ISSN:0035-9009    

DOI:10.1002/qj.49710243302

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractMultiple radar and aircraft observations of a damaging supercell hailstorm in northeastern Colorado are synthesized to show that the airflow and hail growth conformed in many respects to earlier models derived by the lead author. Some features that before had to be inferred indirectly are now substantiated and elaborated upon by direct observation. As a result of our increased confidence in certain aspects of the model we have been able to draw implications regarding the feasibility of suppressing hail by different means.The observations indicate that an important stage in the growth of large hail is the entry of hailstone embryos into the edge of an intense updraught. These embryos are grown near the storm's right flank and get carried around the forward flank under the influence of a strong environmental airstream which divides at the stagnation point and flows around on either side of the main updraught. The main updraught itself is characterized by a weak‐echo vault and the embryos grow into large hail as they follow a simple up‐and‐down trajectory over it.The vault takes on added significance in that it is a symptom of inefficiency in the conversion of cloud water to precipitation. Paradoxically it is this very inefficiency that encourages the growth of large hail by minimizing the effects of competition for the available supercooled droplets. Those recirculating embryos that find their way to the edge of the vault via the embryo curtain are seen to compete unfairly by being the first to encounter the undepleted cloud water in the vault. It is difficult to suppress their growth artificially because seeding in the main updraught at economical rates produces particles that rise above the hail growth region before they can have any significant influence on the available water.The existence of a large vault implies that the flow field is so restricting the number of embryos re‐entering the main updraught that they may be sweeping up only a small proportion of the overall cloud water. Thus there is a danger that, if a mode of seeding were used which caused additional embryos to re‐enter the updraught along with the re‐entering natural embryos, this would lead to the production of additional large hailstones. Hence, at least for some storms, doubt is cast on the credibility of the competing embryo hypothesis for hail suppression, according to which the generation of additional embryos by seeding is supposed to promote competition for a restricted amount of cloud water to the extent that none can

ISSN:0035-9009    

DOI:10.1002/qj.49710243303

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractFrom over 1000 half‐hour observations of near‐surface wind profiles at Hay, NSW, more than 500 high quality sets of data are selected. In unstable conditions, these closely confirm previous analyses and suggest near equality betweenz/LandRi. The data are well described by either the KEYPS relationship (Panofsky) or that of Businger: ϕM= (1–16z/L)−1/4. In stable conditions, a log‐linear formulation (ϕM= 1+az/L) is found to give an adequate description of the wind profile up toz/L≃ 0.5, with some evidence for a slight variation in α between the values 4.0 at neutral and about 6 whenz/L≃ 0.2. The average value of α between these limits is found to be 5.0±0.2.In conditions of very high stablity, a linear profile (du/dz=cu*/(kL)) is suggested above the heightz≃ 10L. In the transition region between the log‐linear and linear profile regimes, the log‐linear formulation appears to tend towards a purely logarithmic law as stability increases. There is no evidence for any sudden change in behaviour, nor is there any suggestion that a purely logarithmic relationship is ever attained as an average situation. The value ofcin the purely linear relationship is found to be between 0.4 and 0.9. The data also indicate that in extremely stable conditions (L≃ 50cm) the dimensionless gradients of heat and of momentum may differ by about a factor of two, with ϕHbeing the larger.The roughness length of the site used is found to be 1.2±0.1 mm, considerably less than the values appropriate to earlier experiments performed in the same general area. There is some evidence for an increase inz0with decreasing wind speed (reaching about 3 mm when the wind at 1 m is about 1 ms−1), in accord with Deacon's hypothesis concerning the form drag of roughness elements. From the point of view of applying a low‐level drag coefficient in order to estimate friction velocities, the errors arising from the change in roughness length are sometimes comparable to those resulting from stability effects.Not surprisingly, the data show that in slightly stable conditions dewfall was low, whereas in extremely stable situations dewfall accounted for most of the heat loss from the air.The data used here form part of a much larger body of information obtained during 1967 and k

ISSN:0035-9009    

DOI:10.1002/qj.49710243304

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractSimultaneous measurements of narrow‐beam vertical infrared radiance (10–12μm), cloud dropsize distributions, temperature and humidity in some stratocumulus clouds are reported.Cloud volume absorption coefficients calculated from mean dropsize distributions are found to be well correlated with the cloud liquid water content. Values of cloud volume absorption coefficient are found to vary from 2 to 20 km−1for a range in liquid water content from 0.02 to 0.27gm−3. The equivalent variation in cloud mass absorption coefficient is from 700 to 1030g−1cm2, with a mean value for all clouds of 765g−1cm2. From the radiance measurements the clouds are found to become ‘black’ for depths ranging from about 100 to 450m. The variation of measured cloud emissivity with liquid water mass is found to be consistent, within the observed variabilities of the clouds, with the emissivities calculated from the above mean absorption coefficient. The results are compared with earlier f

ISSN:0035-9009    

DOI:10.1002/qj.49710243305

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe forced entrainment rate at the top of a stratocumulus deck is derived from the assumption that the negative buoyancy flux there is a particular fraction (1/2) of the average buoyancy flux within the entire mixed layer. A portion of the cloudtop radiative cooling is allowed to occur below the inversion base, thus contributing to the maintenance of the mixed‐layer turbulence. The derived equation clearly shows the dependence of the forced entrainment rate upon the surface heat and moisture fluxes, cloudtop radiative cooling rate, temperature and humidity jumps across the capping inversion, solar absorption rate within the mixed layer, and percentage of the mixed layer occupied by stratocumulus. The forced entrainment rate becomes inapplicable when the thermal jump at cloudtop becomes very small; then either encroachment or free entrainment occurs, and expressions are proposed for each. A useful criterion is that the entrainment rate is the minimum of the calculated rates for forced entrainment, encroachment, and free entrainment. The equation is tested satisfactorily against observations of coastal stratus off California and stratocumulus development over Lake Michiga

ISSN:0035-9009    

DOI:10.1002/qj.49710243306

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe effects of tropical forcing on the atmosphere have been isolated in two general‐circulation models – one hemispheric, one global – by eliminating other zonally asymmetric forcing such as mountains and extratropical ocean/land thermal contrasts. Tropical surface temperatures were found to force upper tropospheric geopotentials in the tropics and geopotentials throughout the troposphere in middle and high latitudes with little scale dependence up to about wavenumber 5. In the hemispheric experiment the tropical forcing generated Rossby–Haurwitz waves similar to tesseral harmonics with 3 or 5 nodes between the poles; the associated phase agreement between tropical forcing and the tropical and extratropical response was also a dominant feature of the global experiment despite the more complicated structure of the forcing field. Some aspects of the results are explicable by wave propagation theory including a strong dependence on wavenumber of the geopotential response in the tropical lower trop

ISSN:0035-9009    

DOI:10.1002/qj.49710243307

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractObserved tropical Atlantic temperature anomalies for January 1963 have been used in a hemispheric model in an attempt to assess the extratropical effects of a tropical ocean temperature anomaly and to compare them with the results of previously reported experiments on tropical forcing. Falls of surface pressure were obtained over the warmed tropical water and extended north at least to 60°N over the east Atlantic, the northward limit of the falls varying considerably between three 80‐day experiment pairs as was to be expected from the tropical forcing experiments. The maximum amplitude of the mid‐latitude response (up to 7mb) was also consistent with that expected. One of the three experiment pairs produced an Atlantic pressure anomaly pattern similar to the average for the winter (1962–1963) in which the ocean anomaly was observed. The model is shown to give results which compared well with observations in winters with anomalous tropical Atlantic temper

ISSN:0035-9009    

DOI:10.1002/qj.49710243308

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractAn areal average data series based on 18 stations scattered throughout south and east England was set up. The incomplete gamma function was used as a model for the frequency distribution of the data, since it gave good agreement with the parameters of the longest record in the area, Kew (1697–1971). The power spectrum of the areal average series shows that non‐linear processes on a time scale of months or years may be involved in rainfall fluctuations. Decade average values of the total monthly rainfall of the area series were used to forecast decade mean monthly rainfall totals for 1970–1979, 1980–1989 and 1990–1999, using a mixed time ser

ISSN:0035-9009    

DOI:10.1002/qj.49710243309

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractThe Southern Oscillation is an important mode of operation of the atmosphere. Several indices of the Southern Oscillation have been recognized, but for many purposes an index that gives a continuous, homogeneous time series is required. Phase differences between various areas complicate the definition of a simple index but weighted values of seasonal surface pressure at Darwin and Tahiti seem suitable for reference purposes. Time series of short period correlation coefficients prove useful for revealing temporal variations in the character and amplitude of the Southern Oscillation and reveal times when it is not very prominent. Lag relationships are determined and used to indicate some aspects of the circulation near times of phase reversal of the oscillation.Spectral analysis showed most variance to be in the range of 2 to 10 year periodicities, and cross spectral calculations further indicated that only for the 2–10 year range, and mainly for the 3–6 year range, of periodicities is there large‐scale coherence and out‐of‐phase relationships between stations in the South Pacific and Darwin. This defines the time scale of the Southern Oscillation.The Southern Oscillation is not confined to tropical and subtropical latitudes but is clearly linked to mid‐latitude systems and long waves in the westerlies. East‐west circulations coupling the South Pacific Convergence Zone and the South Pacific high also appear to play an important role in the Souther

ISSN:0035-9009    

DOI:10.1002/qj.49710243310

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY

摘要:

AbstractCharney's (1975) suggestion that high desert albedo initiates a biogeophysical positive feedback response by the atmosphere has been tested in the 2‐D zonal atmospheric model, ZAM2. The results agree with the earlier test in the NASA GISS 3‐D GCM model (Charneyet al.1975) in confirming a positive feedback through precipitation but indicate a negative feedback in terms of temperature. The latter suggests that tropical deserts may play a role in air conditioning or placing an upper‐bound on the temperature of our p

ISSN:0035-9009    

DOI:10.1002/qj.49710243311

出版商:John Wiley&Sons, Ltd    

年代:1976

数据来源: WILEY