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1.
Meso-gamma scale forecasts using the nonhydrostatic model LM 总被引:10,自引:0,他引:10
J. Steppeler G. Doms U. Schättler H. W. Bitzer A. Gassmann U. Damrath G. Gregoric 《Meteorology and Atmospheric Physics》2003,82(1-4):75-96
Summary ?The nonhydrostatic model LM was developed for small scale operational predictions. Advances in computer development will
give the possibility of operational models of a rather fine scale, which will cover the meso-gamma scale. The LM is currently
applied at a scale of 7 km and an increase of the operational resolution to 2.5 km is planned for the next few years. Predictions
of such high resolution require to abandon the hydrostatic assumption, which is used with most current operational weather
prediction models. The LM was designed to cover all resolutions from 50 m to 50 km with an efficiency making it suitable for
operational use. It is a fully elastic model, using second order centred finite differences. The time integration is done
using the Klemp–Wilhelmson method, treating the slow modes by a larger time step than the fast modes. The vertical propagation
of the fast waves is done implicitly.
After describing the design of the LM, this paper gives examples of model predictions at the meso-γ scale. Some results of
the current operational application at the resolution 7 km are presented. Deficiencies in the localisation of model generated
precipitation are investigated using an idealised bell shaped mountain and applying different resolutions. In this way the
convergence to the correct solution can be investigated. From these results it is concluded, that orographic filtering is
necessary and the effect of such filtering on precipitation forecasts is investigated. Finally, the prediction of a squall
line over northern Germany is shown in order to demonstrate the potential of the model in forecasting the meso-γ scale.
Received May 15, 2001; revised September 21, 2001 相似文献
2.
Summary The National Hurricane Center and Joint Typhoon Warning Center operational tropical cyclone intensity forecasts for the three
major northern hemisphere tropical cyclone basins (Atlantic, eastern North Pacific, and western North Pacific) for the past
two decades are examined for long-term trends. Results show that there has been some marginal improvement in the mean absolute
error at 24 and 48 h for the Atlantic and at 72 h for the east and west Pacific. A new metric that measures the percent variance
of the observed intensity changes that is reduced by the forecast (variance reduction, VR) is defined to help account for
inter-annual variability in forecast difficulty. Results show that there have been significant improvements in the VR of the
official forecasts in the Atlantic, and some marginal improvement in the other two basins. The VR of the intensity guidance
models was also examined. The improvement in the VR is due to the implementation of advanced statistical intensity prediction
models and the operational version of the GFDL hurricane model in the mid-1990s. The skill of the operational intensity forecasts
for the 5-year period ending in 2005 was determined by comparing the errors to those from simple statistical models with input
from climatology and persistence. The intensity forecasts had significant skill out to 96 h in the Atlantic and out to 72 h
in the east and west Pacific. The intensity forecasts are also compared to the operational track forecasts. The skill was
comparable at 12 h, but the track forecasts were 2 to 5 times more skillful by 72 h. The track and intensity forecast error
trends for the two-decade period were also compared. Results showed that the percentage track forecast improvement was almost
an order of magnitude larger than that for intensity, indicating that intensity forecasting still has much room for improvement. 相似文献
3.
Summary A precipitation correction and analysis (PCA) model has been designed and tested during the preparation phase of the BALTEX
main experiment BRIDGE. The PCA model consists of a dynamical bias correction module and a geostatistical module. The bias
correction reduces the systematic undercatch of the rain gauges due to wind-induced, evaporation and wetting losses by taking
instrument-specific properties plus additional information from synoptic observations into account; the mean correction factor
is maximum in February (1.25–1.50) and minimum in August (1.02–1.05). The geostatistical module is an ordinary block kriging
algorithm; it yields gridded daily precipitation values plus error estimates on the 1/6-degree resolution of the meso-scale
BALTEX models.
Here we use 3 years (1996–1998) of 4000 rain gauge observations collected by the BALTEX Meteorological Data Centre for a preliminary
high-resolution climatology of the BALTEX catchment. It comprises: Time-series of area-averaged daily precipitation; and area
distributions of monthly and annual precipitation. Largely independent is the worldwide monthly GPCP data set which includes
also satellite data and is available from 1979 on. GPCP yields, for the years 1996–1998, an average of 2.10 mm/day while our
evaluation yields 2.01 mm/day. The maximum difference (22%) occurs in January; during the summer months the values are about
equal.
Received September 27, 2000 Revised February 6, 2001 相似文献
4.
R. I. C. C. Francis A. B. Mullan J. A. Renwick 《Theoretical and Applied Climatology》2003,75(1-2):1-14
Summary ?We evaluate United Kingdom Meteorological Office (UKMO) one-month ensemble forecasts of mean sea-level pressure (MSLP) in
the southern hemisphere (SH) to 60° S, with a special focus on their utility near New Zealand (NZ). There are 105 9-member
ensembles, at approximately two-week intervals, between 1995 and 1999. Each forecast is averaged over two successive 15-day
periods and verified against the NCEP/NCAR reanalysis data set. Compared to climatology, the skill of the ensemble mean is
slightly positive in days 1–15, and slightly negative in days 16–30. Skill near NZ is slightly lower than the SH averages.
For SH-scale circulation patterns (as seen in the first few principal components), skill is greater than for most individual
grid points, but is still negligible or negative in days 16–30. Moderate skill-spread correlations (ρ ≈−0.5) were found for
some skill scores. The way that skill varies with season and the Southern Oscillation Index is consistent with other research
but not statistically significant for this small data set. Probabilistic forecasts of low and high pressures have skill similar
to that of the ensemble mean. The ensemble spread is generally too small, in that the analysis lies within the ensemble less
often than the theoretically optimum value of 80% of the time. Measured as a fraction of the natural variability, the spread
increases substantially with time and latitude: it is less than 0.5 near the equator in days 1–15, and takes values near 1
only at higher latitudes during days 16–30. The initial sequential structure of the ensembles (a consequence of the use of
time lags in their genesis) is still apparent in days 1–15 but has disappeared by days 16–30. Three potential alternatives
to the ensemble mean were all found to have less skill than it.
Received June 17, 2001; revised July 4, 2002; accepted November 22, 2002
Published online March 17, 2003 相似文献
5.
K. K. W. Cheung 《Meteorology and Atmospheric Physics》2001,78(1-2):23-34
Summary
Random perturbations (RPs) and a modified version for breeding of growing modes are used with a regional baroclinic mesoscale
model to perform ensemble forecasting of tropical cyclone motion. Based on a sample of six cases, similar conclusions are
found as in previous barotropic modeling studies. Even after introducing a larger spatial correlation into the RPs using a
multi-quadric analysis scheme, the skill of this ensemble mean track prediction is almost always lower than that of the control
forecast in the cases considered. The track prediction performance of the ensemble using regional bred modes (RBMs) as perturbations
has a higher average skill. At nearly all forecast intervals except less than 24 h when the initial position error still dominates,
the ensemble mean tracks in all six cases are improved over the control forecast. In the 6 h–24 h range, the success rate
(ratio of the cases with a forecast improvement to the total number of cases) has a value of 10/24. In the 30 h–48 h range,
the success rate increases to 20/24, but drops to 18/24 in the 54 h–72 h range. A relative skill score (RSS) is used to compare
the skills of the two perturbation methodologies. It is found that the average RSSs of using RBMs are significantly higher
than the corresponding ones of RPs at the 99% confidence level in all three 24-h periods. Note that the above conclusion is
only based on ensemble mean forecasts. All of the possibilities from an ensemble-based probabilistic track distribution are
not explored in this paper. The ensemble spreads in these RBM ensembles are large enough to include the verifying tracks in
all the cases considered. It is also found that the ensemble spread is well correlated with the average error in an ensemble
when using RBMs, but not with the ensemble mean forecast error in both methodologies.
Received February 7, 2001/Revised April 18, 2001 相似文献
6.
Summary ?A major limitation in predicting the ultraviolet-B irradiance on humans, plant leaves and flowers and aquatic organisms is
the difficulty in estimating exposure. This study analyzes the spatial variability in the daily exposure of narrow band 300 nm
and 368 nm and broadband 290–315 nm (UVB) solar radiation between twelve paired locations in the United States Department
of Agriculture (USDA) UVB Climate Network over two summer growing seasons (May through August of 2000 and 2001). The spatial
correlation of the UVB, 300 nm and 368 nm daily exposures between locations was approximately 0.7 to 0.8 for spacing distances
of 100 km. The 300 nm daily exposure was typically more highly correlated between locations than the 368 nm daily exposure.
Both the diffuse and direct beam components to the 300 nm daily exposure were similarly correlated with distance between locations.
The 368 nm diffuse component of the daily exposures was less correlated with distance than the direct beam component, limiting
the ability to interpolate daily exposures from measurement locations. In general the variability in daily exposures of UVB
in the USDA UVB Climate Network is too large to interpolate daily exposures of solar radiation, with estimated 300 nm, 368 nm
and broadband UVB errors at one-half the mean station spacing of the USDA Network of 22%, 21% and 16% respectively. More accurate
interpolations of UVB exposure from this network will require either the incorporation of cloud cover variability from satellite
imagery for daily exposure or the use of longer periods of accumulated exposure.
Received May 14, 2002; revised October 25, 2002; accepted November 16, 2002 相似文献
7.
Harsh Oza Virendra Padhya Akash Ganguly K. Saikranthi T. N. Rao R. D. Deshpande 《Climate Dynamics》2020,54(5):2745-2757
Long term (2005–2016) daily precipitation isotope data (δ18O, δD and d-excess) from Ahmedabad in semi-arid Western India are examined in light of various meteorological parameters and air parcel trajectories to identify prominent patterns in the isotopic character and discern the underlying hydrometeorological processes. One of the most prominent and systematic annual patterns is the isotopic depletion (average δ18O: − 2.5‰ in Jun–Jul; − 5.2‰ in Aug–Sept) in the second half of the Indian Summer Monsoon (ISM), which is observed in the 11 out of the 12 years of this study. Four geographically feasible causal factors have been examined if they contribute to observed late monsoon isotopic depletion. These factors are: (1) increased contribution of terrestrially recycled vapor; (2) intra-seasonal change in sea-surface, surface-air and cloud base temperatures; (3) increased rain-out fraction from marine vapor parcel; and (4) increase in relative proportion of convective rain. It is inferred from the present study that isotopic depletion in the second half of ISM is associated with: (1) increased contribution (45% from 36%) of terrestrially recycled moisture; (2) 1.9° C lower cloud base temperature; (3) increased rainout fraction due to decreased wind velocity (6.9 m/s from 8.8 m/s); and (4) an increase of 22.3% in the proportion of convective rain. Daily rain events with atypical isotopic composition (20‰ < d-excess < 0‰) are ascribed mainly to local weather perturbations causing sudden updraft of moist air facilitating terrestrial recycling of water vapor. 相似文献
8.
R. S. Ross A. Chakraborty A. Chen L. Stefanova S. Sirdas T. N. Krishnamurti 《Meteorology and Atmospheric Physics》2007,98(3-4):137-174
Summary Climate variations in the Caribbean, largely manifest in rainfall activity, have important consequences for the large-scale
water budget, natural vegetation, and land use in the region. The wet and dry seasons will be defined, and the important roles
played by the El Ni?o-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) in modulating the rainfall during
these seasons will be discussed.
The seasonal climate forecasts in this paper are made by 13 state of the art coupled atmosphere-ocean general circulation
models (CGCMs) and by the Florida State University Synthetic Superensemble (FSUSSE), whose forecasts are obtained by a weighted
combination of the individual CGCM forecasts based on a training period. The success of the models in simulating the observed
1989–2001 climatology of the various forecast parameters will be examined and linked to the models’ success in predicting
the seasonal climate for individual years. Seasonal forecasts will be examined for precipitation, sea-surface temperature (SST), 2-meter air temperature, and
850 hPa u- and v-wind components during the period 1989–2001. Evaluation metrics include root mean square (RMS) error and Brier skill score.
It will be shown that the FSUSSE is superior to the individual CGCMs and their ensemble mean both in simulating the 1989–2001
climatology for the various parameters and in predicting the seasonal climate of the various parameters for individual years.
The seasonal climate forecasts of the FSUSSE and of the ensemble mean of the 13 state of the art CGCMs will be evaluated for
years (during the period 1989–2001) that have particular ENSO and NAO signals that are known to influence Caribbean weather,
particularly the rainfall. It will be shown that the FSUSSE provides superior forecasts of rainfall, SST, 2-meter air temperature,
and 850 hPa u- and v-wind components during dry summers that are modulated by negative SOI and/or positive NAO indices. Such summers have become
a feature of a twenty-year pattern of drought in the Caribbean region. The results presented in this paper will show that
the FSUSSE is a valuable tool for forecasting rainfall and other atmospheric and oceanic variables during such periods of
drought. 相似文献
9.
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
10.
B. Geerts 《Theoretical and Applied Climatology》2002,72(3-4):157-163
Summary ?Weather station data indicate that large-scale irrigation of an arid region in southeastern Australia decreased the annual
range of monthly-mean temperatures by 1–2 K. Such decrease is consistent with an observed increase in dewpoint, by about 1 K.
Urbanisation may reduce the annual range but generally increases it.
Received August 23, 2001; revised March 6, 2002; accepted April 14, 2002 相似文献
11.
A mesoscale boundary-layer model (BLM) is used for running 12-h low-level wind forecasts for the La Plata River region. Several
experiments are performed with different boundary conditions that include operational forecasts of the Eta/CPTEC model, local
observations, as well as a combination of both. The BLM wind forecasts are compared to the surface wind observations of five
weather stations during the period November 2003–April 2004. Two accuracy measures are used: the hit rate or percentage of
cases with agreement in the wind direction sector, and the root-mean-squared error (RMSE) of the horizontal wind components.
The BLM surface wind forecasts are always more accurate, since its averaged hit rate is three times greater and its averaged
RMSE is one half smaller than the Eta forecasts. Despite the large errors in the surface winds displayed by the Eta forecasts,
its 850 hPa winds and surface temperature forecasts are able to drive the BLM model to obtain surface winds forecasts with
smaller errors than the Eta model. An additional experiment demonstrates that the advantage of using the BLM model for forecasting
low-level winds over the La Plata River region is the result of a more appropriate definition of the land–river surface temperature
contrast. The particular formulation that the BLM model has for the geometry of the river coasts is fundamental for resolving
the smaller scale details of the low-level local circulation. The main conclusion of the study is that operational low-level
wind forecasts for the La Plata River region can be improved by running the BLM model forced by the Eta operational forecasts.
L. Sraibman and G. J. Berri—Members of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina.
An erratum to this article can be found at 相似文献
12.
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
13.
Summary Dubbed Ice Storm ’98, an extreme weather event characterized by two synoptic systems in succession dropped about 70–100 mm
(in terms of water equivalent) of freezing precipitation over southeastern Ontario, southwestern Quebec and northeastern New
York during a 6-day period from January 5 to 10 in 1998. Individually, the two synoptic systems were not dramatically more
extreme in freezing precipitation than other major freezing rain events (4 since 1961) which occurred in the past over the
affected area. Some regions in the target area, however, were impacted more by the second system. Based on an analysis of
the 500 hPa vorticity field during the ’98 event, we suggest that the 1997/98 El Ni?o had a role in creating a flow environment
conducive to the rapid formation of the second synoptic system. In contrast, other major freezing rain events in the last
30 years involved only one synoptic system per event lasting no more than 3 days, and producing 20–50 mm of precipitation.
We have also found that, 3 out of 4 past major freezing rain events since 1958 were associated with the positive phase of
the North Atlantic Oscillation (NAO). Consistent with this usual past association between the NAO and a major freezing rain
event, Ice Storm ’98 also occurred when the phase of the NAO was positive. Analysis of these 3 past and the ’98 events also
indicates an apparent connection between the positive phase of the NAO and the northern Quebec high pressure system, which
is an essential synoptic feature of a major freezing rain occurrence over the southcentral region of Canada. As measured by
their respective indices, the maximum positive NAO state leads the maximum northern Quebec high by about 2 days (5 days in
the ’98 event). There is some suggestive evidence to indicate that the persistence of the northern Quebec high pressure system
is connected to the persistence of the positive phase of the NAO.
Received January 17, 2000 相似文献
14.
R. Hess 《Meteorology and Atmospheric Physics》2001,77(1-4):145-154
Summary Inaccurate specification of soil moisture contents can result in forecast errors up to several degrees centigrade. Since
direct measurements are rarely available, a variational method has been developed that assimilates synoptic measurements of
2 m-temperature in order to specify the moisture contents of the two soil layers of the Local Model at Deutscher Wetterdienst.
The analyzed values minimize a cost functional that expresses the differences between model forecast and observed screen-level
temperatures. The minimization is performed highly efficiently and only two additional forecasts are required but neither
tangent linear nor adjoint. Background state and background error covariance matrix are updated at each analysis step in a
Kalman-filter-like cycled scheme, which takes a model error into account. The soil moisture assimilation shows improved 2 m-temperature
forecasts in case of high radiative forcing by up to 3 °C for small areas in the presented 6-week trial run. It proved stability
and robustness for general weather conditions and has become operational at DWD for the LM on 14 March 2000.
Received August 21, 2000 Revised March 26, 2001 相似文献
15.
Summary ?During recent years, numerous studies have examined the Buenos Aires urban climate, but the relationship between large-scale
weather conditions and the Buenos Aires urban heat island (UHI) intensity has not been studied. The goal of this paper is
to apply an objective synoptic climatological method to identify homogeneous air masses or weather types affecting Buenos
Aires during winter, and to relate the results to the UHI intensity. A K-means clustering method was used to define six different
air masses considering the 03:00, 09:00, 15:00 and 21:00 LT surface observations of dry bulb temperature, dew point, cloud
cover, atmospheric pressure and wind direction and velocity at Ezeiza, the most rural meteorological station of the Buenos
Aires metropolitan area (Fig. 1). Results show that the mean UHI intensity is at its maximum (2.8 °C) a few hours before sunrise
when conditions are dominated by cold air masses associated with cold-core anticyclones, weak winds and low cloud cover. Inverse
heat islands are found during the afternoon for all air masses indicating that surface processes are not dominant at that
time. The relatively infrequent and warmest air mass is the only one that presents a mean negative urban-rural temperature
difference (−0.1 °C) during the afternoon with the smallest diurnal cycle of the UHI intensity probably due to the prevailing
high humidity and cloudy sky conditions. The paper provides an insight into the Buenos Aires urban–rural temperature difference
under a variety of winter weather types and results could be useful to improve local daily temperature forecasts for the metropolitan
area of Buenos Aires on the basis of the routine forecasts of weather types.
Received October 24, 2001; revised June 12, 2002; accepted October 10, 2002 相似文献
16.
Summary ?The LITFASS project (‘Lindenberg Inhomogeneous Terrain – Fluxes between Atmosphere and Surface: a Long-term Study’) of the Deutscher Wetterdienst (DWD, German Meteorological Service) aims to develop and to test a strategy for the determination
and parameterisation of the area-averaged turbulent fluxes of heat, momentum, and water vapour over a heterogeneous land surface.
These fluxes will be representative for an area of about 10 * 10 km2 (while the typical patch size is between 10−1 to 100 km2) corresponding to the size of a grid cell in the present operational numerical weather prediction model of the DWD.
LITFASS consists of three components:
– the development of a non-hydrostatic micro-α-scale model (the LITFASS local model – LLM) with a grid-size of about 100 * 100 m2,
– experimental investigations of land surface – atmosphere exchange processes and boundary layer structure within a 20 * 20 km2 area around the Meteorological Observatory Lindenberg,
– the assimilation of a data base as an interface between measurements and modelling activities.
The overall project strategy was tested over a three-week period in June 1998 during the LITFASS-98 field experiment. This
paper gives an overview on the LITFASS project, on the design and measurement program of the LITFASS-98 experiment, and on
the weather conditions during the period of the experiment. Conclusions are formulated for the operational realisation of
the LITFASS measurement concept and for future field experiments aimed at studying the land surface – atmosphere interaction
in the Lindenberg area. Selected results from both experimental and modelling activities are presented in a series of companion
papers completing this special issue of the journal.
Received June 18, 2001; revised March 18, 2002; accepted April 2, 2002 相似文献
17.
Summary. ?Cyclone track predictions in the Indian seas (Bay of Bengal and Arabian Sea) with a quasi-Lagrangian model (QLM) have been
attempted. QLM has a horizontal resolution of 40 km and 16 sigma levels in the vertical. It is integrated in a domain of about
4400 × 4400 km2. A new initialization procedure to provide initial fields for running the model has been designed. The initialization procedure
consists of updating the global model forecasts, used as first guess, provided by the National Center for Medium Range Weather
Forecasting (NCMRWF), New Delhi. A new version of IMD’s operational optimum interpolation scheme has been created to suit
the QLM grid structure. Lateral boundary conditions are computed from the extended forecasts of NCMRWF. The track forecasts
in each case show a reasonable skill of the forecast model in predicting the direction of movement within acceptable limits
of forecast errors, which are comparable to some of the best models operated by advanced NWP centers of the world. Even the
recurving storms are well predicted. Evolution of the vertical motion fields are also studied which reveal some interesting
features, which are described in detail in the text. The composited vertical motion fields are projected against observed
rainfall distribution, which show a good spatial correspondence.
Received August 9, 2001; revised March 12, 2002; accepted June 17, 2002
Published online: May 8, 2003 相似文献
18.
Jeffrey Shaman Jonathan F. Day Marc Stieglitz Stephen Zebiak Mark Cane 《Climatic change》2006,75(4):495-511
We present a method for the ensemble seasonal prediction of human St. Louis encephalitis (SLE) incidence and SLE virus transmission
in Florida. We combine empirical relationships between modeled land surface wetness and the incidence of human clinical cases
of SLE and modeled land surface wetness and the occurrence of SLE virus transmission throughout south Florida with a previously
developed method for generating ensemble, seasonal hydrologic forecasts. Retrospective seasonal forecasts of human SLE incidence
are made for Indian River County, Florida, and forecast skill is demonstrated for 2–4 months. A sample seasonal forecast of
human SLE incidence is presented. This study establishes the skill of a potential component of an operational SLE forecast
system in south Florida, one that provides information well in advance of transmission and may enable early interventions
that reduce transmission. Future development of this method and operational application of these forecasts are discussed.
The methodology also will be applied to West Nile virus monitoring and forecasting. 相似文献
19.
Summary The increasing use of weather radar quantitative precipitation estimates, particularly in automatic applications such as operational
hydrometeorological modelling or assimilation in numerical weather prediction (NWP) models, has promoted the development of
quality control procedures on radar data.
Anomalous propagation (AP) of the radar beam due to deviation from the standard refractivity vertical profile, is one of the
factors that may affect seriously the quality of radar observations because of the increase in quantity and intensity of non-precipitating
clutter echoes and consequent contamination of the estimated rainfall field. Another undesired effect of AP is the change
in the expected radar echo height, which may be relevant when correcting for beam blockage in radar rainfall estimation in
complex terrain. The aim of this paper is to study the use of NWP mesoscale forecasts to predict and monitor AP events.
A nested 15-km grid resolution version of the MASS model has been used to retrieve refractivity profiles in the coastal area
of Barcelona, near a weather radar and a radiosonde station. Using the refractivity profiles two different magnitudes were
computed: the vertical refractivity profile of the lowest 1000 m layer and a ducting index which describes the existence and
intensity of the most super-refractive layer contained in the lowest 3-km layer. A comparison between model forecasts and
radiosonde diagnostics during a six-month period showed that the model tended to underestimate the degree of super-refraction,
with a bias of 4 km−1 and RMSE of 11 km−1 in the 1-km vertical refractivity gradient. Further analysis of the data showed that a combination of previous observations
and forecasts allowed to produce modified forecasts improving the original direct model output, decreasing substantially the
bias, reducing the RMSE by 20% and improving the skill by 40%, beating also radiosonde observations persistence. 相似文献
20.
Raphael E. Okoola 《Meteorology and Atmospheric Physics》2000,73(3-4):177-187
Summary Climatological statistics of extreme temperature events over Kenya are established from the analysis of daily and monthly
maximum temperatures for a representative station (Nairobi Dagoretti Corner) over the period 1956–1997.
The months of June to August were shown to be the coldest with a mean monthly maximum temperature of less than 22 °C. Seasonal
(June to August) mean maximum temperature was 21.5 °C. Using this seasonal mean temperature for the period 1967–1997 delineated
1968 as the coldest year in this series and 1983 as the warmest year.
Spectral analysis of the seasonal data, for both the coldest and the warmest years, revealed that the major periods were the
quasi-biweekly (10 days) and the Intraseasonal Oscillations (23 days). Secondary peaks occurred at periods of 4–6 and 2.5–3.5
days.
A temperature threshold of 16.7 °C during July was used to define cold air outbreaks over Nairobi. This threshold temperature
of 16.7 °C was obtained from the mean July maximum temperature (20.9 °C) minus two standard deviations. Notable trends include
a decrease in the frequency of station-days, between 1956 and 1997, with temperatures less than 16.7 °C during July.
Surface pressure patterns indicate that the origin of the cold air is near latitude 25° S and to the east of mainland South
Africa. The cold air near 25° S is advected northwards ahead of the surface pressure ridge.
Received July 19, 1999 Revised January 11, 2000 相似文献