Lyle H. Horn

Geographical and Seasonal Distribution of North American Cyclogenesis, 1958–1977

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 ...

Spring Season Colorado Cyclones. Part I: Use of Composites to Relate Upper and Lower Tropospheric Wind Fields

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 propagat...

Static Stability and the 300 mb Isotach Field in the Colorado Cyclogenetic Area

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. S...

Spring Season Colorado Cyclones. Part II: Composites of Atmospheric Moisture and Moist Static Stability

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 transpor...

An evaluation of soundings, analyses and model forecasts derived from TIROS-N and NOAA-6 satellite data

Abstract Evaluations of operational TIROS-N and NOAA-6 temperature soundings over North America are presented for an early January 1980 period one month after completion of the First GARP Global Experiment. In addition to collocated comparisons, synoptic analyses derived only from satellite data and model forecasts initialized from these analyses are compared with those obtained from conventional data. The collocated results, similar to those presented by Phillips et al. (1979) and Schlatter (1981) from TIROS-N soundings, show maximum sounding errors new the surface and tropopause. The analysis comparisons further illustrate that thermal gradients inferred from satellite soundings are too weak, with NOAA-6 gradients slightly weaker than TIROS-N gradients. Difference fields between satellite and conventional thickness analyses propagate eastward with the synoptic patterns, strongly suggesting a correlation of satellite sounding errors to synoptic patterns. These anomalies are also retained in model forecas...

On the Relation Between TIROS Radiation Measurements and Atmospheric Infrared Cooling

Abstract TIROS II channel 2 and channel 4 measurements of infrared radiation are statistically correlated with corresponding radiometersonde measurements of radiation divergence within various layers of the atmosphere. Estimates of the correlation coefficient greater than 0.60 are obtained for all layers extending from the surface of the earth to levels beyond 600 mb. The results of this study indicate that the satellite measured flux is most significantly related to the infrared cooling profile within the surface to 600-mb layer. It is shown that good estimates of the infrared cooling profile might be obtained from satellite measurements if the general sky condition is also known.

An Application of the Satellite Indirect Sounding Technique in Describing the Hyperbaroclinic Zone of a Jet Streak

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 stxeaks thermal support.

THE CONTRIBUTION OF INFRARED COOLING TO THE VERTICAL MOTION FIELD AND ITS IMPLICATION IN ATMOSPHERIC ENERGETICS

Abstract The validity of the adiabatic assumption used in estimating vertical motion is examined by comparing the relative contributions due to adiabatic processes with the diabatic process of infrared cooling. Radiometersonde data are used to prepare vertical profiles of the adiabatic and infrared components of the vertical motion. These data are first filtered to reduce the effect of random errors. Although the adiabatic component of the vertical motion is usually much larger than the infrared component, the profiles indicate that the infrared component can be important in determining the total vertical motion. A comparsion of the contribution of the infrared component in cloudy versus clear sky conditions shows that this component contributes more to downward vertical motion in clear air than in the cloudy situations. The consequences of this systematic variation in estimating energy conversions is discussed. In view of these results the effect of other diabatic processes is very briefly considered.

Atmospheric Energy Transport Over North America For Three Winter Months

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 m...

An Evaluation of Alternatives to the Retrieval Methods Used in Processing FGGE Level II TIROS-N Soundings

Abstract Alternatives to the retrieval techniques applied by NESDIS operations to derive the FGGF Level IIa soundings are examined. A physical iterative retrieval technique is compared to the operational statistical method, and the influence of using higher resolution subsets of the original infrared observations is examined. These alternatives are evaluated using TIROS-N observations from a January 1980 period over the conventional data-rich region of the United States. The evaluations involve colocation statistics and 700–300 mb thickness difference fields. The initial tests using operational (9 × 7 HIRS/2 fields of view) resolution show that the physical iterative retrieval makes substantial corrections to climatological first guesses, but only minor corrections to a first guess based on the operational soundings. Colocation statistics and 700–300 mb thickness difference fields indicate that the physical retrieval method does not offer significant improvements over the FGGE operational soundings. As in...