Subtropical Air Research Articles

A Trajectory Analysis of Tropical Upper-Tropospheric Moisture and Convection

Abstract It is shown that the distribution of upper-tropospheric humidity (UTH) in the cloud-free Tropics can be simulated with a simple model in which air expelled from moist convective regions is dried by subsidence along its trajectory. The distribution of UTH is analyzed in the tropical eastern Pacific using moisture data retrieved from GOES 6.7-μm observations during September 1992. The analysis examines the variation in moisture along horizontal trajectories derived from European Centre for Medium-Range Weather Forecasts wind analyses. Trajectory analysis is used to trace the convective sources of subtropical air. For the eastern subtropical Pacific, convective sources lie entirely outside the dry region, and are predominately in the ITCZ and over South America, with some air tracing to midlatitudes. The analysis also shows that, over large parts of the eastern subtropical Pacific, air has advected horizontally for five or more days since exiting convection. Composites of many trajectories from spec...

Comparison of modeled backscatter using measured aerosol microphysics with focused CW lidar data over Pacific

During NASA's GLObal Backscatter Experiment (GLOBE) II flight mission over the Pacific Ocean in May‐June 1990, extensive aerosol backscatter data sets from two continuous wave, focused CO2 Doppler lidars and an aerosol microphysics data set from a laser optical particle counter (LOPC) were obtained. Changes in aerosol loading in various air masses with associated changes in chemical composition, from sulfuric acid and sulfates to dustlike crustal material, significantly affected aerosol backscatter, causing variation of about 3 to 4 orders of magnitude. Some of the significant backscatter features encountered in different air masses were the low backscatter in subtropical air with even lower values in the tropics near the Intertropical Convergence Zone (ITCZ), highly variable backscatter in the ITCZ, mid‐tropospheric aerosol backscatter background mode, and high backscatter in an Asian dust plume off the Japanese coast. Differences in aerosol composition and backscatter for northern and southern hemisphere also were observed. Using the LOPC measurements of physical and chemical aerosol properties, we determined the complex refractive index from three different aerosol mixture models to calculate backscatter. These values provided a well‐defined envelope of modeled backscatter for various atmospheric conditions, giving good agreement with the lidar data over a horizontal sampling of ∼18,000 km in the mid‐troposphere.

N2O transport in a three‐dimensional model driven by U. K. Meteorological Office winds

A three‐dimensional spectral chemical transport model truncated at T21 is employed to simulate N2O transport. The wind and vertical motion fields are taken from the U.K. Meteorological Office four‐dimensional assimilation data set. UARS cryogenic limb array etalon spectrometer (CLAES) N2O measurements are used to initialize the model in late August 1992. Model results are shown to simulate the CLAES measurements quite well over the first few months: N2O variability is similar at extratropical latitudes in the southern hemisphere over the period September 2–17, 1992, at 4.6 and 10 mbar, and there is good agreement in the synoptic maps of minor warmings during this period. Prior to a large warming event on September 30, minor stratospheric warmings are shown to produce negligible changes in the vortex below 4.6 mbar, but considerable mixing of air from the vortex edge and subtropical air is indicated. This results in a steepening of the N2O gradient at the vortex edge. During a warming event when the vortex center moves away from the pole, downward transport by the residual circulation can be large. This is offset by eddy transport effects, but these terms reverse during the recovery from the warming. From September 2 to 17, there is evidence of continuous mixing at midlatitudes at 4.6 mbar in contrast to more discontinuous, warming‐associated mixing at 10 mbar. The breakup of the vortex is initiated by the September 30 warming, and a warming on October 13 has a strong influence on the breakup. The breakup propagates downward. The climatological distribution of N2O in the tropics follows the seasonal variation of the solar radiation with a maximum, which is determined by the strength of the upward residual motion, shifting towards the summer hemisphere by 10°–15° latitude. The surf zone in both the model and the observations at the middle latitudes is well defined, but the gradients of N2O at the edge of the tropics and at the edge of the vortex are smaller in the model than in the observations. This is probably being caused by excessive mixing in the model.

Remote sensing of carbon monoxide over the continental United States on September 12–13, 1993

Tropospheric carbon monoxide (CO) column densities were retrieved from 491 nadir infrared spectra obtained by the University of Wisconsin's High‐Resolution Interferometer Sounder (HIS) during a cross‐country flight of a NASA ER‐2 on September 12–13, 1993 (UT). A classical, Fourier‐based signal‐processing technique was utilized to retrieve the CO column density from the 2135–2200 cm−1 portion of the CO 1–0 vibration‐rotation band. Error analysis indicates column CO retrieval uncertainties of approximately ±10%. Retrieved CO column densities over the western United States from this flight, ∼1.2×1018 cm−2, agree well with previous ground‐based CO column measurements over the western United States at the same season. Derived CO sensitivity functions for these measurements possess a broad peak centered in the 500–600 mbar region, thus implying the retrieved tropospheric CO mixing ratios are indicative of the mean free troposphere (mft). Across the United States, three spatial groupings of CO were observed: west, mft CO ∼76 parts per billion by volume (ppbv); midwest, mft CO ∼56 ppbv; and east, mft CO ∼71 ppbv. Weather map and back trajectory analyses indicate different source regions for the air parcels in each of these regions. The low CO amounts over the midwest most likely result from the mixing of rapidly subsiding air naturally low in CO with a low‐altitude inflow of similarly clean subtropical air off the Gulf of Mexico. These results demonstrate the utility of satellite infrared spectrometer observations to globally map tropospheric CO abundances and further reveal the impact of synoptic scale atmospheric circulations on CO abundances.

Zonal variance of nitric acid vapor as an indicator of meridional mixing in the subtropical lower stratosphere

Several transport scenarios have been advanced in the literature to account for the exchange of tracers between the extratropics and tropics in the lower stratosphere. Each scenario involves a slow, net upward transport at low latitudes and varying degrees of mixing at middle latitudes plus detrainment across a subtropical region of enhanced tracer gradients. In situ observations for differentiating among these scenarios are generally lacking above about 21 km altitude. Existing satellite data sets can provide useful information throughout the lower stratosphere. In particular, we have looked for such exchange by examining time series of the daily zonal variance of the Nimbus 7 limb infrared monitor of the stratosphere distributions of nitric acid vapor (HNO3) mixing ratio. This species is characterized by strong subtropical meridional gradients between about 10 and 100 hPa, which develop in response to extratropical wave activity and any associated mixing processes. The subtropical variance is a measure of that activity, its region of maximum values moving equatorward during late autumn and then relaxing and retreating in late spring. At and below the 45‐hPa level there is not a clear maximum of variance at a given latitude, perhaps a result of the secondary circulation that was present during the 1978/1979 winter in response to the easterly phase of the quasi‐biennial oscillation cycle. At the 30‐hPa and 50‐hPa levels, while we find occasions in winter when tongues of subtropical air were transported to higher latitudes, we find no clear evidence for a large‐scale transport of extratropical air to lower latitudes through a subtropical region of enhanced gradients.

On the Subtropical Edge of the Stratospheric Surf Zone

The formation of a subtropical “transport barrier” in the wintertime stratosphere is investigated in the context of a high-resolution shallow-water model in which Rossby waves are topographically forced on a zonally symmetric basic state. Two sets of experiments are performed: in the first “adiabatic” set, no dissipation or forcing of the mean state is imposed; in the second set, the layer thickness is relaxed to an equilibrium state taken to be representative of middle stratospheric radiative equilibrium temperatures. It is found that in the adiabatic case only a very weak subtropical barrier forms for forcing amplitudes that generate realistically steep potential vorticity gradients at the edge of the polar vortex; the vigorous wave breaking in the surf zone generates secondary waves that spread and, in turn, break well into the summer hemisphere. In contrast, the inclusion of relaxation to a realistic thermal equilibrium leads to the formation of a subtropical region of steep PV gradients. The strong subtropical shear induced by die diabatic relaxation is shown to be an important factor for the formation of the subtropical edge of the surf zone. Furthermore, the authors demonstrate that a simple one-layer shallow-water model can capture the full process of the formation of a surf zone with both polar and tropical edges starting from conditions typical of the early fall–that is, with a flow in which the polar vortex is not initially present. Finally, the authors quantify the mixing of polar and subtropical air into the midlatitude surf zone with the help of the contour advection technique. Although the quantitative estimates depend sensitively on how the edges of the surf zone are defined, our results indicate that more tropical than polar air is entrained into the surf zone.

Simulation of Extreme New Zealand Precipitation Events. Part II: Mechanisms of Precipitation Development

Abstract The extreme precipitation event that occurred on 27 December 1989 over the South Island of New Zealand was simulated using the DAR hydrostatic mesoscale model nested within the ECMWF analyses. The model simulated nearly half of the peak observed rainfall for this storm (greater than 700 mm) and captured the location and timing of the intense precipitation. The heavy precipitation developed while a deep layer of moist subtropical air along a cold front ascended the high terrain of the South Island. The intense orographic ascent was associated with a low-level jet core with wind speeds of over 20 m s−1 ahead of the cold front. An upper-level trough and jet streak entrance region were also present upstream of the South Island during the event, aiding the ascent over the mountains and deepening the layer of moist air. The air crossing the mountain was nearly saturated throughout the troposphere and had only weak moist vertical stability near the cold front. Almost all of the simulated precipitation f...

Spatial variability of small‐basin paleoflood magnitudes for a southeastern Arizona mountain range

Paleohydrological procedures were used to analyze spatial variations in flood magnitude in drainage basins of the Santa Catalina Mountains (maximum elevation 2791 m), southeastern Arizona. Paleoflood reconstructions in eight watersheds, ranging from 10 to 110 km2, were obtained for the largest floods in stable canyon reaches, yielding discharges ranging from 50 to 270 m3 s−1. Although there is a high correlation between drainage area and flood magnitude, it is also clear that basins located in the southern half of the Santa Catalina Mountains have larger unit discharges (peak discharge divided by drainage area) and that these values decrease toward the northern part of the range. This spatial pattern is probably the result of local hydroclimatological conditions whereby the largest floods are associated with the northward influx of moist, tropical, and subtropical air. For this atmospheric circulation the northern part of the range experiences a rain shadow effect that decreases the unit discharges generated in those basins. The unusually high unit discharge for one of the south facing basins is explained by its basin morphometry. Inverse problem solutions using a rainfall‐runoff model demonstrate that both this basin and another with a comparable drainage area but a much lower estimated peak paleoflood discharge can have their peak flood outputs produced by the same storm intensity. The results demonstrate the potential of paleoflood data collection for understanding spatial variability in small basin flood hydrology.

低気圧周辺の温位•湿度場の地理的比較

Temperature and humidity fields in the low-level atmosphere around cyclones, which are assumed to differ for each individual cyclone, were objectively analyzed through a year on the hemispheric scale to detect their representative spatial patterns. Geographical and seasonal characteristics for the appearance of cyclones as related to these patterns are comparatively shown over the middle latitudes of the Northern Hemisphere.Cyclone positions at 1000 mb and surrounding potential temperature (θ) and specific humidity (q) fields at 850 mb are objectively computed using NMC data from August, 1985 to July, 1986. The distribution of the air-mass fields and the cyclone positions in the analysis year were firstly examined in relation to the former climatological maps. Then, representative spatial patterns of θ and q around cyclones with cloud areas are obtained as a function of the three rotated principal components (RPCs). The first two spatial fields of the RPCs-θ show a relatively symmetric correlative structure with southwest-northeast and southeast-northwest dipole patterns, respectively. However, the first spatial field of RPC-q shows a concentric structure with meridional asymmetry. Case studies indicate that the RPCs-θ pattern represents the actual distribution of temperature and its evolutional variations along the border between air masses, and RPCs-q represents the distribution of absolute humidity gradient, which is recognized as the margin of the sub-tropical air mass in the middle to lower latitudes in summer.The frequency distributions of cyclone appearance with representative plus and minus RPCs are composed for θ and q, and shown with seasonal mean θ and q fields over the Northern Hemisphere. The appearance of cyclones with the first two RPCs-θ patterns was determined by the relative location along the edge of the cold air mass trough, which is formed by the intrusion of a cold air mass from the Arctic region. Large-scale topography, such as straits, large plains and chained mountains, determines the path of the cold air mass from the north, and reflects the strong regional characteristics of the RPCs-θ values affecting the appearance of cyclones. Prevailing regions for critical RPCs-θ values shift longitudinally according to the season and according to the major routes of air mass intrusion. Meanwhile, cyclones with strong RPC-q patterns appear from the southwestern Tibetan Plateau to the southeastern side of Eurasia, from the northeastern Tibetan Plateau to the northwest Pacific and southern North America to Canada along the east side of Rocky Mountains, in summer. These regions overlap with the western margins of maritime Polar Frontal Zones and the Siberian-Canadian Arctic Frontal Zone.

Transport of atmospheric tracers by planetary waves during a winter stratospheric warming event: A three‐dimensional model simulation

In a simple three‐dimensional primitive equation model, a wave number 1 major stratospheric warming is simulated. With the aid of two idealized tracers it is shown that the transport during a major warming event is characterized by a small, well‐organized tongue of subtropical air flowing around the displaced winter vortex into the polar cap and by a wide area with strong quasi‐horizontal mixing (surf zone). The description of these dynamical processes requires a full three‐dimensional space resolution.

極東亜熱帯湿潤気候区の低気圧にともなう豪雨

On June 27, 1972, torrential rains fell over the southwestern Kyushu, the western part of Japan, when a weak frontal depression passed to the north of Kyushu. The large-scale characteristics of heavy rainfalls in the Asian subtropical-humid region are represented through the case study on this heavy rainfall.Synoptic and subsynoptic-scale analyses indicate that 1) the heavy rainfalls occurred within the subtropical air mass to the south of the subtropical jet stream's axis; 2) destabilization of the stratification on the heavy rainfall area was not caused by the cold air intrusion aloft but mainly by the intrusion of the tropical air mass in the lower layer; 3) the extremely moist tropical air mass was transported toward the depression from the south along the western periphery of the Pacific subtropical anticyclone.We also give our attention on the differences between the large-scale features of the heavy rainfalls in the Asian subtropical-humid region and these of severe storms in the subtropical-humid region of U.S.A. In essence, the heavy rainfalls are the convective phenomena within the subtropical air mass occurred under the condition of the intrusion of tropical air, while the severe storms are those within the subtropical air mass occurred under the condition of strong cold advection aloft.

A further study of four-hourly observations of the vertical distribution of atmospheric ozone

Results from a series of six ozonesonde balloon flights taken every 4 hours at a single station during a period when there was a strong confluence of polar air and subtropical air over the station are presented. The soundings in this case are found to be characterized by several highly persistent maxima and minima in the vertical distribution of the partial pressure of ozone in the lower stratosphere. The altitudes above the station at which these maxima and minima occur increase systematically from the beginning to the end of the series. This indicates that the maxima and minima correspond to thin sloping layers of ozone-rich air and ozone-poor air, respectively, that undergo a displacement with time along with the displacement of the pressure system with which they are associated.

Ozone and temperature changes in the lower stratosphere

The vertical profiles of ozone and temperature from a series of balloon soundings at Delhi (28°N), Poona (18°N) and Trivandrum (8°N) were studied with synoptic meteorological data. While both ozone and temperature profiles show similar variations over all three stations, ozone maxima being always associated with thermally stable layers, the variations are most pronounced over Delhi, particularly in winter and in early spring when a series of western disturbances pass over north India. Both ozone and temperature profiles over Delhi show a layer structure characterized by a series of maxima and minima in both the vertical distribution of ozone and temperature and these are most pronounced in the lower stratosphere. These variations are associated with the influx of ozone-rich middle latitude stratospheric air over Delhi replacing subtropical air.

Short-Term Variations in the Vertical Distribution of Atmospheric Ozone at Albuquerque, New Mexico

Variations with time in the vertical distribution of ozone during a period of influx of subtropical air associated with a shortwave ridge and a cold middle tropopause are studied in detail by means of a series of ozone soundings taken at intervals varying from 4–6 hr. In the lower stratosphere, the vertical distributions of the partial pressure of O3 were characterized by secondary maxima and minima that could be followed from sounding to sounding. However, a general decrease in the partial pressure of O3 with time took place at nearly all levels in the lower stratosphere as the ridge advanced over the station. The most significant changes in ozone with time were observed in the layer between altitudes of 9 and 18 km. A pronounced minimum in the partial pressure of O3 persisted at the 18.5-km level throughout the entire period of observations. Such a minimum may he formed when a thin, upper layer of the cold tongue of subtropical air associated with the quasi-permanent, longwave pressure system is left undisturbed as the lower part of the cold tongue is being replaced by ozone-rich polar air associated with a shortwave trough moving through the longwave system. The minimum formed in this way may persist for a long time if not modified by either a significant change in the longwave pressure pattern or by a shortwave disturbance that reaches to an altitude greater than that of the minimum.

Variations in the vertical distribution of atmospheric ozone during the passage of a short wave in the Westerlies

Vertical distributions of ozone and of temperature taken from a series of balloon flights at Albuquerque, New Mexico, approximately every 4 hours over a -day period during which a short-wave upper trough passed over the station are studied together with synoptic meteorological data. The analysis indicates that with the approach of the trough and the simultaneous lowering of the tropopause and a southeastward displacement of the subtropical jet stream over the station, there is an increased influx of ozone-rich polar stratospheric air over the station in the form of thin layers, which replaces ozone-poor subtropical air. This results in a layer structure characterized by several maxima and minima in the vertical distribution of the partial pressure of ozone and by corresponding maxima and minima in the vertical distribution of the thermal stability as indicated by the temperature soundings. The layer structure is most pronounced between the lower tropopause and the 20-km level. The large-scale features of the layer structure, especially with regard to ozone, show a remarkable persistence from sounding to sounding throughout the entire period of observations, which indicates that the layers associated with the long-wave pressure system and the accompanying jet stream have a very large horizontal, or quasi-horizontal, extent. At the same time, variations in the vertical distribution of ozone with respect to time, observed as the short-wave trough is being displaced with respect to the station, indicate that within the layer structure, especially in, or near, a trough, there also may exist horizontal gradients of the partial pressure of ozone of considerable magnitude in the direction of the flow as well as in a transverse direction.

The influence of the weather on the concentration of nuclear bomb debris in air

Daily measurements of the concentration of nuclear bomb debris in air in terms of gross β radioactivity have been made by the Norwegian Defense Research Establishment at a number of places in Norway, The weather developments were examined for samples collected during the period July 1, 1957, to June 30, 1959. It was found that low radioactivity was predominantly associated with centers of low pressure, with influx of arctic air, or with rainy weather. High radioactivity was predominantly associated with centers of high pressure, with influx of warm subtropical air, or with marked frontal passages. This indicates that concentrations of nuclear bomb debris become adjusted to weather conditions fairly rapidly. The statistics seem to be satisfactorily explained if the principal removal agent is precipitation and the principal ways of replenishment are vertical downward transport in high-pressure centers and overturning of air in frontal zones. The low concentration in arctic air may well be due to a short residence time of air in the Arctic. It need not reflect a lack of downward flux in this area.