State University. Libraries, 2003) Seaman, Curtis J., author; Vonder Haar, Thomas H., author; Cooperative Institute for Research in the Atmosphere (Fort Collins, Colo.), publisher
    The University of Wyoming King Air research aircraft was flown into five mid-level clouds that formed over the western Great Plains during the Ninth Complex Layered Cloud Experiment (CLEX-9). Four of the clouds were mixed-phase. This study presents the direct observations made of these clouds as well as the cloud properties that were derived from these observations. In particular, profiles of temperature, water vapor mixing ratio, liquid water content (LWC) and ice water content (IWC) are shown. These profiles were used to calculate profiles of latent heating rate, and long- and shortwave radiative heating rate. In-cloud temperatures were observed between +2 °C and -25 °C. Maximum horizontally averaged LWC and IWC values were between 0.04 - 0.28 g m-3 and 0 - 0.16 g m-3 respectively. Cloud depths ranged from 248 m to 3106 m, with cloud bases between 2.9 and 5.6 km above mean sea level. Direct observations of ice particles made through the use of 2D-C and 2D-P optical imaging probes were analyzed using the methods of Heymsfield et al. (2002) to account for departures from sphericity, which reduces the observed ice water content by as much as 95%. These methods were also used to fit the observed ice particle size distribution into a modified gamma distribution equation, from which the ice particle effective radii were determined. Knowledge of the ice particle effective radii, plus observations of the liquid droplet effective radii made by a Forward Scattering Spectrometer Probe, were used with the profiles of LWC and IWC to calculate liquid and ice water paths and optical depths of these clouds. Ice particle size distributions and profiles of IWC show evidence of growth by the Wegener-Bergeron-Findeisen mechanism and aggregation. This data was input into a simple model to calculate the relative importance of subsidence, radiation, entrainment and precipitation in affecting cloud lifetimes. Results of this model show that subsidence and precipitation are the most important processes. It is also shown that the passage of potential vorticity anomalies may be intricately linked to the lifetimes of isolated, non-frontal and non-orographic mid-level clouds. A selection of previous studies was examined in light of these results to develop a consistent picture of the lives of midlevel clouds. The results of this study are shown to be similar to the results of previous studies of mid-level clouds, particularly those that took place over the continental United States.