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Thomas H. Achtor and Lyle H. Horn

Abstract

A set of 70 cases of spring season Colorado cyclone events is used to form composites which describe the upper (300 mb) and lower (850 mb) tropospheric wind fields during the early stages of cyclone formation. The 70 cases are partitioned into those which persist beyond 72 h (developing) and those which fill after 24 h (nondeveloping). The developing sample reveals a well-defined 300-mb wind maximum embedded in a short wave trough which propagates eastward during the six time periods studied. However, the nondeveloping sample composite exhibits little structure to the 300-mb wind pattern. A subsample of the developing cases, chosen on the basis of the presence of a jet streak over the New Mexico-Texas area, shows a stronger 300-mb wind maximum. The 850-mb composites show southerly flow in the southern Great Plains in the two samples and the subsample. The developing sample and jet streak subsample exhibit a marked increase in 850-mb wind speed as the exit region of the 300-mb jet maximum propagates over the region.

Composites of the departure of the 300-mb observed wind from the geostrophic wind show that in the developing and jet streak cases the normal component of the ageostrophic wind is directed to the right of the wind in the exit region of the jet. This is consistent with the upper transverse branch of an indirect circulation. The 850-mb ageostrophic winds are directed toward the north in all three composites; however, the developing sample and jet streak sub-sample exhibit slightly stronger values and a greater north-south geographical extent than the nondeveloping sample. The relatively stronger 850-mb ageostrophic flow in the presence of a composite 300-mb jet may reflect the existence of the lower transverse branch of an indirect circulation in the exit region of the jet.

The sites of the 39 developing cases are nearly all located in the left front quadrant of the composite 300-mb jet. This favored location is discussed in terms of upper tropospheric divergence resulting from the viewpoints of vorticity advection and a transverse indirect circulation in the exit region of the 300-mb jet.

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ELFORD G. ASTLING and LYLE H. HORN

Abstract

The excellent North American radiosonde network is used to calculate the poleward energy transport for the continental area during the period January–March 1966. The transport of sensible and latent heat and geopotential and kinetic energy is partitioned according to four circulation modes—mean and transient meridional circulations and stationary and transient eddy circulations. In addition, the roles of various synoptic features in the transient eddy flux are examined.

The mean meridional transport was computed in two ways. One involved a calculation of the contribution of the North American sector to the hemispheric mean meridional transport. Because of strong meridional flow at high levels and a lack of compensating flow at low levels, very large transports were obtained. The transports were much greater than the average for the entire hemisphere and point up the helical structure of the meridional cells. To obtain comparisons with other modes of transport, we made another calculation of the mean meridional transport by subtracting the vertical mean component from the longitudinal average. The results show that the energy transports were large and positive in subtropical latitudes and were zero or small and negative in middle latitudes.

Of the remaining modes, the transient eddy mode was the most effective in transporting energy poleward. The maximum transport occurred at 40°N for both the hemisphere and for North America; however, the value for North America was about 50 percent larger and the latitudinal variation was considerably greater than for the hemisphere. Sensible heat transport was largest, with the maximum latent transport amounting to one-half the sensible heat. Energy fluxes by the standing eddy and transient meridional modes were relatively small.

A brief study of the importance of various large-scale synoptic features in transporting energy indicated that large-amplitude troughs with closed 500-mb Lows are most effective in the transient eddy transport. Indications exist that the largest poleward energy transport is accomplished during the intensifying stage of baroclinic disturbances associated with the 500-mb Lows.

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Roberta A. Marshment and Lyle H. Horn

Abstract

A composite based on 39 cases of April and May Colorado cyclogenesis is used to examine the moisture distribution and its evolution during the early stages of Colorado cyclones. Moist static stability and its changes are also examined using the Showalter index.

In the composite, a narrow tongue of relatively higher surface dew points extends northward on the day preceding cyclone development. This tongue expands and shifts eastward on the following two days. Maps displaying the specific humidity at four levels between the surface and 700 mb reveal that the moisture is mainly confined to 850 mb and below. East-west cross sections of isentropes, normal wind component and specific humidity provide increased vertical resolution, which confirms the concentration of moisture at low elevations. The normal wind component (approximately from the south) shows a diurnal tendency (stronger at night) but also the influence of synoptic male forcing. The strongest southerly winds and northward moisture transport occur at 1200 GMT beneath the exit region of a 300-mb jet maximum.

Maps of the Showalter index show the weakest static stability initially in the form of a tongue extending from northeastern Mexico into Oklahoma. As the 300-mb jet propagates into the area, a pattern of weaker stability develops, particularly in the exit region of the jet. The pattern of weaker static stability shifts rapidly east-northeastward as the cyclone develops.

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William E. Togstad and Lyle H. Horn

Abstract

Often important weather-producing features such as the jet streak are not adequately resolved at the input stage of numerical models. The satellite indirect sounding technique offers promise of greatly increasing observational detail. An 18 March 1971 case study is used to test the ability of the SIRS algorithm to resolve the thermal support of a jet streak. Isentropic cross sections through the streak are prepared and 17 synthetic soundings are obtained. The sounding data are reduced to equivalent irradiances, and the SIRS algorithm is used to retrieve the thermal structure for various assumed observational errors, grid spacings, and initial guess profiles. While an observational error of 0.25 erg (cm2 sec sr cm−1)−1 permits the, reconstruction of the general wind field, accuracies to within 0.10 and preferably 0.05 erg are required to resolve the essential structure of the jet stxeak's thermal support.

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Linda M. Whittaker and Lyle H. Horn

Abstract

The isentropic form for available potential energy (APE) is used to analyze the impact of the inclusion of satellite temperature retrieval data on forecasts made with the NASA Goddard Laboratory for Atmospheres (GLA) fourth order model. Two analysis datasets are used for the forecasts, one containing the NEDIS TIROSN retrievals and the other GLA retrievals using the physical inversion method. A third analysis dataset did not contain satellite data and was used as a control. Two analysis datasets, with and without satellite data, were used for verification.

Northern Hemisphere values for the total APE show an increase throughout the 72 h forecast period for all three sets, mostly due to an increase in the zonal component, in contrast to the verification sets, which showed a steady level of total APE. The three forecast sets start with different values of total APE but by 36 h, the differences begin to diminish. At 72 h the total APE values for the three sets are almost identical. The magnitude of the total APE in the Southern Hemisphere does not increase in time and remains within the range of the verification sets.

The vertically integrated grid point distributions of the eddy APE which provide geographical representations of baroclinic zones show little difference between the 0 h forecast fields and the analysis fields. At the end of the forecast period, however, there are moderate differences in the southern Pacific and Indian oceans. The grid point distributions of eddy APE are quite different among the three forecasts as well as the verification sets. In the Northern Hemisphere there are very pronounced differences in grid point distribution over Asia, the North Pacific Ocean and North America. These differences increase in area and magnitude as the forecast period progresses. They show coherence in their eastward progression with time across the eastern Pacific and North America. Examination of the grid point distributions indicates that the forecasts are slow, for example, in developing a low in the northern Rockies and never correctly capture its position or intensity. The differences noted above are a reflection of incorrect timing, position and/or magnitude. Isentropic cross sections for 50°N show that the lime lag exists around the entire parallel. The forecast set using the GLAS rerievals does somewhat better than the other sets in predicting intensity and position of developing lows, but all three forecasts resemble each other rather than the verification sets. This would seem to indicate that model characteristics are overwhelming any differences in the data sets.

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Elford G. Astling and Lyle H. Horn

Abstract

Infrared radiation observations made by TIROS II during 27 days between 26 November 1960 and 6 January 1961 are used to obtain mean latitudinal profiles of outgoing long-wave radiation. The mean profiles revealed an insufficient estimate of limb darkening in the data. Consequently, only subsatellite observations are used to obtain mean latitudinal profiles for oceanic and continental areas. The distinct differences which are noted between the land and ocean profiles are discussed. Finally, the means obtained from the TIROS II data are found to agree well with estimates of the terrestrial radiation based on theoretical considerations and indirect quantitative estimates.

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Byron A. Paulson and Lyle H. Horn

In an attempt to improve the quality of Nimbus-6 soundings, the Man-computer Interactive Data Access System (McIDAS) at the University of Wisconsin is used to manually edit individual (scan spot) High-resolution Infrared Sounder (HIRS) soundings. Unlike the Nimbus-6 Data System Test (DST) soundings that are derived from averages of up to 84 spot radiance measurements and the TIROS-N and NOAA-6 operational soundings that can involve averages of up to 63 spot measurements, the HIRS soundings used in this study were derived from single spot radiances or averages of five single spot soundings. Also, unlike the DST soundings, the HIRS McIDAS retrievals used contemporary surface (instrument shelter) temperatures as a pseudoinfrared window channel to aid in cloud filtering. These McIDAS-generated soundings were used to analyze 1) level temperature fields at 850, 500, and 300 mb; 2) the 850–300 mb thickness field; and 3) the 500 mb height field over eastern Europe for a 20 August 1975 case. The excellent radiosonde network in this area is used for verification purposes.

While the individual spot soundings offer little improvement over the DST data, the averages of five of these soundings provide analyses that are distinctly superior to the operational DST soundings. Although improvement is noted in the root mean square and bias scores, the largest improvements are found in the S 1 score, which is a measure of gradient comparison. The radiosonde verification data are partitioned into subsets and in some tests are combined with the HIRS soundings. Other tests involve comparisons in which the radiosonde data consist of only mandatory-level data. The various tests indicate that the edited HIRS soundings averaged in small groups are comparable to mandatory-level radiosonde data in constructing 500 mb height analyses.

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Linda M. Whittaker and Lyle H. Horn

Abstract

The geographical and seasonal distributions of cyclogenesis are studied for the North American sector for the period 1958–77. The primary zone of cyclogenesis is located between 35 and 40°N with this zone providing a maximum in all seasons except summer when a broad maximum exists between 45 and 55°N. Of the individual months, March provides a maximum and September a minimum. The most active areas are: 1) the East Coast, including the Gulf of Mexico coast in winter, 2) Colorado and Great Basin, 3) Alberta and Northwest Territories. A pronounced maximum exists in the Colorado–Great Basin area in March, while at the same time a minimum is present in Alberta–Northwest Territories. As summer approaches the Colorado activity declines markedly while activity in the Alberta–Northwest Territories region increases somewhat. Of the various regions, the greatest concentration of cyclogenesis occurs in the Colorado region. This is attributed to the sharp eastward decline in the terrain in this region and to the fact that the polar jet stream is present at this latitude through much of the year.

The frequency of North American cyclogenesis exhibits a statistically significant decline between 1958 and 1977. The secular trend is examined for each of the four midseason months. The largest downward trend is in January and July, months for which Zishka and Smith (1980) also noted a significant decline. Analysis of the North American data show that the principal contribution to the decline is provided by the region north of 40°N. While a significant decline is present in the Alberta–Northwest Territories activity, no trend is found in the Colorado activity. Although a downward trend is present in the frequency of North American cyclogenesis, data from the entire extratropics of the Northern Hemisphere do not show a trend, implying that the decrease in North American activity has been compensated for by increases elsewhere.

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Russell D. Hovanec and Lyle H. Horn

Abstract

The static stability and 300 mb isotach fields for 102 cases of Colorado cyclogenesis during spring and fall are studied. The data sample is divided into cases that develop into either major or minor storms (developing cases) and casts which fail to develop into significant cyclones (non-developing cases). Two stability indices are used: the 800–300 mb temperature difference and a slightly modified form of the Showalter index. The seasonal means, based on a sample of 915 days, show relatively weak stability in the lee of the southern Rockies, which is accentuated in developing cyclones situations. The mean 300 mb isotach field for conditions that lead to developing cyclones shows a well developed jet core propagating eastward into the Colorado region. Contingency tables relating the 300 mb wind speeds at two upstream stations to static stability at two downstream stations show a statistically significant joint occurrence of strong 300 mb winds and weak stability in cases of developing cyclones. Student t tests further indicate that the sample means of the static stability indices are highly significant for both developing and non-developing cyclones. The sample means of the 300 mb winds are statistically significant for developing cases but not for non-developing ones. It is postulated that weak stability in the lee of the southern Rockies is an important ingredient in cases of both developing and non-developing cyclogenesis; however, for developing cases a strong 300 mb wind maximum is also present.

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Lyle H. Horn, Ralph A. Petersen, and Thomas M. Whittaker

Abstract

A 23–24 February 1975 case study is made comparing the results achieved using Nimbus 5 satellite temperature, profiles with those obtained from radiosonde data and the initial hour data of the Limited Area Fine Mesh (LFM) model of the National Meteorological Center. An objective analysis technique is used to construct and analyze isotropic cross sections through an intense baroclinic zone. A cross section based on 1700 GMT Nimbus 5 soundings is compared with those based on 1200 and 0000 GMT radiosonde and LFM data. Geostrophic shear calculations are used to compare the thermal gradients for various isobaric layers obtained from the three data acts. The Nimbus 5 results show somewhat less detail than those based on the radiosonde data, but wore than those obtained from the LPM data. Comparisons of the mean temperature in various isobaric layers for the three time periods indicate that the 1700 GMT Nimbus 5 data appear to be consistent with the changing synoptic pattern. Estimates of the wind components normal to the cross sections are obtained using 1) the geostrophic wind calculated from Nimbus 5 data, 2) the observed winds which are enhanced by the geostrophic shear obtained from the radiosonde data, and 3) the initial-hour LFM winds. The wind estimates also indicate that the Nimbus 5 data provide results which agree with the evolving synoptic situation. In general, the results suggest that the insertion into numerical models of satellite-derived mean thermal gradients calculated for 100–200 mb thick layers, rather than mean temperatures, may facilitate the use of satellite soundings.

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