Douglas L. Westphal

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

A multidimensional model for aerosols: description of computational analogs

Abstract The numerical algorithms which we use to simulate the advection, diffusion, sedimentation, coagulation and condensational growth of atmospheric aerosols are described. The model can be used in one, two, or three spatial dimensions. We develop the continuity equation in a generalized horizontal and vertical coordinate system which allows the model to be quickly adapted to a wide variety of dynamical models of global or regional scale. Algorithms are developed to treat the various physical processes and the results of simulations are presented which show the strengths and weaknesses of these algorithms. Although our emphasis is on the modeling of aerosols, the work is also applicable to simulations of the transport of gases.

Global Monitoring and Forecasting of Biomass-Burning Smoke: Description of and Lessons From the Fire Locating and Modeling of Burning Emissions (FLAMBE) Program

Recently, global biomass-burning research has grown from what was primarily a climate field to include a vibrant air quality observation and forecasting community. While new fire monitoring systems are based on fundamental Earth Systems Science (ESS) research, adaptation to the forecasting problem requires special procedures and simplifications. In a reciprocal manner, results from the air quality research community have contributed scientifically to basic ESS. To help exploit research and data products in climate, ESS, meteorology and air quality biomass burning communities, the joint Navy, NASA, NOAA, and University Fire Locating and Modeling of Burning Emissions (FLAMBE) program was formed in 1999. Based upon the operational NOAA/NESDIS Wild-Fire Automated Biomass Burning Algorithm (WF_ABBA) and the near real time University of Maryland/NASA MODIS fire products coupled to the operational Navy Aerosol Analysis and Prediction System (NAAPS) transport model, FLAMBE is a combined ESS and operational system to study the nature of smoke particle emissions and transport at the synoptic to continental scales. In this paper, we give an overview of the FLAMBE system and present fundamental metrics on emission and transport patterns of smoke. We also provide examples on regional smoke transport mechanisms and demonstrate that MODIS optical depth data assimilation provides significant variance reduction against observations. Using FLAMBE as a context, throughout the paper we discuss observability issues surrounding the biomass burning system and the subsequent propagation of error. Current indications are that regional particle emissions estimates still have integer factors of uncertainty.

A case study of mobilization and transport of Saharan dust

Abstract Numerical models of the atmosphere and aerosols are used to investigate mobilization and transport of Saharan dust over West Africa and the tropical Atlantic Ocean for 23–28 August 1974. We have found that mobilization during this period was related to the passage of a shallow easterly wave and was not initiated by dry convective mixing of a midlevel easterly jet, as has been previously suggested, since high static stability beneath the midlevel easterly jet inhibited significant boundary layer development and transport of momentum in the jet down to the surface. Instead, mobilization was done by dry convective mixing of low-level jets associated with the easterly wave. Another easterly wave present in the domain during the period did not contribute significantly to dust mobilization while over Africa yet became a strong tropical storm over the Atlantic Ocean in early September. The periodicity of the outbreak was reinforced by scavenging of dust by precipitation associated with the easterly wave...

Atmospheric Aerosol Optical Properties in the Persian Gulf

Aerosol optical depth measurements over Bahrain acquired through the ground-based Aerosol Robotic Network (AERONET) are analyzed. Optical depths obtained from ground-based sun/sky radiometers showed a pronounced temporal trend, with a maximum dust aerosol loading observed during the March-July period. The aerosol optical depth probability distribution is rather narrow with a modal value of about 0.25. The Angstrom parameter frequency distribution has two peaks. One peak around 0.7 characterizes a situation when dust aerosol is more dominant, the second peak around 1.2 corresponds to relatively dust-free cases. The correlation between aerosol optical depth and water vapor content in the total atmospheric column is strong (correlation coefficient of 0.82) when dust aerosol is almost absent (Angstrom parameter is greater than 0.7), suggesting possible hygroscopic growth of fine mode particles or source region correlation, and much weaker (correlation coefficient of 0.45) in the presence of dust (Angstrom parameter is less than 0.7). Diurnal variations of the aerosol optical depth and precipitable water were insignificant. Angstrom parameter diurnal variability (;20%-25%) is evident during the April-May period, when dust dominated the atmospheric optical conditions. Variations in the aerosol volume size distributions retrieved from spectral sun and sky radiance data are mainly associated with the changes in the concentration of the coarse aerosol fraction (variation coefficient of 61%). Geometric mean radii for the fine and coarse aerosol fractions are 0.14 mm (std dev 5 0.02) and 2.57 mm (std dev 5 0.27), respectively. The geometric standard deviation of each fraction is 0.41 and 0.73, respectively. In dust-free conditions the single scattering albedo (SSA) decreases with wavelength, while in the presence of dust the SSA either stays neutral or increases slightly with wavelength. The changes in the Angstrom parameter derived from a ground-based nephelometer and a collocated sun photometer during the initial checkout period were quite similar.

April 1998 Asian dust event: A southern California perspective

In late April 1998 an extreme Asian dust episode reached the U.S. western seaboard. This event was observed by several in situ and remote sensing atmospheric measurement stations. Dramatic reductions in boundary layer visibility were recorded and the resultant peak backscatter coefficients exceeded prevailing upper tropospheric background conditions by at least 2 orders of magnitude. An analysis of this event is given using lidar vertical backscatter profilometry, concurrent Sun photometer opacity data, and transport modeling. At San Nicolas Island the measured and modeled aerosol optical thickness at 500 nm increased dramatically from 0.15 on April 25 to 0.52 on April 26-27. Volume size distribution on April 27 exhibited a prominent coarse mode at 1-2 μm radius, and single-scattering albedo was observed to increase from 0.90 in the blue to 0.93 in the near infrared. Concurrent lidar observations tracked the evolution of the plume vertical structure, which consisted of up to three well-defined layers distributed throughout the free troposphere.

Microphysical modeling of cirrus: 1. Comparison with 1986 FIRE IFO measurements

We have used a one-dimensional model of cirrus formation to study the development of cirrus clouds during the 1986 First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) intensive field observations (IFO). The cirrus model includes microphysical, dynamical, and radiative processes. Sulfate aerosols, solution drops, ice crystals, and water vapor are all treated as interactive elements in the model. Ice crystal size distributions are fully resolved based on calculations of homogeneous freezing nucleation, growth by water vapor deposition, evaporation, coagulation, and vertical transport. We have focused on the cirrus observed on November 1, 1986. Vertical wind speed for the one-dimensional simulation is taken from a mesoscale model simulation for the appropriate time period. The mesoscale model simulation suggested that strong upward motions over Wyoming and subsequent horizontal transport of upper level moisture were responsible for the cirrus observed over Wisconsin on this date. We assumed that our one-dimensional model could be used to represent a vertical column moving from Wyoming to Wisconsin over a period of several hours. Ice crystal nucleation occurs in our model in the 8 to 10-km region as a result of the strong updrafts (and cooling) early in the simulation. Growth, coagulation, and sedimentation of these ice crystals result in a broad cloud region (5–10 km thick) with an optical depth of 1–2 after a few hours, in agreement with the FIRE measurements. Comparison with aircraft microphysical measurements made over Wisconsin indicates that the simulation generated reasonable ice water content, but the predicted ice number densities are too low, especially for radii less than about 50 μm. Sensitivity tests suggest that better agreement between simulated and observed microphysical properties is achieved if the nucleation rate is higher or stronger vertical mixing (perhaps associated with multidimensional motions) is present.

Simulations of microphysical, radiative, and dynamical processes in a continental-scale forest fire smoke plume

A numerical model of meteorology, aerosols, and radiative transfer is used to study the impact of a large forest fire smoke plume on atmospheric processes. The simulated smoke optical depths at 0.63 μm. wavelength are in agreement with analyses of satellite data and show values as high as 1.8. The smoke has an albedo of 35%, or more than double the clear-sky value, and cools the surface by as much as 5 K. The best agreement with the analyses of optical depth and surface cooling is obtained when a fuel loading more than 10 times that which has been previously suggested is used to calculate the initial smoke mass load. An imaginary refractive index, nim, of 0.01 yields results which closely match the observed cooling, single scattering albedo, and the Angstrom wavelength exponent. An nim of 0.1, typical of smoke from urban fires, produces 9 K cooling. Coagulation causes the geometric mean radius by number to increase from the initial value of 0.08 μm to a final value of 0.15 μm while the specific extinction and absorption increase by 40% and 25%, respectively. After 42 hours, these changes in the smoke optical properties lead to a 32% increase in optical depth and an 11% increase in surface cooling over that found in a simulation where coagulation is not allowed. In the model, 47% of the smoke is removed by scavenging as it is incorporated into the frontal zone over the Great Lakes. Self-lofting of the smoke in a direct, smoke-induced circulation is observed in the baseline simulation and is much stronger in the urban smoke simulation.

A study of the sensitivity of simulated mineral dust production to model resolution

The dependence of dust production on model grid-space resolution is investigated using the Navys operational mesoscale meteorological model with an imbedded dust emission model. The study covers a 2-week period of strong dust storms in April 1998 in the major dust source area of East Asia. The modeled surface winds at grid resolutions of 20, 40, 60, and 80 km are verified against observational data. At all resolutions the model has a positive bias in wind speed that decreases as resolution increases. Dust fluxes that are proportional to the fourth power of the friction velocity (u*) and the third power of the wind speed are calculated at all four resolutions and compared. Compared with the 20-km resolution u*-driven flux, which is deduced to be the most accurate, the u*-driven flux on the coarser grids overestimate the flux, with the 80 km being 60% higher for individual events and nearly 20% higher in the total dust production for the entire study period. The wind-driven flux misses the smaller events due to the lack of a dependence on stability and wind shear, when compared with the timing of surface dust observations, and has differences of up to 70%, when compared with the 20-km u*-driven flux. Averaging over space and time tends to reduce the differences among grids and might support modeling at coarse resolution.

Numerical Simulation of a Low-Level Jet over Complex Terrain in Southern Iran

The Lut Desert of Iran is an elongated valley oriented north-northwest to south-southeast. The valley descends southward to the Jaz Murian dry lake through a pass. The Navy’s Coupled Ocean‐Atmosphere Mesoscale Prediction System is used to study a northerly low-level jet in the valley and across the dry lake. The dynamics of the jet are investigated with force balance and Froude numbers to determine the contribution of various mechanisms to the jet formation and maintenance. The jet is initiated as a channeled gap flow in the convergent topography of the Lut valley by the valley-parallel pressure gradients generated by the large-scale processes and by the presence of cold air over the valley’s sloping terrain. The pressure gradient is mainly counteracted by the frictional force. The imbalance between them controls the intensity and persistence of the jet in the valley. Farther south, the jet evolves into a downslope flow resembling a hydraulic jump on the steep slope of the dry lake. A transition of subcritical situation to supercritical faster flow is found at the mountain crest between the Lut valley and dry lake. The depth of stably stratified cold layer, the static stability of upstream inversion, and magnitude of upstream winds all determine the jet configuration over the dry lake. The lee troughing over the Gulf of Oman and the Persian Gulf, as the large-scale inland flow crosses the coastal mountains, supports this low-level jet through the increased along-jet pressure gradient. The jet is also influenced by diurnal forcing, being strong at night and weak during daytime.