Search Results

You are looking at 1 - 10 of 64 items for

  • Author or Editor: B. L. Smith x
  • Refine by Access: All Content x
Clear All Modify Search
Wendy L. Wolf and Ronald B. Smith

Abstract

Values of mountain pressure torque are calculated for the Rocky and Andes Mountains and for the Tibetan Plateau in order to evaluate their conuibufion to the observed anomalous length-of-day (LOD) and atmospheric angular momentum (AAM) change during the period from late January to mid-February 1983. A period of rapid increase in AAM and LOD is found to coincide with unusually high values of mountain torque on the Rocky Mountains, associated with a midcontinent high-pressure event and a sequence of Pacific frontal cyclone lows hitting California. A subsequent decrease in AAM is not correlated with mountain pressure torquesover the three regions tested, thus implicating other regions or other transfer mechanisms.

Full access
W. L. Smith and H. B. Howell

Abstract

In this paper, the algorithm used for calculating the water vapor distribution from SIRS-B spectral radiances is given. Examples are presented illustrating the effects of errors in the water vapor absorption coefficients and the specified temperature profile on the retrieval of the water vapor profile. Comparisons of satellite-derived and radiosonde-observed water vapor profiles indicate that the errors of the SIRS-derived relative humidity in the middle troposphere (i.e., the 400–600 mb layer) are less than 20%. Relative humidity errors in the lower troposphere (600–1000 mb) are somewhat larger but still less than 30%.

Full access
Steven L. Mullen and Bruce B. Smith

Abstract

A study of sea-level cyclone errors which occurred in 24- and 48-h forecasts of the National Meteorological Center's nested grid model (NGM) is performed for the 1987–88 winter season (1 December 1987–31 March 1988). All available 0000 UTC and 1200 UTC forecast cycles are analyzed for North America and adjacent ocean regions. Errors in forecasted central pressure and position are computed.

NGM forecasts of cyclone central pressure average 0.6 mb too deep at 24 hours and 0.3 mb too deep at 48 hours. The root-mean-square (RMS) errors for central pressure are 5.7 mb at 24 hours and 7.9 mb at 48 hours. The mean systematic displacement errors are 29 km at 24 hours and 51 km at 48 hours, and are directed towards the west at both times. The mean absolute displacement errors are much larger, 268 km at 24 hours and 393 km at 48 hours. Cyclone movement is forecasted too slow more frequently than too fast. Large variability in the skill of successive runs characterizes NGM cyclone forecasts though, with ∼70%–∼75% of the temporal variance being associated with fluctuations having periods shorter than seven days.

The results for the 1987–88 NGM are compared to those for the limited-area fine-mesh model (LFM) for the 1978–79 winter season. The size of the systematic pressure error decreased 50%–75% over the past decade. The absolute displacement error only decreased 10%–15%. Forecast variability, as measured by the RMS error of central pressure, is ∼15% smaller at 48 hours but remains essentially the same at 24 hours.

The comparison of the NGM and LFM results suggests that the nature of the short-range forecast problem for wintertime extratropical cyclones has changed somewhat over the past decade. It now appears that the problem is no longer one of primarily reducing the systematic error but also involves minimizing the impact of variability among individual forecasts.

Full access
Steven L. Mullen and Bruce B. Smith

Abstract

Sea level cyclone errors for two contrasting planetary-scale flow regimes, a long-wave trough verses a long-wave ridge over western North America, are computed for the National Meteorological Center's Nested Grid Model (NGM) and “Aviation Run” of the Global Spectral Model (AVN). The study is performed for the 1987/88 and 1989/90 cool seasons (1 December–31 March). All available 24- and 48-h forecast cycles are analyzed for North America and adjacent ocean regions. Errors in the central pressure and position of the cyclone are computed.

Statistically significant differences in forecast skill are found between the two flow patterns. This finding suggests that the utility of cyclone forecasts can be improved if model performance is documented for other recurrent, persistent flow regimes.

Full access
Bruce B. Smith and Steven L. Mullen

Abstract

Sea level cyclone errors are computed for the National Meteorological Center's Nested-Grid Model (NGM) and the Aviation Run of the Global Spectral Model (AVN). The study is performed for the 1987/88 and 1989/90 cool seasons. All available 24- and 48-h forecast cycles are analyzed for North America and adjacent ocean regions. Forecast errors in the central pressure, position, and 1000-500-mb thickness of the cyclone center are computed.

Aggregate errors can be summarized as follows: NGM forecasts of central pressure are too low (forecast pressure lower than analyzed) by 0.72 mb at 24 h and 0.66 mb at 48 h, while AVN forecasts are too high by 2.06 mb at 24 h and 2.50 mb at 48 h. Variance statistics for the pressure error indicate that AVN forecasts possess less variability than those of the NGM. Both mean absolute displacement errors and mean vector displacement errors are smaller for the AVN. The NGM moves surface cyclones too slowly and places them too far poleward into the cold air; the AVN possesses a smaller, slow bias only. Both models contain a weak cold bias as judged from the 1000-500-mb thickness over the cyclone center.

The aforementioned aggregate error characteristics exhibit significant variability when the data are stratified by geographical region, observed central pressure, and observed 12-h pressure change, however. For most regional, central pressure, and pressure change categories, the AVN performs better than the NGM in terms of smaller mean pressure errors, reduced pressure error variances, and shorter displacement errors. One noteworthy exception is deepening systems where the NGM's systematic pressure errors are generally 2–3 mb smaller than the AVN's errors.

The impact that ensemble averaging of individual NGM and AVN cyclone forecasts has on skill is examined. An equally weighted average of the NGM and AVN increasingly becomes the best forecast (more skillful than both the AVN and NGM individually) as the difference between the two models increases. This finding suggests that ensemble averaging offers increased skill during situations when the NGM and AVN forecasts diverge widely.

Full access
Y. L. Lin and R. B. Smith

Abstract

The response of a stratified atmosphere to local heating is a common element in several problems in mesoscale dynamics. To investigate this response, a time-dependent linearized problem is solved analytically for an elevated, local heat source turned on as a pulse in a stratified, moving fluid. The thermally induced circulation in the vicinity of the drifting disturbance is qualitatively similar to that of a cumulus cloud in mean wind. The updraft at the center of this cloud is surrounded by the compensating downdrafts at early times even if that air has also been heated. Once the updraft at the drifting center weakens, upward motion begins in the adjacent regions. An integration of the pulse solution yields the response to steady heating, turned on at t = 0. As steady state is approached, this solution exhibits a region of positive displacement moving downstream while negative displacement develop near the stationary heat source. The solution offers an explanation to a curious negative phase relationship between heating and displacement and the lack of a true steady state noted by other authors. It is suggested that the nature of this response may help to explain three problems in mesoscale dynamics: cloud interaction, heat island/orographic rain, and the squall line.

Full access
James L. McElroy and Ted B. Smith

Abstract

Airborne lidar and supplementary measurements made during a major study of air chemistry in southern California (SCCCAMP 1985) provided a rare opportunity to examine atmospheric boundary-layer structure in a coastal area with complex terrain. This structure results from a combination of daytime heating or convection in the boundary layer (CBL), the intrusion of a marine layer into the inland areas, the thermal internal boundary layer (TIBL) formed within the marine onshore flow, inland growth of the TIBL, interactions of the CBL and the TIBL, and airflow interactions with terrain features.

Measurements showed offshore mixing-layer thicknesses during SCCCAMP to be quite uniform spatially and day to day at 100–200 m. Movement of this layer onshore occurred readily with terrain that sloped gradually upward (e.g., to 300 m MSL at 50 km inland), but was effectively blocked by a 400–500 m high coastal ridge. In the higher terrain beyond the coastal ridge, aerosol layers aloft were often created as a result of deep convection and of a combination of onshore flow and heated, upslope airflow activity. Such aerosol layers can extend far offshore when embedded in reverse circulations aloft.

The forward boundary of the marine layer was quite sharp, resembling a miniature cold front. Within the marine layer the onshore flow initiates a TIBL at the coastline, which increases in depth with distance inland due to roughness and convective influences. A coherent marine layer with imbedded TIBL was maintained for inland distances of 20–50 km, depending on terrain. Intense heating occurred inland prior to the arrival and undercutting by the marine front. The resulting, effective mixing layer increased in thickness from a few hundred meters to nearly two kilometers in a very short distance.

Comparisons of a representative, physically based TIBL and convective mixing-layer models with observed data indicate that they generally do a credible job of estimating the depth of the marine layer and the CBL when applied appropriately as a function of geographical location and physical situation. Empirical TIBL models usually did not perform as well as physically based models.

Full access
W. L. Smith, H. B. Howell, and H. M. Woolf

Abstract

It is shown that the partial interferogram measurement technique, originally developed to separate the trace gas emissions from a spectral signal dominated by background radiation (from the earth's surface) and emissions from major constituents (H2O and CO2), has application to the vertical sounding problem. The interferometric technique will enable relatively high vertical temperature profile resolution to be achieved and will provide absolute accuracies of temperature approaching, and at same levels exceeding, 1°C.

Full access
W. L. Smith, W. C. Shen, and H. B. Howell

Abstract

A seven-channel Multi-spectral Scanning Radiometer (MSR) was flown aboard the NASA Convair-990 aircraft during the GARP Atlantic Tropical Experiment (GATE) from June–September, 1974. The radiometer measures the total shortwave (0.2–5 μm) and longwave (5–50 μm) components of radiation and the radiation in specific absorption band and window regions that modulate the total radiation flux. Measurements of the angular distribution of radiation, including the upward and downward components, were obtained. The principal scientific objective of the MSR experiment was to obtain the atmospheric absorption data required for precise computations of radiative heating profiles from atmospheric state parameters. The method used to construct the infrared radiation heating computational model based on in situ GATE MSR observations is described. Radiative heating profiles computed with this model for both cloudy and cloudless atmospheres were compared with direct observations by flux radiometers and with profiles computed with the Rodgers and Walshaw model. The results indicate that the empirically based computational model should provide tropospheric radiative heating profiles sufficiently accurate for diagnostic and prognostic applications of GATE data.

Full access
H. P. BENNER, R. E. HAMBIDGE, L. B. OVERAAS, D. B. SMITH, and J. A. YOUNGBERG

Abstract

Previous studies of convective activity over northern and central California were based on data from scattered weather reporting stations. Radar, however, by maintaining a large area under constant surveillance, displays a more complete picture of convective cell formation and patterns. With the installation of the WSR-57 radar in Sacramento, Calif. in early 1960, the opportunity to study the formation and patterns of convective cells over northern and central California was made available. This article summarizes the findings of such a study for the summer months of 1960 and 1961.

Full access