Graduate Thesis Or Dissertation


Marine Low Clouds : Radiation, Turbulence, and Forecasting Public Deposited

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  • High-resolution ship-based observations of a nearly uniform stratocumulus deck in the southeast Pacific during 17 days of the 2008 VOCALS regional experiment elucidate radiation and turbulence in the marine boundary layer (MBL). A new method for prescribing observations-based cloud properties to the Rapid Radiative Transfer Model is presented and applied to the VOCALS dataset. The simulated radiative fluxes compare well with surface observations and with simulations from several other commonly employed methods, particularly during times of high drizzle content and/or precipitation. Clear-sky and cloudy scenarios illustrate the radiative effect of clouds on the MBL. Turbulence estimates are made for the stratocumulus layer using the simulated radiative fluxes and vertical velocity observations from the NOAA W-band Doppler cloud radar. The averages and diurnal cycles of the terms in the turbulent kinetic energy (TKE) budget are presented. 10-minute vertical velocity statistics and estimates of TKE budget terms reveal a robust diurnal cycle of weak turbulence during daylight hours with a rapid increase near sundown followed by sustained strong turbulence at night. Buoyant production is driven by cloud-top longwave radiative cooling during the night, but is offset and dominated by absorption of solar radiation from ~0900-1600 LT. At night, dissipation increases toward cloud base while vertical velocity variance is nearly constant with height. This suggests larger-scale turbulence occurs near cloud-top while dissipation interacts with smaller-scale turbulence near cloud base. Cloud-top entrainment rates are estimated from the entrainment heat flux calculated as a residual from the heat budget. Entrainment, with an average value of 4.6 mm s-1, has a diurnal cycle similar to turbulence with larger values at night. The third part of this dissertation presents one of the first studies using reforecast data to produce downscaled low cloud probabilistic forecasts for aviation applications. Probability of low cloud ceiling and visibility are forecast using analogs chosen from an ensemble of reforecasts, on the basis of their agreement with thermodynamic soundings. Skillful forecasts are produced for up to 30-hour leads times using reforecasts of 1° resolution at major airports across the continental United States. These probabilistic forecasts outperform climatology at all lead times and trained forecasters at nine-hour and longer lead times. These results suggest that real-time forecasts based on analog reforecasts is a useful approach for aviation applications with potential to enhance safety and commerce.
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