THE ROLE
OF HYDRODYNAMIC INSTABILITY IN A WEATHER PREDICTION
Marta Mann
Geofizički zavod Prirodoslovno-matematičkog fakulteta
Sveučilišta u Zagrebu
Abstract: The aim of
this paper is to answer the question why our weather prediction for Croatia
failed on June 22, 1990? It deals with the application of the meridional wind
profile in estimating the direction of movement of a cyclone in a
hydrodynamically unstable atmosphere. Nonlinear meridional variation of zonal
flow – barotropic wind shear – has been proved to govern the movement of
cyclones and to support their amplification in the vicinity of the inflexional
parts of the zonal flow meridional profiles.
Key words:
hydrodynamic instability, baroclinic and barotropic instability, meridional
profiles of zonal flow, barotropic governing
MOUNTAIN
DRAG ESTIMATION FROM THE OPERATIONAL SYNOPTIC NETWORK
Abstract: This paper
presents the time series of the three-hourly average pressure drag vectors per
unit volume for the Dinaric Alps computed separately from microbarographic and
conventional pressure data. Regardless of magnitude, the pressure drag vectors
seem to be aligned almost perpendicularly to the main mouton ridge. The
pressure drag maxima during SOP are always connected with the Bora periods and
the magnitudes of the drag values indicate that during these events there is a
major sink of atmospheric momentum over the Dinaric Alpine region.
The main aim of this
paper is to provide a means of drag estimation for period in which no
microbarographic data are available. It investigates the correlation between
the time series of pressure drag computed from microbarographic and synoptic
pressure data. The results suggest the possibility of application of the linear
regression method in drag estimation for periods without microbarographic data.
Key words: mountain
drag, pressure drag vector, microbarographic data, synoptic network,
correlation coefficients, ALPEX-SOP
APPLICATION
OF THE HYDRAULIC THEORY TO NATURALLY OCCURRING BORA FLOW
Vesna Jurčec and Dražen Glasnović
Abstract: This paper
deals with the application of Smith’s (1985) internal hydraulic theory to some
special cases of Adriatic bora during ALPEX SOP. In particular, the case of 15
April 1982 is analysed for which the numerical simulation by Klemp and Durran
(1987) is available. Their conclusion is that in this case neither inversion
nor critical levels are important for the bora occurrence, but the low-level
wave breaking due to weak flow in the upstream region. While this conclusion
seems reasonable, from the present analysis it is argued that these results are
valid only for the characteristics of the local bora in Senj and not for the
flow across the higher mountains along the northern Adriatic coast.
This conclusion is
based on a comparison with the results of the theory application to the
stronger bora condition on the previous day and especially to the case of the
strongest SOP bora on 6 March 1982 for which the hydraulic theory offered much
better results.
Key words: Adriatic
bora, hydraulic flow, wave breaking, severe winds.
STATISTICAL
ANALYSIS OF SEVERE ADRIATIC BORA
Višnja Vučetić
Abstract: This paper
presents the results of a statistical analysis of severe bora occurrence at two
locations, Split and Dubrovnik, in mid-Adriatic. A situation with severe bora
is defined as a period with wind, the direction of which is continuously
between 360° and 90°, with a mean hourly speed ³17.0 ms-1 during at least one hour. In the
30-year period of observation (1958-1987) 116 situations with severe bora were
registered in Split. This is four times less than in Senj, on the northern
Adriatic. In Dubrovnik, severe bora is not so frequent. It is shown that severe
bora with maximum gusts ³40.0 ms-1 appears along the entire Adriatic
coast, but its duration and frequency decrease from north to south.
Key words: severe and
strong bora, Split and Dubrovnik, statistical analysis of wind.
WIND PROFILE
IN SURFACE LAYER OF THE ATMOSPHERE
Lidija Cvitan
Abstract: A wind model
based on the Monin-Obukhov theory has been prepared for operational use over
flat terrain in northern Croatia. The input parameters are air temperature at 2
and 35 m, wind speed at 35 m and a constant value for surface roughness length
zo determined on the basis of terrain classification by Kondo and
Yamayawa (1986). The iterative method was used to improve the accuracy of some
similarity parameters. The wind profiles showed considerable dependence upon
stability. Therefore special stability categories were introduced, based on the
values of bulk Richardson number. Based on the similarity theory universal
functions, the local functions were derived for each stability class. These
functions were included into the model and only then a satisfactory speed
profile simulation was achieved.
Key words:
Monin-Obukhov theory, universal functions, iterative method, stability
classification
CORRELATION
BETWEEN METEOROLOGICAL FACTORS AND BLOOD COAGULATION PARAMETERS IN CEREBROVASCULAR
PATIENTS
Nada Pleško1, Miroslava Klein-Pudar2,
Ksenija Zaninović1
Abstract: This article
presents the results of measurements of the various blood coagulation parameters
and blood pressure of cerebrovascular patients in different weather conditions
during the year. The air temperature involved was from 2-25°C and barometric
pressure form 987-1004 hPa. Measurements have been performed through 6
three-day series, with a cold front passage over Zagreb in the central day of
every series.
The results show that
a significant correlation exists only between air temperature and both
prothrombine and thrombine time. Prothrombine and thrombine times were longer
with the higher air temperature. A highly significant correlation (r = -0.465,
P<0.01) was obtained between the fibrinogen quantity and barometric
pressure, showing that if the weather situation has a lower barometric
pressure, the fibrinogen quantity is higher.
With regard to cold
front passage, the mean prothrombine time is shortened from “n-1” to “n+1” day,
while the mean thrombine time oscillates around the day with front passage and
it is the shortest one day after front passage. The mean fibrinogen quantity is
at its highest on the day before the front passage and at its lowest on the day
of front passage.
Key words:
cerebrovascular patients, cold front passage, air temperature, barometric
pressure, blood coagulation parameters, blood pressure
CORRELATION
OF PSYCHOSIS AND SUICIDE ATTEMPTS WITH METEOROLOGICAL FACTORS
Nada Pleško1, Vlasta Jasprica-Hrelec2,
Ingrid Bošan-Kilibarda2, Berislav Banek2
Abstract: In this
study the authors have tried to find out if there is any correlation between
weather characteristics and acute psychoses and suicidal attempts (APSA). This
research was based on SHMP data about the number of interventions in cases of
APSA over a period of one year (VI/ 1988-V/1989) in Zagreb and on the
meteorological data registered at the Observatory Zagreb-Maksimir in the same
period as well as on weather charts prepared daily or collected in the
Hydrometeorological Institute of Croatia in Zagreb. A possible correlation with
weather types and passages of cold fronts and especially with meteorological elements
as: cloudiness, barometric pressure, air temperature and relative humidity, was
studied. Linear correlation coefficients were calculated for daily values and
for moving three-, five- and seven-day periods.
The results revealed
that the number of psychoses and suicidal attempts increased from month to
month during the period considered, especially in females of the 15-20, 21-30
and 31-40 age groups. The acute psychosis and suicidal attempt number does not
significantly differ with respect to weather type. The daily frequency of these
pathological reactions increases in all seasons typically a day or two before
and after a cold font passage, but differences are not significant. A
significant correlation was obtained only with the duration of a high daily
cloudiness amount. Suicidal attempts and psychoses increase significantly if
the cloudy period lasts at least three days, but their significance is higher
if the cloudy weather period is longer.
Key words: suicidal
attempts, acute psychoses, weather types, passages of cold fronts, duration and
cloudiness amount.
A STATISTICAL
ANALYSIS OF INVERSIONS AND MIXING LAYER HIGHS IN ZAGREB AREA
Abstract: A
statistical analysis of surface inversions, elevated inversions and mixing
heights over Zagreb region has been performed by means of radio-sonde
observations at Zagreb-Maksimir at 00 and 12 UTC (1977-82).
Annual frequency of
all night inversions is 90%, and of all day inversions are about 65%. During
all seasons the surface inversions most frequently reach the high of about 100
m. Height surface inversions are winter characteristics and having a
persistency up to several days. Elevated inversion heights show a considerable
seasonal and diurnal variability with their lower basis being much lower during
the colder part of a year.
Mixing layer heights
are greater during summer than during winter time.
Key words: ground
(surface) inversions, elevated inversion, mixing height, statistical analysis,
the area of Zagreb.
ANALYSIS
OF PHENOLOGICAL CHARACTERISTICS IN THE REGION OF MOLVE
Višnja Vučetić
Abstract: The
development stages of perennial trees (apricot, walnut, common spruce and
locust-tree) al well as stages of honey bees were analysed in Đurđevac and
Bjelovar in the periods before (1971-1980) and after (1981-1990) the marsh gas
station Molve was putting in operation.
It has been shown
that differences between the considered periods do exist. Colder winters in the
second period caused late leafing and blooming of the common spruce, apricot
and walnut trees. Warmer springs provoked earlier vegetation of the
locust-trees and earlier stages of bees. However, the variations established
were not of local character having been observed on both locations at the same
time.
Key words:
Phenological observations, the marsh gas station Molve.
CLIMATOLOGICAL
OBSERVATION TIMES AT 07, 14 AND 21 LMT ARE RETAINED
Ivo Lukšić
Abstract: Yugoslavia
has for a long time been considering whether it would be justified to change
the climatological observation times at 07, 14 and 21 LMT to 06, 12 and 18 GMT.
Finally, it has been decided to retain the present climatological observation
times at 07, 14 and 21 LMT. The main reasons for this decision are: the special
importance of data collected at these three climatological observation times, a
better daily distribution of the climatological observation times at 07, 14 and
21 LMT, difficulties with recording instruments, satisfying climatological
needs, homogeneity, and the fact that more frequent objections concerning the
21 LMT data quality are not justified.
Key words:
climatological observation times.
Čedomir Branković
D.Sc.Thesis – Zagreb University, Faculty of Natural Science,
Zagreb, 1990, 111 pp.
Summary: The results
from a set of 30-day integrations made with the ECMWF model over a period of
two years are discussed. The impact of spectral horizontal resolution and of
model changes on the model systematic errors and forecast skill in the extended
range is studied.
The
30-day mean systematic errors in wind and geopotential heights reveal that,
among the four horizontal resolutions (T21, T42, T63 and T106) considered,
there is a distinct difference in behaviour between the T21 and other models,
and a number of similarities between T42, T63 and T106 models. Due to
unrealistic representation of eddy momentum flux, the T21 model significantly
underestimates zonal flow in the southern hemisphere. On the other hand, too
strong westerlies in the northern hemisphere at T42, T63 and T106 resolutions
are the consequence of poor representation of the effects of small and
subgrid-scale orography. In higher resolution models, these effects act, via
better representation of planetary waves and sensible heat flux, as to improve
thermal balance in the model stratosphere, ultimately leading to smaller errors
than in T21 and T42 models.
Systematic
errors in the tropics are large and the influence of horizontal resolution on
tropical flow is generally weaker than in the extratropics. Errors in the tropics
are related to inadequate simulation of diabatic forcing and are primarily
associated with the parameterisation of deep convection and partially with
model radiation. Apart form the rainfall, tropical errors are not much
dependent on representation of orography in the model.
It
is concluded that, because of its strong climate drift, the T21 model is the
least suitable for extended-range and climate integrations. Both T106 and T63
models have smaller climate drift when compared with T21; however, they are
probably too detailed for extended-range and climate integrations, especially
having in the mind the computational burden for long integrations at these
resolutions. Tropospheric systematic errors in the T42 model are quite
comparable with those of higher resolution models, and in view of computational
coast the T42 is a good compromise for climate runs.
Changes
in model formulation, parameterisation of orographic gravity wave drag and an
increase in vertical resolution, act as to reduce the model systematic errors.
The effects of these changes are most pronounced beyond the medium range, thus
confirming the importance of diagnosing systematic errors in the extended range
for the model primarily designed for medium-range forecasting.
The
skill scores are comparable at higher resolution models and are better than
those at T21. Time averaging improves the model skill scores, the largest
improvement being for T106 model at days 10-15. This is certainly encouraging,
since it coincides with the limit of theoretical predictability. Skill scores
are largely variable and depend on the development of the large scale
atmospheric systems such as blocking. The model did not prove successful in
predicting realistic transitions between various large scale flow types. Interannual
variability of the mean flow also affects forecast skill, and therefore
improvements in the model climatology do not necessarily reflect in improved
skill scores. Whilst, in principle, spread can be used as a predictor for
skill, it was found that the relationship between spread and skill is not
always unique and reflects the influence of the annual cycle.
A TWO-LAYER
SCALAR MODEL FOR POLLUTANT DISPERSION IN URBAN CONDITIONS
Marina Grčić
M.Sc. Thesis – Zagreb University, Faculty of Natural
Sciences,
Zagreb, 1990, 116 pp.
Summary: A two-layer model (DSM) of urban air pollution is
developed which describes temporal variations of pollutant concentrations.
Any
model for the dispersion of pollutants over a city must take into consideration
the effects of the city on the large-scale air flow and the complexity of urban
pollutant sources. A qualitative description of the urban effect on climate and
dispersion mechanisms is first given, and then a review is made of the
capabilities of modern modelling techniques to simulate and elucidate these
processes.
The
DSM introduced the concept of two distinct urban layers in order to
characterise the energy processes in an urban atmosphere. One, called the urban
canopy layer (UCL), extends from the ground up to about mean roof level; the
other, called the urban boundary layer (UBL), is a mesoscale internal boundary
layer the characteristics of which are determined, at least partially, by the presence
of the city beneath.
The
DSM is formulated by using the concentration equation based on the principle of
mass conservation. The equations are analytically solved for each layer with
respect to the vertical turbulent pollutant flux between the layers.
Based
on the flux-gradient theory an iterative procedure has been developed for the
estimation of the turbulent pollutant flux which is expressed here as a
function of atmospheric stability and difference of concentration between the
layers.
The
important input parameters of the DSM are: sensible heat flux, wind, air
temperature, UBL height, emission rate, urban size and structure.
Where
there is a lack of measured data several techniques are recommended for their
evaluation.
The
DSM has been used to simulate temporal variations of pollutant concentration in
the UCL and UBL in a synthetic city during the observed meteorological
conditions which included clear skies, low humidity and high pressure.
The
simple, one-layer scalar model (often used as control model) has been applied
to the same set of input parameters. A comparison of the results shows several
advantages of the DSM.
The
complexity of the relationship between input parameters, mathematical handing
of physical processes and output values often makes it difficult to assess
exactly what effect the change in a particular input should have had.
Sensitivity
analyses were conducted on all inputs by varying a specific input over a range
of typical urban values while all others were at chosen values. These analyses
were undertaken because of uncertainty about the values to be assigned to some
of the input parameters and to assess the model in terms of the physical
reality of the results.
The
sensitivity analyses showed the DSM’s general flexibility and applicability in
handling complex emissions and atmospheric conditions in an urban area.