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This topical volume of the Journal of Pure and Applied Geophysics utilizes new information not previously accessible for fog related research. It focuses on surface and remote sensing observations of fog, various numerical model applications using new parameterizations, fog climatology, and new statistical methods. The results presented in this special issue come from research efforts in North America and Europe.
This volume presents the history of marine fog research and applications, and discusses the physical processes leading to fog's formation, evolution, and dissipation. A special emphasis is on the challenges and advancements of fog observation and modeling as well as on efforts toward operational fog forecasting and linkages and feedbacks between marine fog and the environment.
A two-dimensional boundary layer model is described. This model is designed to predict and study the effects of meteorological changes on the formation and dissipation of fog and stratus. Radiational heat loss along with the transport of static energy, moisture and momentum are treated in the model. Cloud droplet distributions are parameterized using a gamma distribution from which radiative properties and droplet fall velocities are computed. Turbulent exchange coefficients are calculated using the Monin-Obukhov theory of similitude which accounts for variations in atmospheric stability. Several experiments are presented which demonstrate the effects of various meteorological parameters on the formation and duration of stratus and fog. Energy budget analyses show the importance of each of the physical processes being modeled.
In response to Air Weather Service requirements, the Air Force Geophysics Lab has been involved in research in the development of mesoscale advection fog prediction techniques. A two-dimensional fog prediction model developed at the Naval Environmental Prediction Research Facility (NEPRF) was selected for evaluation because it can operate on a mini-computer of the size planned for the Air Force's Automated Weather Distribution System (AWDS). Six case studies developed by Calspan Advanced Technology Center were used to test the model's accuracy. These case studies covered a wide range of fog/stratus formation and dissipation stages. Four major weaknesses were identified in the model. The most important was that cloud tops increased in temperature through infrared radiative heat processes rather than decreased. The other weaknesses include lack of solar radiation processes, unreliable treatment of the height of mixed layer during stable conditions, and insufficient handling of vertical motions. The model may have potential in AWDS. However, these weaknesses must first be corrected.
This topical issue of the Journal of Pure and Applied Geophysics (PAAG) on ice fog, ice clouds, and remote sensing focuses on cold fog and ice cloud microphysics and dynamics in earth’s boundary layer. The measurements from state of art in-situ instruments, remote sensing platforms, and simulation results from numerical weather prediction (NWP) models are used in this book. Use of remote sensing platforms, including satellites, radiometers, ceilometers, sodars, and lidars, as well as in-situ sensors for the monitoring, and short term forecasting of cold fog and ice clouds are required to improve present knowledge. The new scientific challenges in addition to present knowledge on cold fog and ice clouds are also considered. University students, postgraduates, and researchers interested in cold fog and ice clouds, related to forecasting and nowcasting, aviation meteorology, remote sensing, climate, hydrometeorology, and agriculture meteorology can benefit extensively from this topical issue.
The ability of the U.S. Navy's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) (Trademark) to accurately forecast the height and structure of the Marine Boundary Layer (MBL) in the coastal zone is analyzed and compared to surface and aircraft observations from the Dynamics and Evolution of Coastal Stratus (DECS) field study conducted along the central coast of California from June 16 to July 22, 1999. The stratus field was found to have significant mesoscale variability within 100 km of the coast due to interaction between the mean flow and the coastal terrain. This structure is consistent with general hydraulic flow theory and the development of a low-level coastal jet. However, the specific characteristics on any given day were very sensitive to flow direction, inversion height, and synoptic conditions. With some modifications, the model predicted the general evolution of these events with qualitative fidelity, but was slow to dissipate the cloud and frequently produced surface fog versus stratus. A consistent tendency was found in the model's predictions of inversion heights 200-300 meters too low, weak inversion strengths, high integrated liquid water content, and weak buoyancy flux near the cloud top. These observed biases are consistent with underestimating the cloud top entrainment velocity and entrainment fluxes in the modeled boundary layer. An explicit entrainment parameterization was developed to better represent the sub-grid scale processes at cloud top and was tested in the single column and 3D versions of COAMPS. The entrainment parameterization was found to improve the boundary layer height and cloud liquid water content as compared to field observations, but the modeled boundary layer still exhibited a low bias, and the entrainment velocity was higher than is generally expected from field studies for this regime. (2 tables, 53 figures. 80 refs.) ANNOTATION: The Role of Cloud-Top Entrainment in Coastal Stratocumulus-Topped Boundary Layers