Showing papers in "International Journal of Climatology in 1982"

Drought in the United States: 1895–1981

Abstract: Statewide averages of temperature and precipitation, from January 1895 to April 1981, were interpolated to a grid and Palmer Drought Severity Indices (PDSI) calculated for each of the grid points. Principal component (PC) analysis was performed on the gridded values of PDSI. By rotating the PC, nine readily identifiable patterns of drought are delineated. Each PC relates to different regions of the country, and is characterized by a distinct annual oscillation of monthly average precipitation. A high degree of confidence is placed in the state gridded data as patterns compare favourably to those derived using smaller averaging areas. Univariate and multivariate autoregressive spectral estimation procedures together with the traditional spectral estimation techniques were used to investigate the spectra and coherence of the PC scores. No significant cycles were found in the 86-year (1036 months) data set. Coherence was very low (<0.30) for all frequencies between the various patterns depicted by the PC. Both the spectra of PC scores and drought duration statistics point to the major difference in droughts across the United States; droughts persist much longer in the interior portions of the country than in areas closer to the coasts. Sensitivity tests on the PDSI indicate that this is not directly attributable to the spatially varying available soil water capacity parameter used in the PDSI calculations. This is not to say that soil moisture evaporation does not indirectly influence drought duration, but merely that the differing lengths of droughts in the United States are real and not an artefact of the mechanics in the PDSI calculations.

A study of the influence of surface albedo on July circulation in semi‐arid regions using the glas GCM

Abstract: A numerical simulation study of the influence of surface albedo on July circulation in semi-arid regions, using the general circulation model (GCM) of the Goddard Laboratory of Atmospheric Sciences (GLAS) is presented. The results are based on two 47-day integrations. In the first integration, called the control run, the surface albedo was normally prescribed, whereas in the second integration, called the anomaly run, the surface albedo was modified in four regions: the Sahel in Africa, the western Great Plains in the United States, the Thar Desert border in the Indian subcontinent, and northeast Brazil in South America. This experiment is similar to that of Charney et al. (1977); however, it was performed with the GLAS model with vastly different boundary forcings and several changes in the physical parameterizations. Each run was started from observed initial conditions for 15 June 1979, based on NMC analysis. An analysis of the two simulations shows that in the Sahel, and the Thar Desert border regions the current results again show reduced precipitation with increased surface albedo in accordance with Charney et al. (1977) and Charney (1975). The semi-arid northeast Brazil region, which had a winter circulation, also conforms with Charney's (1975) hypothesis. However, the Great Plains region was an exception. There the total precipitation in two simulations was unchanged, as was the total cloudiness. In this region, the convective precipitation was reduced somewhat, while the large-scale precipitation compensated this decrease by approximately the same amount. The lack of albedo impact on precipitation in the Great Plains region could be due to the influence of the Rocky Mountains in generating variations in the large-scale flow. The purpose of repeating Charney's albedo experiment was to test the sensitivity of his earlier results with the current GLAS GCM which has substantially modified physical parameterizations, particularly the planetary boundary layer (PBL), crucially important for such impact studies. Taking into account that both the GCM and the initial conditions of the atmosphere were different, it is worth noting that this experiment still provides support for Charney's hypothesis (1975) regarding the influence of surface albedo on mean-monthly climatology in the subtropical desert margin regions. Another important influence noted was the variability of mean monthly simulation in areas far away from the albedo anomaly regions. Because the differences occur in regions of high observed climatological variability, it is necessary to separate the contribution of albedo anomalies as opposed to the model variability. The influence of surface albedo changes to produce large changes in the mean montly circulation elsewhere is very intriguing and needs further investigation.

Climatology of tropical cyclone genesis in the Australian region

Abstract: Statistics of tropical cyclone origin in the Australian region are presented for the period July 1959-June 1979. Cyclone genesis is found to occur in preferred areas and to have a high incidence of coastline developments, often with the precursor system forming over land. There is an average of 1.3 occurrences per year of ‘regeneration’ whereby a cyclone develops from a pre-existing cyclone which has lost intensity after crossing on to the continent. A synoptic analysis of individual developments over five years reveals that 84 per cent of the pre-cyclone cloud clusters first appear on the gradient level monsoon shear line, whereas 97 per cent are on the shearline at the point of development. Statistical evidence is given for the enhancement of lower and upper level vorticity fields by mid-latitude systems prior to cyclone genesis, but on a case by case basis the role of these systems is highly variable. The structure of the pre-cyclone systems for individual cases and composites shows the upper level warm core and large scale cyclonic surrounding circulation in the middle and lower troposphere, known to exist for similar systems in other parts of the world. At upper levels the systems are surrounded by tangential winds in the anticyclonic sense beyond 6 degrees latitude radius.

Climate and energy balance on the arctic tundra

Abstract: Energy balance measurement on the arctic tundra on the midwest coast of Axel Heiberg Island, N.W.T., Canada, is summarized. The methods and the instruments as well as their difficulties in the arctic field conditions are described. The seasonal characteristics of the energy balance are presented for the dry snow, the melt and the postmelt periods. The diurnal variations of the energy balance for each of these periods are also presented. The climatic differences for the regions of the tundra, glaciers and the arctic seas become prominent during the three summer months of June, July and August. The energy balance at the present site is compared with that of Barrow, Alaska, revealing a high degree of similarity. The slight differences between them are more net radiation, more sensible and latent heat fluxes, larger Bowen ratio and less soil heat flux at Barrow. These differences are considered to be due to the larger distance of the present station from the coast. The energy balance on and in the dry snow cover is investigated. The stability of the snow cover is attributed to the high reflectivity as well as to the large absorptivity of its surface with respect to solar radiation. The absorption occurs mainly at the surface of the snow cover rather than its interior. This condition facilitates the efficient removal of the absorbed energy from the surface by the atmosphere. The energy balance on the tundra is compared with those on the Central Arctic Ocean, the ablation area of the polar glaciers, the accumulation area of the glaciers and the boreal forests. The surfaces of the tundra, ablation area of the glaciers and the Central Arctic Ocean are found to receive similar net radiation, despite the albedo differences. The fundamental difference among these surfaces is the magnitude of the latent heat of melt. This component on the ablation area of the glaciers and the Central Arctic Ocean is four to six times larger than on the tundra, which results in differential heating and evaporation in these regions. This difference is considered to be the basis of the milder climate in the tundra region in the Arctic. More substantial differences are found between these low altitude arctic surfaces and the accumulation areas of the glaciers and the boreal forests. The differences are due to the large variation in albedo for the former and to the difference in global radiation for the latter. These differences ultimately regulate the regional variations in net radiation, sensible and latent heat fluxes.

Cave climate: Assessment of airflow and ventilation

Abstract: Study of cave climates is a neglected aspect of microclimatology. An understanding of the processes that control cave climate is required in the study of cave flora and fauna, thermodynamic conditions for karst processes underground, hydrogeologic features of speleothems as well as human use of caves for recreational and commercial purposes. An understanding of cave airflow is of central importance as the climate of a cave is a function of air exchange with the outside environment. The study site is the Glowworm Cave of New Zealand and is of particular interest because of its heavy visitor use and the presence of climate sensitive cave fauna on which the value of the cave as a tourist attraction depends. The study examines airflow using conventional methods together with experimental procedures using SF6 and CCl2F2 tracer gases. The results show that the rate and direction of airflow are a direct response to a thermally induced disequilibrium in air density between the cave and outside air. Both diurnal and seasonal patterns in the direction of airflow were identified. Reversal of airflow occurs when the cave to outside air density gradient is zero rather than when thermal conditions of the cave and external air are the same. Major and minor airflow routes through the cave were identified. The results of the gas tracer experiments allowed estimation of rates of air change in different parts of the cave and confirm the marked difference in ventilation that occurs within the cave. Information of this type is important because of the relationship between natural ventilation and CO2 buildup. There is also the effect of ventilation on differences in climate within the cave as the thermal and moisture effects of outside air will not be as pronounced in parts of the cave where air change is limited.

Response' of the antarctic ice" sheet to a climatic warming,: a model study

Abstract: It is generally believed that the increasing C02 content of the atmosphere will lead to a substantial climatic warming in the polar regions. In this study the effect of consequent changes in the ice accumulation rate over the Antarctic Ice Sheet is investigated by means of a numerical ice flow model. In the model runs, temperature increases linearly with time during 100 years, and is kept constant afterwards. The results indicate that a climatic warming will probably lead to a sea-level lowering of some tens of centimetres in the next centuries. This is because for Antarctic conditions the increase in snow accumulation exceeds the increase in melting. This estimate does not take into account the effects of possible surging of parts of the Antarctic Ice Sheet and the response of the Greenland Ice Sheet (which may be quite different).

Nature and possible causes of droughts on the canadian prairies‐case studies

Abstract: There has been increasing interest in recent years in the study of droughts because of their economic, social and political impacts on society. Two summer droughts that affected the Canadian Prairies in 1961 and 1967 are examined. Based on a synoptic climatological approach, the investigation reveals that droughts occurred when the weather type WR (mid-tropospheric ridge dominant) was persistent with a ridge over western Canada. A study of the 1976–1977 winter drought supports this interpretation. In all three cases, a quaststationary mid-tropospheric ridge over western Canada acted as a block which displaced the jet stream, cyclones and moist air masses to a location north of the Canadian Prairies, while anticyclonic circulation under the high pressure ridge gave atmospheric stability and dry conditions on the Canadian Prairies.

Principal component analysis of the lamb catalogue of daily weather types: Part 1, annual frequencies

Abstract: Principal component analysis is applied to annual weather type frequencies for the British Isles. The results objectively define characteristic relationships between the different types, and are used to determine changes in the atmospheric circulation in the region of the British Isles over the period 1861 to 1980. It is shown that, as far as annual frequencies are concerned, only six (of the 27) weather types are needed to define and monitor changes in the atmospheric circulation. The component amplitudes are correlated with temperature and rainfall data, and physically reasonable relationships are identified. The analysis is extended to the period 1781 to 1785, and similarities are found between the extremes experienced during that period and during the 1970s.

The southern oscillation: Sea surface temperature and wind relationships in a 100‐year data set

Abstract: The development of sea surface temperature (SST) and surface wind anomalies during the course of the Southern Oscillation is examined with a 100-year data set covering most of the Pacific Ocean region. Correlation coefficients of SST and winds with Wright's (1975) Southern Oscillation Index (SOI) are presented for lags and leads up to four seasons relative to the Index. The primary SST response is in the eastern and central equatorial ocean, the regions usually associated with the El Nino phenomenon, but there are important SST and wind changes elsewhere in the Pacific. Significant precursors in the SST include warming (cooling) in the northwest subtropical Pacific up to one year preceding positive (negative) SOI and like-signed anomalies near New Guinea one season later, in addition to the cooling (warming) along the Chilean coast and in the eastern equatorial region beginning 3 seasons before positive (negative) SOI. (Positive SOI corresponds to pressures higher than average over the south-east Pacific and lower than average over the western equatorial Pacific.) The major wind response is an area of easterly (westerly) anomalies in the central and western equatorial ocean beginning (in the latter portion) north of the equator 2 seasons before positive (negative) SOI. There is evidence for increased (decreased) westerlies in the southern mid-latitudes as well as easterly and southerly (westerly and northerly) anomalies in the southeast subtropics 1-3 seasons before positive (negative) SOI. For both SST and surface winds, little seasonal dependence of the correlation results is found for the equatorial anomalies, but in the Indonesian-New Guinea region positive correlations are found in the latter portion of the year; mid-latitude SST anomalies in both hemispheres are restricted to June to August. The mechanisms that determine the response of the SST and winds to the SOI are examined and compared with the factors that control the seasonal cycle in selected regions. Subtropical precursors to positive SOI are consistent with enhanced subsidence, higher anticyclonic vorticity and stronger oceanic gyres. Advection seems to account for the spreading of the SST anomalies westward along the equator. SST anomalies in the Indonesian region may be due to solar radiation changes accompanying cloud cover variations.

The effect of altitude on the daily range of temperature

Abstract: Although it has been claimed that the daily range of screen temperature increases with altitude on a mountain, this appears not to be true generally. Data from sixteen sources indicate a confusion of evidence, though there appears to be some evidence of an increase of daily temperature range with height from 0-200 m, and of a decrease between 750 m and 3400 m. The low-altitude increase is attributed to the sea's moderating influence at the coast. The higher altitude decrease may be due to stronger winds or to more cloud. The evidence for a decrease may be confounded in some places by either peculiarities of locale or the increased daily fluctuation of radiation at highest altitudes.

Correlations between the tropical atlantic trade winds and precipitation in northeastern Brazil

Abstract: Surface wind and sea surface temperature data for the tropical Atlantic Ocean for the period 1911-1972 are analysed in relation to the precipitation at Quixeramobim, Ceara in northeastern (NE) Brazil. Stratified correlations between the March-April-May Quixeramobim rainfall and the December, January, February, and March values of SST, and u- and v-components of surface wind are presented for the tropical Atlantic. The implications of the correlations are that the months preceding an abnormally dry rainy season in NE Brazil are characterized by a strengthened South Atlantic trade flow and a North Atlantic circulation slightly weaker than normal. Cold SST anomalies are found in the South Atlantic and warm anomalies in the North Atlantic, suggesting a wind-induced change in surface mixing or in evaporation from the surface. The divergence over the South Atlantic is seen to be stronger and more extensive preceding a dry season, and the equatorial convergence appears to be compressed northward and also strengthened. This northward compression is concluded to be responsible for the lack of rainfall in Ceara.

Zonal index and circulation change in the north Atlantic area, 1873–1972

Abstract: Monthly and annual values are calculated of a zonal index based on surface pressure at 35°N and 55°N between 60°W and 10°W. The period under study, 1873-1972, is divided into three sub-periods which approximate with three epochs proposed by Lamb (1972). A comparison of our indices with Lamb's results is followed by a discussion of our monthly index, and the rarity of easterly flow in the 100-year period is noted. The results are subjected to a trend and a power spectrum analysis and the latter indicates a number of periodicities some of which are not easily explicable.

Fluctuations in northeast monsoon rainfall of Tamil Nadu

Abstract: The average rainfall series of Tamil Nadu for the NE monsoon months of October-December and the season as a whole were analysed for trends, periodicities and variability using standard statistical methods. The trend analysis showed that there are no long-term trends of increasing or decreasing rainfall in the individual months or the season as a whole. The power spectrum analysis of rainfall series showed peaks at frequencies corresponding to the time periods of 2-2.44 yr and 3.66-4.40 yr.

Snow cover conditions in the northern Hemisphere during the winter of 1981

Abstract: During the winter of 1981 the mean snow covered area in the Northern Hemisphere was less than any other year in the 15-year record (1966-1981) except 1970. For North America, the mean snow cover area was the lowest on record, and for Eurasia the mean snow cover area was the second lowest since records began in 1966. It has been observed that the average winter snow cover in both North America and Eurasia for both 1980 and 1981 decreased from that of the previous year, the first time this has been observed in the 15-year period of record. Assessment of any long term climatic trends must wait extension of the snow cover data base.

Protection from cold stress in a Eucalyptus shelterwood

Abstract: Nocturnal measurements of air, temperature and longwave radiation were taken in a Eucalyptus delegatensis shelterwood and in a clearfelled control site. On average the shelterwood exhibited temperatures between 1°C and 2°C warmer. Longwave radiation correlated well with percentage sky view as determined from fish eye lens photographs. The shelterwood seems to decrease the radiation loss by an average figure of 21 per cent on a cloudless night. The Brunt cooling theory is used to relate the air temperature and radiation figures.

Analysis of the spatial pattern of July temperature departures (1943 1972) over Canada and estimates of the 700 MB midsummer circulation during middle and late holocene

Abstract: The pattern of July temperature variability over Canada north of ∼50°N for the period 1943–1972 has been analysed by means of principal component analysis Our objective was to assess the spatial similarities between modern temperature fluctuations at the interannual scale and those derived from paieotempe nature records encompassing a period of the last 6000 14C years A comparison of the first four principal components from each data set suggests that contemporary midsummer temperature variations offer a reasonable analogue to Holocene fluctuations on the order of a few centuries The Keewatin-Baffin Island-Ungava Peninsula region appears to be within the area of maximum temperature response

On the intra‐annual variation in rainfall over the sub‐continent of southern Africa

Abstract: The variation in intra-annual variance in rainfall over Southern Africa is examined for evidence to support the contention that it has undergone a systematic upward change over the period 1921-1974. A principal components analysis on the data shows that the time series of intra-annual variances may be represented by four components. Although each of the four principal component time series possesses an oscillation in the region of the biennial wave, there is little evidence to suggest that the within-year variation in rainfall is undergoing any type of upward trend.

A historical review of studies on the energy balance of arctic tundra

Abstract: The main field experiments and climatological calculations of the surface energy balance on the arctic tundra are reviewed, following the historical development of theories and instruments. The scientific significance and problem areas of individual studies are discussed and the present status of understanding the energy balance on the arctic tundra is described.

Studies of the Indian summer monsoon with a coarse‐mesh general circulation model, part II

Abstract: A previous paper (Part I) discussed the performance of a coarse-mesh general circulation model in studies of the Indian summer monsoon. Part II herein describes impacts on the simulated monsoon climate due to prescribed changes in the lower boundary conditions: lowering and flattening of the Himalayan Mountains and cold and warm sea-surface temperature (SST) anomalies in the Arabian Sea. Results indicate that it is the Himalayas that are responsible for the observed north-westerly flow aloft over northern India before onset and the delay in precipitation onset there until late June or July. SST anomalies, which are held constant throughout four-month simulations, affect local precipitation computations via changes in stability and cause impacts of the opposite sign immediately downwind. Neither of the two anomalies tested resulted in statistically significant precipitation impacts over India.

Comparison of normal and below normal July precipitation patterns in central Iowa

Abstract: Previous discussions of wet, dry and normal years in Iowa have been related primarily to the departure of rainfall from a climatological average. Rainfall during the month of July is examined because of its importance on the local cash crop, maize. The first section of the paper deals with the variability of areal precipitation patterns within the central Iowa climatological network (1000 km2/gauge) for the years 1930-1978. Results of this study seemed to indicate that examination of a fine-grid rain gauge network (1-64 km2/gauge) was needed. Weather radar data also were used to provide a better data base to study the size and variability of July precipitation areas. In the second section, July 1976 (dry) and July 1977 (normal) were chosen to provide a comparision of precipitation patterns between a dry and normal precipitation year. The results show that, during a dry year (40 per cent below normal), there are nearly the same number of days with rain occurring as in a normal year. The most important factor in the comparison of the two years is that the size and number of individual precipitation echoes are both about 40 per cent less during 1976 than in 1977.

Snow and ice balance in britain at the present time, and during the last glacial maximum and late glacial periods

Abstract: Simple parameterizations are used to estimate monthly mean ablation of snow and ice, and also the distribution of monthly mean precipitation in Britain. Together these enable estimates to be made of snow storage at various levels and the gradients of days with snow cover with elevation. Small semi-permanent snow patches below the regional snow line are included in the discussion. The theoretical estimates fit the actual observations very closely. A similar approach is then used to estimate snow balances for the British Devensian ice sheets. Probable snow accumulation/elevation relationships are produced for 24,000 yr BP. Estimates suggest that the British ice sheet was just in balance at the lime of its maximum advance around 18,000 yr BP. The growth and decay of British ice sheets are considered and two stable states are suggested for ice-age climates, one without an ice-sheei and the other with a massive ice-sheet.

Multiple interpolation in climatological spectra

Abstract: The variable length record method of spectral analysis is considered as a composite of perturbations of a simple harmonic analysis. By smoothing over near neighbours, the composite spectrum can be made to approach the true spectrum. Since the method is computationally expensive, a less costly technique of multiple interpolation is considered and shown to produce equivalent results. This method is suitable for examining the low frequency end of cli ma to logical spectra.

Associations among the tropical pacific wind and sea surface temperature fields and higher latitude circulation

Abstract: An empirical study was conducted examining associations between seasonal trade wind and sea surface temperature (SST) fields in the eastern half of the tropical Pacific Ocean for the period 1949-1979. In addition, the tropical fields were statistically related to the extratropical circulation of the Northern Hemisphere (i.e. 700 mb heights). This study differs from most prior studies in that the trade wind field was examined primarily in terms of the horizontal trade wind divergence and seasonally stratified lag relationships were computed. Seasonally stratified autocorrelations of empirical orthogonal function (EOF) amplitudes (out to four seasons) indicated significant persistence of EOF1 of SST and divergence until spring-summer for any initial season; accordingly, spring and summer (summer and autumn) EOF1 for SST (divergence) exhibited the greatest persistence. For divergence some tendency for a reversal of sign was seen from the autumn, winter, or spring seasons to the following autumn, which may imply negative air-sea feedback mechanisms operating. Seasonally stratified cross correlations betwen the first EOF for SST and divergence indicated a significant contemporaneous relationship. When divergence leads SST, the relationship is significant at short lags (1-3 seasons) until spring as a target season is reached. The same is true when SST leads divergence except that a significant relationship returns with opposite sign in the subsequent summer and auiumn. Winter SST and divergence, in particular, are well correlated with prior values of the other parameter. When the tropical parameters were related to the extratropical circulation through correlation with gridded 700 mb heights, significant patterns were found only when winter 700 mb heights were the target parameter (i.e. tropical parameter leading winter 700 mb heights). Summer through winter tropical SST and divergence (EOF1) displays a good association with subsequent winter extratropical circulation in the form of an expanded/contracted circumpolar vortex. The relationship between trade wind divergence and subsequent winter circulation is significant using the prior winter (4 season lag), vanishes during spring, and returns with the opposite sign for summer through winter (lags 0-2). Using either SST or divergence, the observed relationships are stronger at lags 1 and 2 than at lag 0.