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John L. Stanford

Abstract

Satellite microwave brightness temperature (radiance) measurements represent weighted mean temperatures in a certain atmospheric layer. The high density of measurements along a polar-orbiting satellite track allows estimates of the brightness temperature variance spectrum at high meridional wavenumbers. Global data from the Nimbus 6 SCAMS microwave instrument sensitive to the 100–300 mb layer have been analyzed for 30 days in August 1975. The results reveal that the brightness temperature variance spectrum can be fit to the form km over the range k=22 to 59 (horizontal wavelengths 1800 to 700 km). Using 95% confidence intervals, the fit yields m = −3.1±0.3

These results appear to have significance for estimates of energy cascade between different motion scales in the very high wavenumber regime.

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John L. Stanford

Abstract

Observational data and theoretical calculations are presented which suggest a possible sink for stratospheric water vapor in the winter antarctic.

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John L. Stanford

Abstract

A method is proposed whereby it should be possible to obtain valuable information about the amount of water vapor in the lower stratosphere from high-latitude observing locations in winter. Laser-radar scattering returns vs height would be examined for regions exhibiting enhanced returns (over the background due to stratospheric aerosols and atmospheric density fluctuations). These data would be compared with water vapor saturation mixing ratios calculated from radiosonde measurements. Climatological aspects of cold regions of the stratosphere are reviewed and possible locations for such observations are presented. The best such locations will be in Antarctica in winter months. Estimates of the stratospheric condensation particle radii and concentrations suggest that existing laser-radar systems used in studies of stratospheric aerosols have the sensitivity needed to undertake the proposed measurements. Aircraft-borne laser-radar systems would provide particularly interesting information, especially in flight across the winter Antarctic. Careful interpretation of the data should yield upper limits on the stratospheric water vapor mixing ratio on days when no enhanced scattering returns are received, and should provide detailed information on the spatial and temporal variation of the mixing ratio when enhanced returns are observed.

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John L. Stanford

Abstract

It has been widely accepted that electromagnetic noise at low radio frequencies from thunderstorms is mainly vertically polarized. This paper re-examines the existing 500 kHz data of Kohl, taking into account the electrical conductivity of the earth. The corrections involved are shown to be crucial, revealing that Kohl's data actually infer that the vertical component of the rf electric field is several times smaller than its horizontal component. This result is of importance in understanding the relationship between the electromagnetic noise spectrum generated by lightning and the details of the stroke process. The physics of the calculated correction is discussed, as well as the limits of validity for the calculation. Experiments to measure the polarization ratio for other types of terrain are suggested and the theory is used to predict a value for the expected ratio for the midwestern United States.

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John L. Stanford

Abstract

Daily global grids of satellite radiance data over a 24-month period (1973–74) are decomposed into spherical harmonic functions. It is shown that a certain function of the expansion coefficients can be interpreted as the latitudinal wavenumber (k) dependence for global stratospheric temperature variance spectra. Beyond a forcing region, the spectra can be fit to the form k m , with m=−4.1±0.6 for the upper and lower stratosphere, respectively. To within the data scatter, no seasonal dependence is found for m. The onset of the inertial subrange occurs at lowers k for the upper stratosphere than for the lower stratosphere. From a theoretical standpoint it is shown that a sufficient condition for isotropy/homogeneity of the fluctuation spectrum for a two-dimensional scalar on the surface of a sphere is that the variance matrix elements be independent of zonal wavenumber. This leads to a prediction of m=−3.6 for the parameters appropriate to the present investigation. The detailed results suggest that the discrepancy between observation and theory is due in part to significant anisotropy effects on large-scale fluctuations. These results are thought to be the first measurements of latitudinal power spectra for large-scale atmospheric motions and, in addition, to represent the first spectral measurements of any kind for the inertial region of large-scale motions in the stratosphere.

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John L. Stanford

Observational data from the lower stratosphere are examined to determine to what extent an upper limit can be placed on the water-vapor content of that region of the atmosphere, a quantity which is presently in controversy by half an order of magnitude. Presented herein are the details of a systematic search of 37 winter months (November 1964–December 1971) of Northern Hemispheric 30-mb and 50-mb synoptic maps, as well as a search of over 170,000 individual radiosonde 30-mb temperature soundings for the same period. In addition, the results of a search of all available high-latitude Northern Hemispheric meteorological rocket data and a study of the 30-mb climatology for Jan Mayen Island are presented. The results of these studies are compared with the results of a careful literature search, as well as an inquiry to an airline pilot's volunteer organization, for reports of stratospheric cloud observations. The extreme sparsity of such observations, even with allowance for possible obscuring effects of tropospheric cloud cover, leads to the conclusion that water-vapor saturation is seldom reached in the lower stratosphere of the Northern Hemisphere. This conclusion, coupled with the results of the fairly extensive stratospheric observational data mentioned above, appears to be inconsistent with the concept of an average “moist” (10 ppm or greater) lower stratosphere. From the results of the present investigation, it may be inferred that the average 30-mb and 50-mb water-vapor mixing ratios, for the dates investigated, is almost certainly less than 11 ppm, and probably less than 6–8 ppm. The data appear to constitute reasonable evidence for setting an upper limit on the average 30-mb water-vapor mixing ratio at approximately the 6–8 ppm level for high latitudes in the Northern Hemispheric winter.

These 30-mb results fall in between the values quoted by investigators reporting a “dry” (a few ppm mixing ratio) and “moist” (≥ 10 ppm mixing ratio) lower stratosphere, and thus appear to provide a reasonably definitive choice on the question, independent of instrument contamination and calibration problems that have led to discrepancies in stratospheric water-vapor measurements in the past.

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Tsing-Chang Chen
and
John L. Stanford

Abstract

It is shown that the variance of satellite-derived 15 μm radiance measurements can be utilized to provide a relative measurement of stratospheric zonal (Az ) and eddy (AE ) available potential energy. Daily global grids of radiance from the Nimbus 5 Selective Chopper Radiometer are used to calculate daily radiance variances for the two-year period 1973–74. Seasonal and interhemispheric variations are examined as a function of altitude. The results reveal the following:

1) The maximum value of Az in the upper and middle stratosphere shows pronounced variations between winter and summer; in contrast, little variation occurs in the lower stratosphere Az .

2) The aperiodic occurrence of sudden warmings complicates the seasonal variation of Az and AE in the stratosphere, making the energetics irregular. Sudden warmings are observed to drastically enhance AE and precipitously decrease Az in the stratosphere.

3) The lower stratosphere exhibits a distinct interhemispheric difference for Az and AE :

In the upper stratosphere, only slight hemispheric differences occur.

4) Time-Fourier analysis reveals that the primary variation of AE and Az in the stratosphere is annual and semiannual, respectively. In the NH, the phase of the semiannual component of Az occurs slightly before solstices in the upper and middle stratosphere, and slightly after solstices in the lower stratosphere: The phase lag between the two layers is 2–4 weeks. The phase of the AE annual component occurs 2–4 weeks after winter solstice in the NH, but considerably longer lags occur in the SH.

The results suggest differences between hemispheres in the relative importance of vertical propagation of energy by planetary waves and that eddy activity in the upper stratosphere may be due to an in situ mechanism.

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Charles Notis
and
John L. Stanford

Abstract

The synoptic and physical characteristics of Oklahoma tornadoes are studied for the 16-year period 1959–74. When the Oklahoma results are compared with tornadoes in Iowa (Notis and Stanford, 1973), grouping tornadoes into classes based upon direction of movement reveals a number of differences in tornado characteristics between the southern and central portions of the Midwest. In both states a strong correlation between tornado direction of movement and 500 mb flow direction is found. These results, based on 534 tornadoes in Oklahoma and Iowa, provide a statistical basis for tornado movement forecasts. Since it is widely believed by the public that tornadoes move from the southwest, such forecasts may be of greatest value in the northern portions of the Midwest, under conditions of northwesterly flow aloft.

Median and expected path lengths for Oklahoma tornadoes are found to be ∼1 mi and 4.2 mi, respectively. A χ2 test reveals that the tornado path length distributions are not independent of 500 mb speed in the 30–70 kt range.

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Anne R. Douglass
and
John L. Stanford

Abstract

It is shown that the gradient of satellite-derived radiance is proportional to a certain vertical integral of the geostrophic wind. The radiance gradient is shown to be useful in locating the upper tropospheric jet maxima in middle latitudes; the major uncertainty results from lack of specific knowledge of the lower stratospheric winds above the area in question. At high latitudes, the radiance gradient delineates regions of stratospheric jet maxima.

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Charles Notis
and
John L. Stanford

Abstract

Results are presented from a systematic study of 386 Iowa tornadoes reported over a 13-year period (1959–71). The tornado occurrences are studied as a function of time of day, date, path length, width, 500-mb flow, and their relationship to associated fronts and low-pressure centers. The data surprisingly reveal that Iowa tornadoes fall naturally into two rather distinct classes: northeast-advancing (NE) and southeast-advancing (SE) tornadoes. There also appear to be subgroups of tornadoes occurring ahead of the front and on or behind the front. Considering all the tornadoes studied, the tornado direction of movement is found to be closely tied to the 500-mb flow (statistically significant at the 99.99% level). Tornado path lengths tend to be longest during the spring.

Some contrasts found between the two main classes of Iowa tornadoes are the following:

  1. NE and SE tornadoes comprise approximately 70% and 30%, respectively, of the total in Iowa.

  2. NE tornadoes exhibit a bimodal annual distribution occurring predominantly in the spring and again in the fall, while SE tornadoes have a peak occurrence in the summer.

  3. NE tornadoes tend to occur ahead of the front, while SE tornadoes tend to occur on or behind the front.

  4. NE tornadoes are more closely associated with a low than SE tornadoes.

  5. SE tornadoes tend to occur slightly later in the day than NE tornadoes.

  6. The relationship between path length and width is rather complicated and is independent of direction of movement but depends upon the location of the tornado relative to the front.

Because of the results of recent tornado-damage studies which showed that first-floor rooms facing the tornado direction of approach were statistically less safe than other areas, application of some of the present results promises to lead to increased public safety in upper-midwest tornadic storms.

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