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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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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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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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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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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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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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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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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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Robert L. Hendrick
,
Bruce D. Filgate
, and
Walter M. Adams

Abstract

The role of environmental characteristics of elevation, slope-aspect and forest cover in spatially differ-entiating rates of snowmelt is examined over a 43-mi2 northern Vermont watershed. Both energy balance considerations and selected melt observations indicate the operation of an effective natural snowmelt runoff control mechanism. The more spatially diverse the watershed combinations of elevation slope-aspect and forest cover are, the more effective the natural snowmelt control mechanism is apt to be.

Previous snowmelt models developed by the U.S. Corps of Engineers are modified to fit topographic and forest conditions in New England. The modified model is used to compare average snowmelt rates over a typical upland Vermont watershed, a highly diverse watershed in the White Mountains of New Hampshire, and a watershed of maximum homogeneity in the open flat Champlain Valley of Vermont. The profound role of environment in regulating snow-water release is apparent.

Simulated weather sequences of rapid melt days are run through the model to estimate effects of watershed environment on snowmelt. Rapid snowmelt runoff is extremely unlikely except in environmentally homogeneous watersheds or during unusual weather sequences in which sensible heat and condensation heat energy are added rapidly and uniformly to the entire watershed snow cover. As most of the snowmelt in northern New England results from solar radiation over watersheds of great environmental diversity, the region appears to enjoy a highly effective natural snowmelt runoff control mechanism.

Although soil types, geology, ground frost and other factors affect runoff, relatively simple analyses of forest, elevation and slope-aspect distributions over ungaged watersheds provide considerable information on their snowmelt runoff characteristics.

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