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Walter D. Meyer

Abstract

Semiannual variations in the temperature and zonal wind have been found to be the dominant oscillations of the circulation in the tropical upper stratosphere and mesophere. Many hypotheses have been presented in an attempt to explain these semiannual variations. In this study a number of these hypotheses are examined through the use of a diagnostic numerical model for the zonally symmetric flow. It is found that a semiannually varying momentum source is required to drive the zonal wind oscillation. Although the precise form of this momentum source is not determined, some evidence is presented relating this source to the eddy momentum flux by tidal motions in the region above 40 km.

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Walter L. Jones and David D. Houghton

Abstract

A numerical model of internal gravity waves allows momentum transport by the waves to interact with the mean flow. Momentum deposited at a critical level develops a “shelf” in the mean flow. Mean flow acceleration Doppler-shifts the wave frequency, allowing more penetration of wave energy than expected from linear theory.

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Kenneth D. Leppert II and Walter A. Petersen

Abstract

It has been hypothesized that intense convective-scale “hot” towers play a role in tropical cyclogenesis via dynamic and thermodynamic feedbacks on the larger-scale circulation. In this study the authors investigate the role that widespread and/or intense lightning-producing convection (i.e., electrically hot towers) present in African easterly waves (AEWs) may play in tropical cyclogenesis over the east Atlantic Ocean.

The 700-hPa meridional wind from the NCEP–NCAR reanalysis dataset was analyzed to divide the waves into northerly, southerly, trough, and ridge phases. The AEWs were subsequently divided into waves that developed into tropical storms (i.e., developing) and those that did not develop into tropical storms (i.e., nondeveloping). Finally, composites were created using various NCEP variables, lightning data gathered with the Zeus network and worldwide lightning location network (WWLLN), and brightness temperature data extracted from the NASA global-merged infrared brightness temperature dataset.

Results indicate that in all regions examined the developing waves seem to be associated with more widespread and/or intense lightning-producing convection. This increased convection associated with the developing waves might be related to the increased midlevel moisture, low-level vorticity, low-level convergence, upper-level divergence, and increased upward vertical motion found to be associated with the developing waves. In addition, the phasing of the convection with the AEWs as they move from East Africa to the central Atlantic shows some variability, which may have implications for tropical cyclogenesis.

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Pierre D. Mourad and Bernard A. Walter

Abstract

The existence of synoptically distributed, coherent, linear mesoscale features with wavelengths of 12–18 km in a shallow (z l ≈ 150 m) atmospheric boundary layer is documented. These were observed north of Alaska over the ice-covered Beaufort Sea during the LEADEX program in April 1992. These banded features appear both in satellite infrared (but not visible) images and in concomitant in situ aircraft measurements. Those measurements were of cloud condensation nuclei (CCN), potential temperature (θ), and meridional velocity (v) and were taken within and above the arctic atmospheric boundary layer. These aircraft data also exhibit smaller-scale circulations with scales of 3–8 times the boundary layer depth. Based on analysis of our dataset, we argue that the 12–18-km motions may be due to a hybrid form of slantwise convection within the boundary layer. The authors use the term hybrid because some of the energy, scale selection, and orientation of the linear features may be modulated by the nonlinear mean wind profile in the boundary layer. The strongest arguments for slantwise convection are as follows: 1) a significant meridional (cross-band) flux of beat and CCN; 2) long-wavelength, two-dimensional circulation patterns aligned perpendicular to the strong, horizontal temperature gradient; 3) thin, warm bands parallel to thick, cold bands in the IR image, consistent with convection in the boundary layer; and 4) generally weak correlations between lead signals in a downward-looking radiometer and θ, v, and CCN. The data also suggest that at least the influence of the circulations can reach up beyond the well-mixed boundary layer into the stable, lower troposphere. (However, this signal cannot be dismissed as, nor definitely identified with, gravity waves.) It is noted that if slantwise convection is present as described, then it represents another mechanism with mesoscale organization over synoptic-scale regions by which the Arctic's atmospheric boundary layer and the overlying, stably stratified lower troposphere may exchange heat, momentum, and particulates. This is in addition to large leads and shear-generated turbulence in the boundary layer, both of which create vertical mixing in the Arctic's lower atmosphere that is spatially and temporally intermittent.

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Walter L. Jones and David D. Houghton

Abstract

A simple numerical model is used to demonstrate that momentum exchange between wave and mean flow can substantially modify the process of “breaking” of internal gravity waves at great height. The momentum exchange results in appreciable transfer of energy from wave to mean flow.

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Amir Shabbar, Walter Skinner, and Mike D. Flannigan

Abstract

An empirical scheme for predicting the meteorological conditions that lead to summer forest fire severity for Canada using the multivariate singular value decomposition (SVD) has been developed for the 1953–2007 period. The levels and sources of predictive skill have been estimated using a cross-validation design. The predictor fields are global sea surface temperatures (SST) and Palmer drought severity index. Two consecutive 3-month predictor periods are used to detect evolving conditions in the predictor fields. Correlation, mean absolute error, and percent correct verification statistics are used to assess forecast model performance. Nationally averaged skills are shown to be statistically significant, which suggests that they are suitable for application to forest fire prediction and for management purposes. These forecasts average a 0.33 correlation skill across Canada and greater than 0.6 in the forested regions from the Yukon, through northern Prairie Provinces, northern Ontario, and central Quebec into Newfoundland. SVD forecasts generally outperform persistence forecasts. The importance of the leading two SVD modes to Canadian summer forest fire severity, accounting for approximately 95% of the squared covariance, is emphasized. The first mode relates strongly to interdecadal trend in global SST. Between 1953 and 2007 the western tropical Pacific, the Indian, and the North Atlantic Oceans have tended to warm while the northeastern Pacific and the extreme Southern Hemisphere oceans have shown a cooling trend. During the same period, summer forest fire exhibited increased severity across the large boreal forest region of Canada. The SVD diagnostics also indicate that the El Niño–Southern Oscillation and the Pacific decadal oscillation play a significant role in Canadian fire severity. Warm episodes (El Niño) tend to be associated with severe fire conditions over the Yukon, parts of the northern Prairie Provinces, and central Quebec. The linearity of the SVD manifests opposite response during the cold (La Niña) events.

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Walter M. Hannah and Eric D. Maloney

Abstract

The sensitivity of a simulated Madden–Julian oscillation (MJO) was investigated in the NCAR Community Atmosphere Model 3.1 with the relaxed Arakawa–Schubert convection scheme by analyzing the model’s response to varying the strength of two moisture sensitivity parameters. A higher value of either the minimum entrainment rate or rain evaporation fraction results in increased intraseasonal variability, a more coherent MJO, and enhanced moisture–convection feedbacks in the model. Changes to the mean state are inconsistent between the two methods. Increasing the minimum entrainment leads to a cooler and drier troposphere, whereas increasing the rain evaporation fraction causes warming and moistening. These results suggest that no straightforward correspondence exists between the MJO and the mean humidity, contrary to previous studies.

Analysis of the mean column-integrated and normalized moist static energy (MSE) budget reveals a substantial reduction of gross moist stability (GMS) for increased minimum entrainment, while no significant changes are found for an increased evaporation fraction. However, when considering fluctuations of the normalized MSE budget terms during MJO events, both methods result in negative GMS prior to the deep convective phase of the MJO. Intraseasonal fluctuations of GMS, rather than the mean, appear to be a better diagnostic quantity for testing a model’s ability to produce an MJO.

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Albert D. Anderson and Walter E. Hoehne

Strips of metal foil (window), dispersed by balloon and aircraft, have been tracked by radar to measure wind velocities at altitudes up to 74,000 feet. These wind velocities have been compared with those measured over the same altitude range by GMD-1A equipment and radar-target tracking. The results indicate promise for obtaining high-altitude winds by this new technique. Further experiments envisioned for the 100,000 to 200,000 foot altitude range will necessitate the use of rockets to carry and eject the window.

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Lawrence D. Carey and Walter A. Petersen

Abstract

Estimating raindrop size has been a long-standing objective of polarimetric radar–based precipitation retrieval methods. The relationship between the differential reflectivity Z dr and the median volume diameter D 0 is typically derived empirically using raindrop size distribution observations from a disdrometer, a raindrop physical model, and a radar scattering model. Because disdrometers are known to undersample large raindrops, the maximum drop diameter D max is often an assumed parameter in the rain physical model. C-band Z dr is sensitive to resonance scattering at drop diameters larger than 5 mm, which falls in the region of uncertainty for D max. Prior studies have not accounted for resonance scattering at C band and D max uncertainty in assessing potential errors in drop size retrievals. As such, a series of experiments are conducted that evaluate the effect of D max parameterization on the retrieval error of D 0 from a fourth-order polynomial function of C-band Z dr by varying the assumed D max through the range of assumptions found in the literature. Normalized bias errors for estimating D 0 from C-band Z dr range from −8% to 15%, depending on the postulated error in D max. The absolute normalized bias error increases with C-band Z dr, can reach 10% for Z dr as low as 1–1.75 dB, and can increase from there to values as large as 15%–45% for larger Z dr, which is a larger potential bias error than is found at S and X band. Uncertainty in D max assumptions and the associated potential D 0 retrieval errors should be noted and accounted for in future C-band polarimetric radar studies.

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Michael K. Walters, Jeffrey D. Shull, and Robert P. Asbury III

Abstract

The Schrader and Schumann contrail forecast algorithms and a third algorithm are evaluated under low relative humidity conditions using a dataset of asynoptic atmospheric soundings and 318 coincident ground-based aircraft and contrail observations collected near Dayton, Ohio. Aircraft were positively identified and their flight altitudes were determined using either Federal Aviation Administration flight logs or data collected with the Wright–Patterson Air Force Base air traffic control radar. The sounding data were used with assumed aircraft performance parameters to prepare nowcasts of contrail critical temperatures. The nowcasts were examined subjectively by comparing the distribution of correct and incorrect forecasts as a function of the difference between critical temperature and ambient temperature. Objective evaluation against the contrail observations also was done using several common statistical measures. The third algorithm produced critical temperatures that were systematically too cold, leading to forecasts with low skill in comparison with unbiased random forecasts. The Schumann and Schrader algorithms were skillful in comparison with unbiased random forecasts and performed similarly to one another. The distribution of critical temperature deviations from ambient temperature for incorrect forecasts made with the Schumann and Schrader algorithms (but not the third algorithm) can be explained by potential errors in the input data. All results were statistically significant.

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