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1.
Forecasting the equatorial Pacific sea surface temperatures by neural network models 总被引:2,自引:0,他引:2
We used neural network models to seasonally forecast the tropical Pacific sea surface temperature anomalies (SSTA) in the
Ni?o 3.4 region (6 °S–6 °N, 120 °W–170 °W). The inputs to the neural networks (i.e., the predictors) were the first seven wind stress empirical orthogonal function
(EOF) modes of the tropical Pacific (20 °S–20 °N, 120 °E–70 °W) for four seasons and the Ni?o 3.4 SSTA itself for the final season. The period of 1952–1981 was used for training the neural
network models, and the period 1982–1992 for forecast validation. At 6-month lead time, neural networks attained forecast
skills comparable to the other El Ni?o-Southern Oscillation (ENSO) models. Our results suggested that neural network models
were viable for ENSO forecasting even at longer lead times of 9 to 12 months. We hypothesized that at these longer leads,
the underlying relationship between the wind stress and Ni?o 3.4 SSTA became increasingly nonlinear. The neural network results
were interpreted in light of current theories, e.g., the role of the “off-equatorial” Rossby waves in triggering the onset
of an ENSO event and the delayed-oscillator theory in the development and termination of an ENSO event.
Received: 31 October 1995 / Accepted: 25 July 1996 相似文献
2.
I. Bischoff-Gauß N. Kalthoff M. Fiebig-Wittmaack 《Theoretical and Applied Climatology》2006,85(3-4):227-241
Summary High-resolution model simulations were performed with the quadruple-nested version of the mesoscale model KAMM to investigate
the impact of the new storage lake ‘Embalse Puclaro’ on the arid environment. The storage lake covers an area of 1 to 2 km
in width and about 7 km in length. Model simulations were performed for a summer and a winter day.
Due to a change in the surface properties, the installation of the storage lake resulted in a modification of the energy balance.
Above the lake area, a stably stratified atmosphere establishes during the day and unstable stratification during the night.
During the day, the latent heat flux is similar to that of the replaced cultivated ground, but is higher at the night. The
influence of the storage lake on temperature and humidity can be seen to a height of about 300 m above ground level. During
the night, water vapour accumulation results in relative humidity values of 100%, as a result a greater number of days with
fog above the storage lake is likely when compared to the surrounding area. The storage lake does not produce its own lake
breeze during the day, because the larger-scale up-valley wind is too dominant. However, a significant modification of the
nocturnal down-valley wind above the lake area can be observed, especially in summer. As a consequence of the larger-scale
valley wind system, the influence of the storage lake on the temperature, humidity, and wind field can be identified up to
about 4 km on the downwind side. 相似文献
3.
Summary ?The dependence of aerosol optical depth on wavelength as well as the fit of the ?ngstr?m approximation have been investigated
under different air masses at a sub-Arctic location (Abisko, Sweden; 68° 21′ N, 18° 49′ E) and a tropical environment (Ife,
Nigeria; 7° 30′ N, 4° 31′ E). The study is based on spectral data acquired with a high resolution spectral radiometer (spectral
range: 300–1100 nm) in absorption-free regions.
The wavelength dependence of the aerosols under different air mass conditions at the sub-arctic location offer significant
contrasts to aerosols of Saharan origin at Ife. A general characteristic of the aerosol optical depth spectra after the Pinatubo
volcanic eruption was a much weaker wavelength dependence relative to pre-Pinatubo conditions.
Categorising the features of the optical depth spectra according to their wavelength dependence, three main groups were observed
at Abisko, while two main classes have been discussed for the harmattan season in the tropical climate of Ife and environs.
For the first two groups in Abisko (and the first group at Ife), aerosol optical depth generally decreased with wavelength
while the third group (second group at Ife) exhibited strong curvatures.
The correlation coefficient obtained from the regression equation of the ?ngstr?m equation, has been shown to be a good index
of the general fit of the ?ngstr?m approximation for the three groups at Abisko, but much weaker for the harmattan conditions
at the tropical location. Although the probability of systematic deviations from the ?ngstr?m law is highest under intense
harmattan conditions with considerably high β and low α, it has been observed that the ?ngstr?m fit was good in many highly
turbid conditions at the tropical site. Hence, apart from the level of turbidity, the applicability of the ?ngstr?m approximation
is strongly dependent on aerosol characteristics and source region.
Formerly Adeyefa.
Received May 18, 2001; revised June 20, 2002; accepted August 5, 2002 相似文献
4.
Summary Seasonal variations of gravity wave characteristics are investigated using rawinsonde data observed at Pohang observatory,
Korea (36°2′N, 129°23′E) during the one-year period of 1998. Analysis is carried out for two atmospheric layers representing
the troposphere (2–9 km) and stratosphere (17–30 km). There exist clear seasonal variations in amplitudes of temperature and
wind perturbations and wave energy in the stratosphere, with their maxima in wintertime and minima in summertime. A strong
correlation is found between the wave activity and the strength of the jet stream, but there is no clear correlation between
the wave activity and the vertical gradient of static stability. The intrinsic frequency and vertical and horizontal wavelengths
of gravity waves in the stratosphere are 2f–3f, where f is the Coriolis parameter, and 2–3 km and 300–500 km, respectively. The intrinsic phase velocity directs westward in January
and northeastward in July. The vertical flux of the stratospheric zonal momentum is mostly negative except in summertime with
a maximum magnitude in January. Topography seems to be a major source of stratospheric gravity waves in wintertime. Convection
can be a source of gravity waves in summertime, but it is required to know convective sources at nearby stations, due to their
intermittency and locations relative to floating balloons. 相似文献
5.
Annual methane emission from Finnish mires estimated from eddy covariance campaign measurements 总被引:4,自引:0,他引:4
K. J. Hargreaves D. Fowler C. E. R. Pitcairn M. Aurela 《Theoretical and Applied Climatology》2001,70(1-4):203-213
Summary Measurements of landscape-scale methane emission were made over an aapa mire near Kaamanen in Finnish Lapland (69° 8′ N,
27° 16′ E, 155 m ASL). Emissions were measured during the spring thaw, in summer and in autumn. No effect of water table position
on CH4 emission was found as the water table remained at or above the surface of the peat. Methane emission fluxes increased with
surface temperature from which an activation energy of −99 kJ mol−1 was obtained. Annual emission from the site, modelled from temperature regression and short-term flux measurements made in
three separate years, was calculated to be 5.5 ± 0.4 g CH4 m−2 y−1 of which 0.6 ± 0.1 g CH4 m−2 y−1 (11%) was released during the spring thaw which lasted 20 to 30 days.
The effect of global warming on the CH4 budget of the site was estimated using the central scenario of the SILMU (Finnish Research Programme on Climate Change) model
which predicts annual mean temperature increases of 1.2, 2.4 and 4.4 °C in 2020, 2050 and 2100, respectively. Maximum enhancements
in CH4 emission due to warming were calculated to be 18, 40 and 84% for 2020, 2050 and 2100, respectively. Actual increases may
be smaller because prediction of changes in water table are highly uncertain.
Received September 17, 1999 Revised October 16, 2000 相似文献
6.
Medha Khole 《Meteorology and Atmospheric Physics》2000,75(1-2):1-9
Summary The year 1997 witnessed one of the most severe El-Ni?o events of the century. However, the All-India Summer Monsoon Rainfall
(AISMR) was 102% of its long period average. In view of recent studies (Tourre and White, 1995, 1997) of detection of ENSO
signal over Indian Ocean, the Sea-Surface Temperature (SST) variation over Indian Ocean (20° N–10° S/50° E–100° E), concurrent
to El-Ni?o event of 1997 is examined. It is observed that during the developing, mature and decaying stages of El-Ni?o, the
North Indian Ocean was abnormally warm. This anomalous warming may be one of the factors responsible for anomalous precipitation
over India during October to December of 1997.
Received August 24, 1999/Revised February 15, 2000 相似文献
7.
Summary ?The paper considers a meso-scale, adiabatic, inviscid and Boussisnesq flow of a stably stratified fluid over a three-dimensional
(3-D) meso-scale orographic barrier with elliptic contour, with special reference to a part of the Western ghats mountain
along west coast of India and on the Khasi-Jayantia hill in the northeast India. The airstream characteristics are simplified
by assuming that the upstream wind velocity (U) and buoyancy frequency (N) are constant with height. Solutions for perturbation vertical velocity (w′) and streamline displacement (η′) are expressed in terms of double integrals. These integrals cannot be evaluated exactly,
hence they have been approximated by asymptotic expansion method. Side by side solutions using numerical method have also
been obtained. The results of the study indicate that the updraft regions in the asymptotic solution are crescent shaped,
symmetrical about the axis y = 0, tilting upwind and spreading laterally with height. The study also shows that in both asymptotic solution and numerical
solution w′ and η′ fall off down wind of the barrier in the central plane (y = 0), further more in the asymptotic solution w′ and η′ fall off as x
−1. The study also indicates that the discrete updraft regions obtained in the numerical solution, when joined, take a crescent
shape.
Received November 26, 2001; accepted February 27, 2002 相似文献
8.
Seasonal carbon dioxide balance and respiration of a high-arctic fen ecosystem in NE-Greenland 总被引:2,自引:0,他引:2
C. Nordstroem H. Soegaard T. R. Christensen T. Friborg B. U. Hansen 《Theoretical and Applied Climatology》2001,70(1-4):149-166
Summary Turbulent fluxes of CO2 were continuously measured by eddy correlation for three months in 1997 over a gramineous fen in a high-arctic environment
at Zackenberg (74°28′12″N, 20°34′23″W) in NE-Greenland. The measurements started on 1 June, when there was still a 1–2 m cover
of dry snow, and ended 26 August at a time that corresponds to late autumn at this high-arctic site. During the 20-day period
with snow cover, fluxes of CO2 to the atmosphere were small, typically 0.005 mg CO2 m−2 s−1 (0.41 g CO2 m−2 d−1), wheres during the thawed period, the fluxes displayed a clear diurnal variation. During the snow-free period, before the
onset of vegetation growth, fluxes of CO2 to the atmosphere were typically 0.1 mg CO2 m−2 s−1 in the afternoon, and daily sums reached values up to almost 9 g CO2 m−2 d−1. After 4 July, downward fluxes of CO2 increased, and on sunny days in the middle of the growing season, the net ecosystem exchange rates attained typical values
of about −0.23 mg m−2 s−1 at midday and max values of daily sums of −12 g CO2 m−2 d−1. Throughout the measured period the fen ecosystem acted as a net-sink of 130 g CO2 m−2. Modelling the ecosystem respiration during the season corresponded well with eddy correlation and chamber measurements.
On the basis of the eddy correlation data and the predicted respiration effluxes, an estimate of the annual CO2 balance the calender year 1997 was calculated to be a net-sink of 20 g CO2 m−2 yr−1.
Received October 6, 1999 Revised May 2, 2000 相似文献
9.
F. J. Olmo J. Vida I. Foyo-Moreno J. Tovar L. Alados-Arboledas 《Theoretical and Applied Climatology》2001,69(3-4):253-263
Summary
Knowledge of the partition of global solar irradiance in its diffuse and direct beam components is required in different areas
of applied meteorology. In the absence of solar irradiance measurements parametric approaches have to be used instead. In
the present work, the parametric CPCR2 model has been analysed at Granada (37.18° N, 3.58° W, 660 m a.m.s.l), an inland location,
covering a period greater than three years. Only cloudless conditions are analysed. Angstr?m’s α and β coefficients have been
computed from measurements carried out with a sunphotometer. As the study reveals, the best performance of the parametric
model is conditioned to the availability of appropriate information on aerosols, especially when the interest is focused on
the direct and diffuse irradiance.
Received October 18, 1999/Revised December 18, 2000 相似文献
10.
Gesa A. Weyhenmeyer 《Climatic change》2008,88(2):199-208
Evaluating trends over time (nonparametric Mann–Kendall test) for 18 water chemical variables from 79 reference lakes, distributed
all over Sweden, during spring since 1984 showed most significant trends for atmospheric deposition driven sulfate (SO4) concentrations. The decrease in SO4 concentrations was on average 2.7 times higher at lower (56°N to 59°N) than at higher latitudes (60°N to 68°N). This large
difference in the rate of change between lower and higher latitudes could not solely be explained by atmospheric deposition
as the rates of change in SO4 wet deposition differed by a factor of only 1.5 between lower and higher latitudes. Significantly higher rates of change
at lower than at higher latitudes are known from the timing of lake ice breakup, a typical climate change indicator. The rates
of change in the timing of lake ice breakup differed by a factor of 2.3 between lower and higher latitudes. Other water chemical
variables showing significantly higher rates of change at lower than at higher latitudes were water color (a factor of 3.5),
calcium (a factor of 2.9), magnesium (a factor of 5.5) and conductivity (a factor of 5.9). The rates of change of all these
variables were strongly related to the rates of change in the timing of lake ice breakup along a latitudinal gradient (R
2 = 0.41–0.78, p < 0.05), suggesting that climatic changes can accelerate atmospheric driven changes at especially lower latitudes. This acceleration
will result in more heterogeneous lake ecosystems along a latitudinal gradient. 相似文献
11.
Mesoscale aspects of the Urban Heat Island around New York City 总被引:15,自引:1,他引:15
S. D. Gedzelman S. Austin R. Cermak N. Stefano S. Partridge S. Quesenberry D. A. Robinson 《Theoretical and Applied Climatology》2003,75(1-2):29-42
Summary ?A mesoscale analysis of the Urban Heat Island (UHI) of New York City (NYC) is performed using a mesoscale network of weather
stations. In all seasons the UHI switches on rapidly in late afternoon and shuts down even more rapidly shortly after dawn.
It averages about 4 °C in summer and autumn and 3 °C in winter and spring. It is largest on nights with clear skies, low relative
humidity through much of the troposphere, and weak northwest winds, when it may exceed 8 °C. The synoptic meteorological situation
associated with the largest UHI occurs roughly two to three nights after cold front passages.
During spring and summer, sea breezes commonly reduce and delay the UHI and displace it about 10 km to the west. Backdoor
cold fronts, which occur most frequently in spring and early summer, reduce or even reverse the UHI, as cold air from the
water to the northeast keeps NYC colder than the western suburbs. Cases documenting the sensitivity and rapidity of changes
of the UHI to changes in parameters such as cloud cover, ceiling, and wind speed and direction are presented.
Received August 16, 2001; revised October 6, 2002; accepted November 20, 2002
Published online March 17, 2003 相似文献
12.
Summary Three cyclones developing between 28 August and 6 September 1995 were studied with respect to the temporal evolution of their
water budget components. The cyclones were simulated with the regional model REMO. Water budget values were determined from
hourly model output for circle areas with 500 km radius around the pressure minimum. The results show a maximum liquid water
path of about 0.12 kg m−2 and a maximum ice water path of 0.16 kg m−2. In the vertical cloud structure the medium cloud layer disappears at the end of the life cycle for all three cyclones. The
release of precipitation onto the Baltic Sea drainage basin is different for each cyclone. It lies between 13 and 22 · 1012 kg. This is about 50% of the total precipitation in the whole area for the strongest cyclone and 65% for the others. The
P — E (precipitation minus evaporation) is 15 · 1012 kg for two of the cyclones and 10 · 1012 kg for the third one.
Received August 7, 2000 Revised March 19, 2001 相似文献
13.
Summary Degree-days as a measure of accumulated temperature deviations from a base temperature have many practical applications in
various human related activities such as home cooling, heating, plant growth in agriculture and power generation in addition
to energy requirement. Long temperature records are necessary for their reliable estimations at given stations. In this paper,
degree-day measure has been applied to monthly temperature records for systematically changed base temperature values from
− 25 °C to + 35 °C with 5 °C increments at 255 meteorology stations in Turkey. The results are represented in the form of
spatial degree-day distribution maps, which are then related to various climatic, meteorological and topographic features
of Turkey. For instance, free surface water bodies in forms of surrounding seas, lakes and rivers insert retardation in the
expansion of heating degree-days over large regions. On the other hand, cold air penetration from polar regions in the northeastern
Turkey originating from Siberia appears at moderate base temperature heating degree-days.
Received August 20, 1998 Revised June 21, 1999 相似文献
14.
A. S. Broad 《Meteorology and Atmospheric Physics》2000,72(1):1-11
Summary. ?A hydrostatic numerical model is used to simulate the lee wave event IOP3 (0000 GMT to 1200 GMT 15th October 1990) from the PYREX mountain experiment. Results from integrations at different horizontal resolutions are used
to investigate the effect on surface pressure drag and the vertical flux of horizontal momentum due to orographically forced
gravity waves. In particular, results showing the dependence on resolution of the partitioning between resolved and parametrized
wave drag and fluxes are presented. With the model horizontal gridlength changing from 50 km to 10 km the majority of wave
momentum flux changes from being parametrized to becoming resolved. More significantly, there is a change in the profile of
flux with height. At 50 km resolution the largest inferred mean flow decelerations are at lower stratospheric level due to
the parametrization scheme. At 10 km resolution this is shifted, with less deceleration high up and more wave deceleration
lower down in the troposphere. Numerical weather prediction models are now beginning to take account of such low level drag
with beneficial results.
Received March 2, 1999/Revised July 15, 1999 相似文献
15.
O. R. Sánchez-Ccoyllo P. L. Silva Dias M. de Fátima Andrade S. R. Freitas 《Meteorology and Atmospheric Physics》2006,92(1-2):83-93
Summary This study is aimed to qualitatively analyze the impact of remote sources on air pollution in the Metropolitan Area of S?o
Paulo (MASP). Air-mass back trajectories from June to August of 1999 were calculated using a three-dimensional kinematic trajectory
model and grouped into trajectory clusters. Correlations of individual trajectory clusters with O3, CO and PM10 concentrations were determined. In this model, trajectories were obtained using the means of the three wind velocity components
(U, V and W). The three-dimensional wind field was derived from the Regional Atmospheric Modeling System, and downscaling
was employed. Coarse and fine nested grids (64-km and 16-km horizontal resolution, respectively) were used. Every 12 h (at
00 and 12 UTC), a back-trajectory ensemble, using the 64-km grid, was calculated for five defined endpoints at intervals of
0.5° N, S, E and W of the MASP (λ = 23° 33′S, ϕ = 46° 45′W), that last endpoint being centered in the MASP. To analyze cluster
trajectories, the five trajectory ensembles from each day were allocated into one of four clusters (northeast, southeast,
southwest or northwest quadrant) based on the origin of the trajectory over 4 days. Days on which all five trajectories originated
from the same quadrant were classified as “core” days. Core day concentrations of CO, O3 and PM10 during the study period were evaluated. The results show that, during the study period, air-mass back trajectories in the
MASP originated from all four quadrants: northeast (32%), southeast (12%), southwest (19%) and northwest (37%). Our analysis
of back-trajectory clusters in the MASP suggests a transport to ambient air of O3 precursors and O3 from the northeast region, which is associated with agricultural activities involving biomass burning. 相似文献
16.
Using a database of spectra collected with an airborne infrared spectrometer between 1978 and 2005, the longest record of
this type, we have searched for a temporal trend in the stratospheric OCS amount. The total column above 200 hPa, in latitudes
from 30° to 60°N, shows a change of about 0.77 ± 0.80% per year relative to the 2010 value which is 1.34 × 1015 molecules cm−2; thus not a significant change. Observations are made from the base of the stratosphere and are uniquely suited to determining
the stratospheric OCS abundance. 相似文献
17.
Summary The aim of this study is the evaluation of the sea breeze speed on the basis of its energy. Energetics of the sea breeze can
be studied by means of the available potential energy (APE). Part of this energy is transformed into the kinetic energy of
the sea breeze. Some similarity exists between the large scale processes of the circulation and the small coastal air circulation
due to the fact that both circulations are triggered by the same physics, i.e., solenoidal activity of the baroclinic atmosphere.
To evaluate the sea breeze speed, APE was calculated by use of the Lorenz’s equation (1955), and which is possible if the
coastal circulation is considered to be a closed system in a hydrostatic equilibrium. For calculations and verifications hourly
sea-surface temperatures, near-ground air temperatures and wind speed measurements, as well as the radio-sounding measurements
at 12 UTC were used at the Zadar station (ϕ = 44° 08′ N, λ = 15° 13′ E), which is situated in the central part of the eastern
Adriatic coast. Two days with an undisturbed sea breeze circulation were extracted using the methods for minimizing other
atmospheric influences. Calculated hourly near ground sea breeze speeds obtained in this way were higher than the measured
ones. With the assumption that some of the APE is transformed into the kinetic energy it is possible to obtain characteristic
speed of the developed sea breeze with small discrepancies to the near-ground measurements. If 6.6% of the mean daily near
ground APE was taken to be transformed to the mean daily kinetic sea breeze energy on the 29th and 4.2% on the 30th September
2002, the best agreement was obtained with the mean daily measured near ground sea breeze speed. This range of values can
be attributed to inability to extract precise values for the lapse-rate needed in the APE sea breeze calculations. Results
show similarities to the general circulation of the atmosphere, since about 10% of the APE is transformed to the kinetic energy
of the sea breeze. On the other hand calculated wind speed at the lower branch of the borderline coastal circulation was not
dependent on the integral value of the APE over the land, but on its value at the near-ground level. 相似文献
18.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献
19.
Summary Two UV-Biometer 501A instruments were used to estimate global erythemal irradiance at two locations in southwest Sweden;
the Earth Sciences Centre, University of G?teborg (57.69° N; 11.92° E) and the island of Nordkoster, 200 km to the north (58.83° N;
10.72° E).
A semi-empirical radiative transfer model was used to calculate the global erythemally effective irradiance under clear skies.
A ratio of the hourly measured to clear-sky modelled irradiance was then derived for zenith angles 35–70°. Subsequent comparisons
were then made with routine measurements of sunshine duration at G?teborg and sunshine duration, cloud cover, type and height
at Nordkoster.
Cloud transmission of UV-B irradiance decreases with increasing solar zenith angle, with cloud attenuation being 8% stronger
at Nordkoster Island for zenith angles >>;60°. Transmission also decreases with increasing cloud cover such that overcast
cloud conditions reduce transmissions by an average of 75%. In addition, cloud type affects the amount of ground incident
irradiant flux. Fractus cloud afforded the least UV-B transmission (0.16), while cirrus filaments afforded the most (0.95).
The spatial and temporal distribution of clouds appears tobe non-random. Under conditions of 1 to 3 octas, sky cover, clouds
appear to be concentrated in line with the sensor and Sun on more occasions than that expected given a random cloud distribution.
The same cloud cover condition also resulted in many instances of ground incident irradiance above clear-sky values. The presence
of cumuliform clouds appears to increase the likelihood of the latter phenomena.
Received January 4, 1998 相似文献
20.
K. Prasad 《Meteorology and Atmospheric Physics》2006,91(1-4):183-199
Summary Results of an earlier study of cyclone track prediction using a quasi-Lagrangian model (QLM) to generate track forecasts of
up to 36 hours were reported by Prasad and Rama Rao (2003). Further experiments to produce track forecasts of up to 72 hours
with an updated version of the same model have been carried out in the present study. In this case, the ability of the model
to predict recent historical cyclones in the Bay of Bengal and Arabian Sea has been assessed. Analysis of some of the structural
features of analyzed and predicted fields has been carried out. Such fields include wind distribution and vertical motion
around the cyclone centre. In addition, the merging of an idealized vortex with the large scale initial fields provided by
a global model, has been carried out for a particular case study of a May 1997 storm, which hit the Bangladesh coast. This
current study has demonstrated that the model generates a realistic structure of a tropical cyclone with an idealized vortex.
Performance evaluation has been carried out by computing the direct position errors (DPE). The results of which show that
the mean error for a 24 h forecast is about 122 km, which increases to about 256 km for a 48 h forecast and 286 km for a 72 h
forecast. These figures are comparable to similar errors in respect of tropical cyclone forecasts produced by an advanced
NWP centre, viz., the UKMO global model during the corresponding period, 1997–2000 (obtained from UKMO web site). The average
forecast errors of the UKMO model are 160 km for 24 h, 265 km for 48 h, 415 km for 72 h forecast ranges. 相似文献