Ismail Gultepe

THE FOG REMOTE SENSING AND MODELING FIELD PROJECT

The main purpose of this work is to describe a major field project on fog and summarize the preliminary results. Three field phases of the Fog Remote Sensing and Modeling (FRAM) project were conducted over the following two regions of Canada: 1) the Center for Atmospheric Research Experiments (CARE), in Toronto, Ontario (FRAM-C), during the winter of 2005/06, and 2) Lunenburg, Nova Scotia (FRAM-L), during June 2006 and June 2007. Fog conditions observed during FRAM-C were continental in nature, while those conditions observed during FRAM-L were of marine origin. The main objectives of the project were to attain 1) a better description of fog environments, 2) the development of microphysical parameterizations for model applications, 3) the development of remote sensing methods for fog nowcasting/forecasting, 4) an understanding of issues related to instrument capabilities and improvement of the analysis, and 5) an integration of model data with observations to predict and detect fog areas and particle phas...

A New Visibility Parameterization for Warm-Fog Applications in Numerical Weather Prediction Models

Abstract The objective of this work is to suggest a new warm-fog visibility parameterization scheme for numerical weather prediction (NWP) models. In situ observations collected during the Radiation and Aerosol Cloud Experiment, representing boundary layer low-level clouds, were used to develop a parameterization scheme between visibility and a combined parameter as a function of both droplet number concentration Nd and liquid water content (LWC). The current NWP models usually use relationships between extinction coefficient and LWC. A newly developed parameterization scheme for visibility, Vis = f (LWC, Nd), is applied to the NOAA Nonhydrostatic Mesoscale Model. In this model, the microphysics of fog was adapted from the 1D Parameterized Fog (PAFOG) model and then was used in the lower 1.5 km of the atmosphere. Simulations for testing the new parameterization scheme are performed in a 50-km innermost-nested simulation domain using a horizontal grid spacing of 1 km centered on Zurich Unique Airport in Sw...

Analysis and Modeling of an Extremely Dense Fog Event in Southern Ontario

In this study, a dense fog episode that occurred near Windsor, Ontario, Canada, on 3 September 1999 is investigated. The fog patch, with a spatial scale of several kilometers, reduced visibility on a major highway to a few meters and led to a series of collisions and loss of life. Satellite imagery and surface observations are used to analyze the physics of the event, and several hypotheses on the origin of the fog are presented. A series of simulations of the event with the fifth-generation Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model (MM5) suggest that the fog formed because of convergence of land breezes developing along the shores of a lake and subsequent advection of moisture over the site of the accident. Tests indicate that the small scale of the modeled event contributes to sensitivity of the results to a broad range of factors. Sensitivity to the initial and boundary conditions, including initial soil moisture content and parameterization of turbulence, is discussed.

Dynamical Structure and Turbulence in Cirrus Clouds: Aircraft Observations during FIRE

Abstract Aircraft data collected during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE)I are used to examine dynamical processes operating in cirrus cloud systems observed on 19 and 28 October 1986. Measurements from Lagrangian spiral soundings and constant-altitude flight legs are analyzed. Comparisons are made with observations in clear air. Each cirrus case contained a statically stable layer, a conditionally unstable or neutrally stratified layer (ice pseudoadiabatic) in which convection was prevalent, and a neutral layer in which convection was intermittent. The analysis indicates that a mixture of phenomena occurred including small-scale convective cells, gravity waves (λ≈2–9 km), quasi-two-dimensional waves (λ≈10–20 km), and larger two-dimensional mesoscale waves (λ≈100 km). The intermediate-scale waves, observed both in clear air and in the cloud systems, likely played an important role in the development of the cloud systems given the magnitude of the associ...

Ice Fog in Arctic During FRAM–Ice Fog Project: Aviation and Nowcasting Applications

Ice fog and frost occur commonly (at least 26% of the time) in the northern latitudes and Arctic regions during winter at temperatures usually less than about –15°C. Ice fog is strongly related to frost formation—a major aviation hazard in the northern latitudes. In fact, it may be considered a more dangerous event than snow because of the stronger aircraft surface adhesion compared to snow particles. In the winter of 2010/11, the Fog Remote Sensing and Modeling–Ice Fog (FRAM-IF) project was organized near Yellowknife International Airport, Northwest Territories, Canada, with the main goals of advancing understanding of ice fog microphysical and visibility characteristics, and improving its prediction using forecast models and remotesensing retrievals. Approximately 40 different sensors were used to measure visibility, precipitation, ice particle spectra, vertical thermodynamic profiles, and ceiling height. Fog coverage and visibility parameters were estimated using both Geostationary Operational Environm...

Dynamical Characteristics of Cirrus Clouds from Aircraft and Radar Observations in Micro and Meso-γ Scales

Abstract Cirrus clouds that formed on 26 November and 6 December 1991 during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) II, which took place over the Kansas region. are studied because of significant dynamic activity in the micro (<1 km) and meso γ (<25 km) scales within the cloud. Observations are obtained from the NCAR King Air, NOAA Doppler, and PSU conventional radar. For this reason coherent structures (e.g., cells, vortex) that transfer significant heat, moisture, and turbulence are analyzed using aircraft and radar observations. Aircraft data is collected at 20 Hz, and calculations are made at two different scales. Scale separation is chosen at about 1 km. A coherence analysis technique is used to specify the correlation between temperature and vertical velocity w fluctuations. A swirling coefficient, indicating spirality, is calculated to better understand cloud dynamics. Sensible heat, latent heat, and radiative fluxes are compared with each other in tw...

A comparison of vertical velocity in cirrus obtained from aircraft and lidar divergence measurements during FIRE

Abstract Techniques are presented to obtain vertical velocity in cirrus clouds from in situ aircraft lateral wind measurements and from ground-based remote Doppler lidar measurements. In general, direct measurements of absolute vertical velocity w from aircraft are currently not feasible because of offsets in the air velocity sensors. An alternative to direct measurement is to calculate w from the integral of the divergence of the horizontal velocity around a closed path. We discuss divergence measurements from both aircraft and Doppler lidar. The principal errors in the calculation of w from aircraft lateral wind measurements are bias in the lateral wind, ground speed errors, and error due to vertical shear of the horizontal wind. For Doppler lidar measurements the principal errors are in the estimate of mean terminal velocity and the zeroth order coefficient of the Fourier series that is fitted to the data. The technique is applied to a cirrus cloud investigated during the FIRE (First International Sate...

Simulations of ice clouds during FIRE ACE using the CCCMA single‐column model

The single-column model (SCM) of the Canadian Centre for Climate Modelling and Analysis (CCCMA) solves prognostic equations for the number concentration and mass mixing ratios of ice crystals. Ice crystal formation is specified via different ice nucleation mechanisms. The CCCMA SCM was used to simulate the evolution of ice clouds for three different flights during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) in April 1998, where measurements of cloud droplets, ice crystals, and aerosols > 0.08 μm in radius were conducted. The CCCMA SCM in its original setup predicts larger ice crystal concentrations than measured with the 2DC probe. The agreement with observations of ice crystals detectable with the 2DC probe was improved if a condensation freezing parameterization depending on supersaturation with respect to ice was used. The best correlation between observed and simulated ice crystal number, which still showed a lot of scatter, was obtained if an empirical linear relationship between the number of aerosols and the number of ice crystals deduced from FIRE ACE was used.

Characterization of Arctic ice cloud properties observed during ISDAC

Extensive measurements from ground-based sites and satellite remote sensing (CloudSat and CALIPSO) reveal the existence of two types of ice clouds (TICs) in the Arctic during the polar night and early spring. The first type (TIC-2A), being topped by a cover of nonprecipitating very small (radar unseen) ice crystals (TIC-1), is found more frequently in pristine environment, whereas the second type (TIC-2B), detected by both sensors, is associated preferentially with a high concentration of aerosols. To further investigate the microphysical properties of TIC-1/2A and TIC-2B, airborne in situ and satellite measurements of specific cases observed during Indirect and Semi-Direct Aerosol Campaign (ISDAC) have been analyzed. For the first time, Arctic TIC-1/2A and TIC-2B microstructures are compared using in situ cloud observations. Results show that the differences between them are confined in the upper part of the clouds where ice nucleation occurs. TIC-2B clouds are characterized by fewer (by more than 1 order of magnitude) and larger (by a factor of 2 to 3) ice crystals and a larger ice supersaturation (of 15-20%) compared to TIC-1/2A. Ice crystal growth in TIC-2B clouds seems explosive, whereas it seems more gradual in TIC-1/2A. It is hypothesized that these differences are linked to the number concentration and the chemical composition of aerosols. The ice crystal growth rate in very cold conditions impinges on the precipitation efficiency, dehydration and radiation balance. These results represent an essential and important first step to relate previous modeling, remote sensing and laboratory studies with TICs cloud in situ observations.

Mid-Mountain Clouds at Whistler During the Vancouver 2010 Winter Olympics and Paralympics

A comprehensive study of mid-mountain clouds and their impacts on the Vancouver 2010 Winter Olympics and Paralympics is presented. Mid-mountain clouds were frequently present on the Whistler alpine venue, as identified in an extensive archive of webcam images over a 45-day period from February 5 to March 21, 2010. These clouds posed serious forecast challenges and had significant impacts on some Olympic and Paralympic alpine skiing competitions. Under fair weather conditions, a diurnal upslope (anabatic) flow can work in concert with a diurnal temperature inversion aloft to produce a localized phenomenon known as “Harvey’s Cloud” at Whistler. Two detailed case studies in this paper suggest that mid-mountain clouds can also develop in the area as a result of a moist valley flow interacting with a downslope flow descending from the mountaintop. A southerly inflow through the Sea-to-Sky corridor can be channeled by the local topography into a westerly upslope flow toward Whistler Mountain, resulting in orographic clouds on the alpine venue. Under favorable circumstances, these clouds are trapped to the mid-mountain zone by the leeward subsidence of an elevated southerly flow. The presence of the downslope subsidence was manifested by a distinguished dry layer observed on the top of the mid-mountain clouds in both cases. It is the subsidence-induced adiabatic warming that imposes a strong buoyant suppression to trap the mid-mountain cloud. On the other hand, the subsidence-induced dry layer has the potential to trigger evaporative instability to periodically breakup the mid-mountain cloud.