Search Results

You are looking at 1 - 10 of 31 items for

  • Author or Editor: D. R. Wilson x
  • Refine by Access: All Content x
Clear All Modify Search
A. Leetmaa, D. R. Quadfasel, and D. Wilson

Abstract

During the spring and summer of 1979 a multi-ship survey studied the changes in currents along the East African coast in response to the transition from the northeast to the southwest monsoon. The Somali Current in late April and early May flowed southwestward along the coast from northern Somalia to about the equator. Surface currents were 50–100 cm s−1 and the transport was 3×106 m3 s−1. South of the equator the East African Coast Current (EACC) flowed northward. The two currents met in the vicinity of the equator and turned offshore to the southeast. Surface velocities in the EACC were about 200 cm s−1, and its transport in the top 100 m was 15×106 m3 s−1. With the initial onset of southerly winds in early May the Somali Current reversed. By mid-May at 3°N surface speeds of 200 cm s−1 were observed. The flow did not continue up the coast, but turned offshore by 4°N. The second onset of the monsoon took place in mid-June. Shortly after this, surface currents >350 cm s−1 were observed at the turn-off region. The transport at 3°N was 27×106 m3 s−1 in the top 100 m. Farther to the north, northeasterly flow was observed as early as March. With the monsoon onset in June a strong anticyclonic circulation developed between 5 and 10°N. This continued to strengthen into July. The transport in the top 100 m in late June was 22×106 m3 s−1.

Energetic, organized flows were observed below the surface layer. North of the equator between 100 and 450 m, the flow was already to the northeast in mid-April. This coastal flow was fed by westward flow along the equator at this level. Little change was seen in this portion of the water column with the monsoon onsets. Around 700 m along the coast the flow was to the southwest with a speed of 50 cm s−1 in late April to mid-May. Flow at 700 m along the equator was to the west. Close to the coast this turned to the southwest. With the reversal in the surface current, the deep southwestward flow also reversed north of the equator. This deep northeastward flow increases the net transport at 3°N to about 80×106 m3 s−1. A persistent, nearshore, southwestward undercurrent was seen in the northern gyre in May–July.

The current changes in the surface layer were primarily related to changes in the local winds and a northward intrusion of the EACC into the Northern Hemisphere. No direct evidence for strong remote forcing was seen. It is not clear what causes the changes in the deep water.

Full access
R. G. Wilson and D. E. Petzold

Abstract

A mathematical model is presented which is used to calculate the flux of solar radiation to a melting snowpack in sub-arctic woodlands. Using tree height, branch radius, and distance between trees as input data, the model calculates both hemispherical and angular view factors. The latter are checked by a special photographic technique and good agreement is obtained. The view factors are used in conjunction with measurements of global and diffuse solar radiation at a base station to calculate the corresponding fluxes in nearby woodland. A test of the model at Schefferville, Quebec, showed that calculated and observed values were highly correlated, although the model consistently produced slight underestimates.

Full access
R. D. Hill, R. G. Rinker, and H. Dale Wilson

Abstract

The production Of nitrogen oxides (NO and NO2) by lightning flashes has been computed from a model of gaseous molecular reactions occurring as heated lightning-channel air cools by mixing with surrounding ambient air. The effect of ozone (O3) on the production of nitrogen oxides has also been investigated in this model and it has been found that the O3 oxidizes NO to NO2 mainly at the end of the cooling process. The maximum total global production rate of nitrogen oxides by lightning is estimated to be ∼6×1027 molecules per second, or 14.4×106 tonnes of NO2, per year.

Full access
G. Szeicz, D. E. Petzold, and R. G. Wilson

Abstract

No abstract available.

Full access
G. Szeicz, D. E. Petzold, and R. G. Wilson

Abstract

In the subarctic region of central Labrador wind speeds were measured at 2 m height in open lichen woodlands of various stand densities and were related to standard winds recorded at the same level on the local airport site. The resulting reduction in wind speeds are shown to be closely related to stand parameter h * which is a function of average tree height, stand density and shrub cover; variables that can easily be obtained from airphotos or from direct ground surveys. The equation giving the ratio of wind in the woodland u(s)to that measured at the air field u(s) is u(s)=u(s)(1+βh *)−2 with β=1.16 for s=2, and s=2.0 m. The equation seems applicable to various types of stands that do not streamline in wind, ranging from open lichen cover without trees, to a dense but leafless deciduous winter hardwood forest stand. For the typical and geographically widespread open lichen woodland of the subarctic, h * was related to the usual silvicultural measure of trunk diameter at breast height (DBH) offering a useful short cut in possible ground surveys.

Full access
James W. Wilson, R. E. Carbone, J. D. Tuttle, and T. D. Keenan

Abstract

This paper examines influences on the short-range prediction of organized convection under conditions of strong diurnal forcing. The analyses are based on data provided by the Maritime Continent Thunderstorm Experiment, which was conducted in 1995 over the Tiwi Islands (11°S) 50–100 km north of the Australian continent. Organized convection over the Tiwi Islands is often dubbed “Hector” by residents and researchers alike. The authors' purpose is to utilize results from these analyses to improve convective storm “nowcast” systems and their associated forecast and warning products.

The environmental near-surface wind direction is shown to be singularly influential in statistically predicting the location of convection and its time of occurrence over the Tiwi Islands. This finding is robust, despite the fact that mean wind speeds were merely 1–4 m s−1. The island boundary layer water vapor mixing ratio, derived from a morning sounding and inland surface stations, is shown to be correlated with measures of overall convective activity. Mesonet inferences of ambient water vapor anomalies, together with geostationary satellite cloud imagery and radar data, point to favored locations for individual thunderstorm initiation nowcasts.

The detailed evolution of Hector thunderstorms on any given day is dependent upon the location and movement of quasi-chaotic interactions among sea breezes, gust fronts, cumulus clouds, and existing storms. The primary mechanism for increasing the size of Hector was an excitation of new convection and merging with existing storms in response to forcing by a westward-propagating gust front. This is fully consistent with previous works, which have examined the effects of convergence lines, cold pools, shear, and the merging of radar echoes. Dissipation of Hector may be predicted as it moves westward from land to ocean and, occasionally, when it moves over land to areas that have been cooled by earlier storms.

The results of this study have important implications for expert and numerically based forecasting methods concerned with thunderstorm prediction in the 0–6-h range. A twofold approach exhibits promise in the Tiwi Islands: 1) use of statistical information provided by a dynamically based climatology (∼6 h forecast) and 2) monitoring and extrapolation of existing convergence lines, storms, and cumulus clouds for individual thunderstorm predictions (0–2 h). Variational assimilation of such information into high-resolution forecast models should lead to improved dynamical predictions in the 2–6-h range.

Full access
J. W. Wilson, S. B. Trier, D. W. Reif, R. D. Roberts, and T. M. Weckwerth

Abstract

During the Plains Elevated Convection at Night (PECAN) experiment, an isolated hailstorm developed on the western side of the PECAN study area on the night of 3–4 July 2015. One of the objectives of PECAN was to advance knowledge of the processes and conditions leading to pristine nocturnal convection initiation (CI). This nocturnal hailstorm developed more than 160 km from any other convective storms and in the absence of any surface fronts or bores. The storm initiated within 110 km of the S-Pol radar; directly over a vertically pointing Doppler lidar; within 25 km of the University of Wyoming King Air flight track; within a network of nine sounding sites taking 2-hourly soundings; and near a mobile mesonet track. Importantly, even beyond 100 km in range, S-Pol observed the preconvection initiation cloud that was collocated with the satellite infrared cloud image and provided information on the evolution of cloud growth. The multiple observations of cloud base, thermodynamic stability, and direct updraft observations were used to determine that the updraft roots were elevated. Diagnostic analysis presented in the paper suggests that CI was aided by lower-tropospheric gravity waves occurring in an environment of weak but persistent mesoscale lifting.

Full access
E. P. Lozowski, R. B. Charlton, C. D. Nguyen, and J. D. Wilson

Abstract

The Edmonton monthly mean temperature record has been examined using the concept of the cumulative high frequency monthly mean temperature anomaly, I. The time sequence of I is shown to exhibit bounded, oscillatory, nonperiodic behavior.

At times features such as annual and quasi-triennial cycles and sudden reversals are exhibited. Some implications of these observations for interannual climate modeling and forecasting are discussed.

Full access
T. K. Flesch, J. D. Wilson, L. A. Harper, B. P. Crenna, and R. R. Sharpe

Abstract

The gas emission rate Q from an artificial 36-m2 surface area source was inferred from line-average concentration C L measured by an open-path laser situated up to 100 m downwind. Using a backward Lagrangian stochastic (bLS) model, a theoretical C L/Q relationship was established for each experimental trial by simulating an ensemble of fluid-element paths arriving in the laser beam under the prevailing micrometeorological conditions. The diagnosed emission rates (Q bLS) were satisfactory for trials done when Monin–Obukhov similarity theory gave a good description of the surface layer, but were poor during periods of extreme atmospheric stability (|L| ≤ 2 m) and transition periods in stratification. With such periods eliminated, the average value of the 15-min ratios Q bLS/Q over n = 77 fifteen-minute trials spanning 6 days was 1.02. Individual 15-min estimates, however, exhibited sizable variability about the true rate, with the standard deviation in Q bLS/Q being σ Q/Q = 0.36. This variability is lessened (σ Q/Q = 0.22, n = 46) if one excludes cases in which the detecting laser path lay above or immediately downwind from the source—a circumstance in which the laser path lies at the edge of the gas plume.

Full access
Kevin Hamilton, R. John Wilson, J. D. Mahlman, and L. J. Umscheid

Abstract

The long-term mean climatology obtained from integrations conducted with different resolutions of the GFDL “SKYHI” finite-difference general circulation model is examined. A number of improvements that have been made recently in the model are also described. The versions considered have 3° × 3.6°, 2° × 2.4°, and 1° × 1.2° latitude–longitude resolution, and in each case the model is run with 40 levels from the ground to 0.0096 mb. The integrations all employ a fixed climatological cycle of sea surface temperature. Over 25 years of integration with the 3° model and shorter integrations with the higher-resolution versions are analyzed. Attention is focused on the December–February and June–August periods.

The model does a reasonable job of representing the atmospheric flow in the troposphere and lower stratosphere. The simulated tropospheric climatology has an interesting sensitivity to horizontal resolution. In common with several spectral GCMs that have been examined earlier, the surface zonal-mean westerlies in the SKYHI extratropics become stronger with increasing horizontal resolution. However, this “zonalization” of the flow with resolution is not as prominent in the upper troposphere of SKYHI as it is in some spectral models. It is noteworthy that—without parameterized gravity wave drag—the SKYHI model at all three resolutions can simulate a realistic separation of the subtropical and polar night jet streams and a fairly realistic strength of the lower-stratospheric winter polar vortex.

The geographical distribution of the annual-mean and seasonal precipitation are reasonably well simulated. When compared against observations in an objective manner, the SKYHI global precipitation simulation is found to be as good or better than that obtained by other state-of-the-art general circulation models. However, some significant shortcomings remain, most notably in the summer extratropical land areas and in the tropical summer monsoon regions. The time-mean precipitation simulation is remarkably insensitive to the horizontal model resolution employed. The other tropospheric feature examined in detail is the tropopause temperature. The whole troposphere suffers from a cold bias of the order of a few degrees Celcius, but in the 3° SKYHI model this grows to about 6°C at 100 mb. Interestingly, the upper-tropospheric bias is reduced with increasing horizontal resolution, despite that the cloud parameters in the radiation code are specified identically in each version.

The simulated polar vortex in the Northern Hemisphere winter in the upper stratosphere is unrealistically confined to high latitudes, although the maximum zonal-mean zonal wind is close to observed values. Near the stratopause the June–August mean temperatures at the South Pole are colder than observations by ∼65°C, 50°C, and 30°C in the 3°, 2°, and 1° simulations, respectively. The corresponding zonal-mean zonal wind patterns display an unrealistically strong polar vortex. The extratropical stratosphere stationary wave field in the Northern Hemisphere winter is examined in some detail using the multiyear averages available from the 3° SKYHI integration. Comparison with comparable long-term mean observations suggests that the model captures the amplitude and phase of the stationary waves rather well.

The SKYHI model simulates the reversed equator-pole temperature gradient near the summer mesopause. The simulated summer polar mesopause temperatures decrease with increasing, horizontal resolution, although even at 1° resolution the predicted temperatures are still warmer than observed. The increasing resolution is accompanied by increased westerly driving of the mean flow in the summer mesosphere by dissipating gravity waves. The present results suggest that the SKYHI model does explicitly resolve a significant component of the gravity waves required to produce the observed summer mesopause structure.

The semiannual oscillation near the tropical stratopause is reasonably well simulated in the 3° version. The main deficiency is in the westerly phase, which is not as strong as observed. There is also a second peak in the amplitude of the semiannual wind oscillation at the top model level (0.0096 mb) corresponding to the observed mesopause semiannual oscillation. This simulated mesopause oscillation is weaker (by a factor of ∼3) than that observed. The simulation in the tropical stratopause and mesosphere changes quite significantly with increasing resolution, however. In the tropical lower stratosphere of the 3° model the zonal-mean zonal wind displays a very weak (∼3 m s−1 peak to peak) interannual variation, which-while rather irregular-does display a roughly biennial period and the downward phase propagation that is characteristic of the observed quasi-biennial oscillation.

Full access