Roland B. Stull

Long-range transport of Asian dust to the Lower Fraser Valley, British Columbia, Canada

For the first time, long-range transport of “Kosa” mineral aerosol from western China to southwestern British Columbia is documented. This late April 1998 event coincided with an episode of photochemical smog and reduced dispersion in the Lower Fraser Valley (LFV). Filter samples in the region show a massive injection of crustal elements (Si, Fe, Al, and Ca) with concentrations of Si approximately double those previously recorded. Ratios of these elements to Fe are shown to be statistically similar to ratios observed in mineral aerosol events in Hawaii and China. On the basis of the difference between observed and expected elemental concentrations and reconstructed soil mass in the episode, it is estimated that Asian dust contributed 38–55% to observed PM10 in the LFV, the remainder being attributed to local sources. Comparison of the April 1998 event with two spring meteorological analogs is consistent with this estimate. Mesoscale model simulations suggest that mineral dust was incorporated into the planetary boundary layer as a result of strong subsidence over the interior of southern British Columbia and Washington State which permitted interception of lower tropospheric elevated aerosol layers by surface-based mixing processes over mountainous terrain. Surface easterly (“outflow”) winds then transported this material into the Lower Fraser Valley where it contributed significantly to total particulate loadings and an intense haze. This mechanism is consistent with the observed spatial and temporal distribution of PM10.

Transilient Turbulence Theory. Part I: The Concept of Eddy-Mixing across Finite Distances

Abstract A first-order turbulence theory is developed that describes eddy-like mixing. Named transilient theory after a Latin word meaning “leap across” this approach models the turbulent mixing between arrays of points separated in space. It differs from eddy-diffusivity theory in that it is not restricted to turbulent transfer between adjacent points. By explicitly including “large eddy” effects it can handle mixing across zero-gradient and counter-gradient situations such as found in convective mixed layers. Applications might include pollutant dispersion, boundary layer modeling and cloud entrainment studies.

Wet-Bulb Temperature from Relative Humidity and Air Temperature.

AbstractAn equation is presented for wet-bulb temperature as a function of air temperature and relative humidity at standard sea level pressure. It was found as an empirical fit using gene-expression programming. This equation is valid for relative humidities between 5% and 99% and for air temperatures between −20° and 50°C, except for situations having both low humidity and cold temperature. Over the valid range, errors in wet-bulb temperature range from −1° to +0.65°C, with mean absolute error of less than 0.3°C.

Internal Gravity Waves Generated by Penetrative Convection

Abstract Penetrative convection is modeled by a series of idealized disturbances at the base of an atmospheric temperature inversion. Internal gravity waves excited in the inversion by the disturbances are theoretically found to drain away a fraction of the initial energy of the disturbances. The portion of energy lost upward is significant when the temperature inversion is weak. Vertical energy fluxes due to internal gravity waves are mathematically described using wave group approaches.

A Fair-Weather Cumulus Cloud Classification Scheme for Mixed-Layer Studies

Abstract Not all cumulus clouds can vent mixed-layer air into the free atmosphere. Therefore, three subtypes of fair-weather cumulus clouds are identified based on the nature of their interaction with the mixed layer: forced, active and passive clouds. Forced clouds, the visible tracers within the tops of some mixed-layer thermals, are totally embedded within the mixed layer. Active clouds reach above their level of free convection and are responsible for inhibiting mixed-layer growth and for venting pollutants from the mixed layer. Passive clouds are the decaying remnants of formerly active clouds, and are disconnected from the mixed layer.

Kalman Filter and Analog Schemes to Postprocess Numerical Weather Predictions

Two new postprocessing methods are proposed to reduce numerical weather prediction’s systematic and random errors. The first method consists of running a postprocessing algorithm inspired by the Kalman filter (KF) through an ordered set of analog forecasts rather than a sequence of forecasts in time (ANKF). The analog of a forecast for a given location and time is defined as a past prediction that matches selected features of the current forecast. The second method is the weighted average of the observations that verified when the 10bestanalogswerevalid(AN).ANKFandANaretestedfor10-mwindspeedpredictionsfromtheWeather ResearchandForecasting(WRF)model,withobservations from400surfacestationsoverthewesternUnited States for a 6-month period. Both AN and ANKF predict drastic changes in forecast error (e.g., associated with rapid weather regime changes), a feature lacking in KF and a 7-day running-mean correction (7-Day). TheANalmosteliminates thebiasoftherawprediction(Raw),whileANKFdrasticallyreducesitwithvalues slightly worse than KF. Both analog-based methods are also able to reduce random errors, therefore improvingthepredictiveskill ofRaw. TheAN isconsistentlythebest, withaverageimprovements of10%,20%, 25%,and 35%with respecttoANKF,KF, 7-Day,andRaw, asmeasuredby centeredroot-mean-squareerror, and of 5%, 20%, 25%, and 40%, as measured by rank correlation. Moreover, being a prediction based solely on observations, AN results in an efficient downscaling procedure that eliminates representativeness discrepancies between observations and predictions.

A Convective Transport Theory for Surface Fluxes

Abstract For a boundary layer in free convection where turbulent thermal structures communicate information between the surface and the interior of the mixed layer, it is hypothesized that the surface momentum flux can be parameterized by u*2 = bDwBMML, the heat flux by w′θ′s = bHwB(θskin−θML), and the moisture flux by w′r′s = bHwB(rskin−rML). In these expressions u* is the friction velocity, M is mean wind speed, θ is potential temperature, r is mixing ratio, subscript ML denotes the interior of the mixed layer, and subscript skin denotes the characteristics of the underlying solid or liquid surface. A buoyancy velocity scale is defined by wB≈[(g/θv)zi(θvskin−θvML)]½, where zi is mixed-layer depth, θv is virtual potential temperature, and g is gravitational acceleration. Using data from the BLX83 field experiment in Oklahoma (roughness length: 0.05 m, latitude: 35.03°N, vegetation: mixed pasture and crops, season: spring), the convective transport coefficients are empirically found to be bH = 5.0×10−4 fo...

The Effects of Nonhomogeneous Surface Fluxes on the Convective Boundary Layer: A Case Study Using Large-Eddy Simulation

Abstract Most land surfaces are quasi-randomly nonhomogeneous, yet most boundary-layer studies assume homogeneous or simply varying surface conditions. In this study, nonhomogeneous surface fluxes of realistic scale and amplitude are applied to a large-eddy-simulation (LES) model. The boundary conditions are representative of conditions observed near Chickasha, Oklahoma during the Boundary Layer Experiment 1983 (BLX83), while the model is a modified version of Moengs LES model. The afternoon of 28 May 1983, which had light winds and cloud-free skies, is simulated by using data collected during BLX83 to provide initial and boundary conditions; the simulation verifies fairly well against observations. A second simulation of the case-study afternoon uses horizontally homogeneous surface fluxes. The results of the two runs are compared to see what effect the quasi-random nonhomogeneous conditions have on mixed-layer development. The inclusion of realistic size (450–900 m) and amplitude (∼2 (°C)2 variance in ...

Mixed-Layer Depth Model Based on Turbulent Energetics

Abstract Mixed layer depths are predicted using an entrainment equation with conservation equations. The entrainment equation is based on the turbulent kinetic energy equation for the mixed layer. The atmosphere is idealized as having temperatures, humidities and winds constant with height in the boundary layer with a step discontinuity marking the top of the mixed layer. This model is tested with mixed layer depth observations made during the 1953 Great Plains experiment, the 1967 Australian Wangara experiment, and the 1972 Puerto Rican tropical experiment. Model calculations of inversion rise and mixed layer depth offer good agreement with the observations. It is found that none of the turbulence generation and loss mechanisms for the mixed layer (such as buoyancy, wind shear and gravity waves) should be neglected a priori.

The LCL Zone and Cumulus Onset

Abstract Variations of the lifting condensation level (LCL) of surface layer air are documented based on data from the BLX83 field experiment in Oklahoma. For example, within a 25 km long region near Chickasha, the local LCL height was found to vary by 15–30% of its average height. This zone of variation, centered on the mean LCL height, is identified as the “LCL zone”. It is analogous to the entrainment zone for the local mixed layer depth. Cumulus clouds first form when the top of the entrainment zone reaches the bottom of the LCL zone. As more of the entrainment zone overlaps and reaches above the LCL zone, the cloud cover increases. Two case studies are presented to demonstrate the diagnosis of cumulus onset time and cloud cover amount using this overlap technique. Combined radar, aircraft, rawinsonde, and surface observations indicate that some of the air observed at cloud base has the same low LCL as that of the mean surface layer air. This leads us to speculate that some surface layer air is rising...