22 research outputs found
Dugoperiodički modovi u tropskoj atmosferi Drugi dio: Analitičko modeliranje Kelvinovih valova povezanih s konvekcijom uz konvekcijsku raspoloživu potencijalnu energiju
The thermal assumption of the model is based on the convective available potential energy (CAPE) closure, i.e. increased CAPE, represented by decreased midlevel potential temperature, results in increased precipitation. The dynamic assumption of the model is that the vertical heating profile has the shape of the first baroclinic mode, while the vertical dependence of modeled fields is calculated, i.e. the model is vertically resolved. The modeled modes are free Kelvin waves and convectively coupled Kelvin waves. It is shown that the CAPE closure is not sufficient to produce the observed destabilization of the Kelvin mode, but that the dynamical properties of the model give the observed phase speeds.Termalna pretpostavka modela je da konvekcijski raspoloživa potencijalna
energija utječe na količinu oborine. Konvekcijski raspoloživa potencijalna energija ovisi
o potencijalnoj temperaturi u srednjoj troposferi. Uzrok povećane količine oborine je
smanjena potencijalna temperatura u srednjoj troposferi. Dinamika modela je razvijena
uz pretpostavku da vertikalni profil grijanja ima oblik prvog baroklinog moda dok se u
modelu računa ovisnost termodinamičkih polja po vertikali. Modelirani valovi su
slobodni Kelvinovi valovi i Kelvinovi valovi povezani s konvekcijom. Iz modela se vidi
da konvekcijski raspoloživa potencijalna energija nije dovoljna za opaženu nestabilnost
Kelvinovih valova povezanih s konvekcijom, ali dinamika modela daje opaženu faznu brzinu
Polarna svjetlost viđena iz Hrvatske i dijela susjednih zemalja
Descriptions of 37 auroral (or polar light) events were analyzed that were published mostly since the middle of the 18th century up to present days and observed within latitudes from 42° to 45.5°N and longitudes 13° to 19°E (mainly on the territory of Republic of Croatia). The list of auroral events includes date, place and source of information and, for the most cases, visually observed parameters such as: color, shape, distribution on the sky and time duration of the phenomenon. Basic statistical results allowed the conclusions about the most frequent occurrence of a particular parameter. The auroral events correlate highly with the solar activity and in mentioned latitudes they correlate also with the secular period of sunspots.Analizirani su zapisi o 37 pojava polarne svjetlosti, viđene od sredine 18. stoljeća do danas, nad područjem od 42 do 45,5 °N širine i od 13 do 19 °E dužine (pretežno područje današnje Republike Hrvatske). Izrađen je popis svih pojava u obliku povijesnog kalendara polarne svjetlosti, koji sadrži datum i mjesto viđenja, izvor informacije
te opis opaženih parametara kao što su boja, oblik, raspodjela na nebu i trajanje pojave, gdje je to bilo moguće odrediti. Kalendar pojava je omogućio zaključke o najčešćim i bitnim značajkama polarne svjetlosti viđene iz naših krajeva. Pokazalo se da je čestina pojave povezana sa Sunčevom aktivnošću
Large-Scale Modes of the Tropical Atmosphere. Part I: Analytical Modeling of Convectively Coupled Kelvin Waves Using the Boundary-Layer Quasiequilibrium Approximation
One way of modeling the convectively coupled Kelvin waves in an equatorial non-rotating atmosphere is presented. It implements a simple linear, analytical model using the boundary-layer quasiequilibrium approximation and wind-induced surface heat exchange. The dynamics of the model are based on the assumption that the vertical heating profile has the shape of the first baroclinic mode. The vertical velocity has two sinusoidal components of different vertical wavelengths. One component corresponds to deep convection and the imposed heating profile while the other component, with shallower vertical wavelength, defines the phase speed of the convectively coupled Kelvin wave.
The results of the model show fast Kelvin waves that resemble adiabatic modes with the vertical wavelength being twice the depth of the troposphere and convectively coupled Kelvin waves that are damped and propagate with phase speed of 18 m/s. Wind-induced surface heat exchange causes the instability of the convectively coupled Kelvin waves, but only for very long wavelengths.
The value of the model is that under the single dynamical assumption of the vertical heating profile and using the boundary-layer quasiequilibrium assumption it yields the observed phase speed for the convectively couple Kelvin waves.U radu se prezentira jedan način modeliranja konvektivno združenih Kelvinovih
valova u ekvatorijalnoj nerotirajućoj atmosferi. Ono uključuje jednostavni linearni,
analitički model koristeći kvaziravnotežnu aproksimaciju u graničnom sloju i površinsku
izmjenu topline uzrokovanu vjetrom. Dinamika modela temelji se na pretpostavci da vertikalni profil grijanja ima oblik prvog baroklinog moda. Vertikalna brzina ima dvije sinusoidalne komponente različitih vertikalnih valnih duljina. Jedna odgovara dubokoj konvekciji i nametnutom profilu zagrijavanja, dok druga komponenta kraće
vertikalne valne duljine definira faznu brzinu konvektivno zdru`enog Kelvinovog vala. Rezultati modela su brzi Kelvinovi valovi koji sliče adijabatskim modovima s vertikalnom valnom duljinom dvostruko većom od dubine troposfere i konvektivno združenih Kelvinovih valova koji su prigušeni i propagiraju faznom brzinom od 18 m/s.
Površiinska izmjena topline uzrokovana vjetrom čini konvektivno združene Kelvinove valove nestabilnim samo za vrlo duge valove.
Vrijednost modela je u tome da pod jednostavnom pretpostavkom vertikalnog profila grijanja i kori{tenjem kvaziravnotežne aproksimacije graničnog sloja daje opaženu faznu brzinu konvektivno združenih Kelvinovih valova
Konvektivno udruženi Kelvinovi valovi i konvektivna inhibicija
We use a thermodynamic assumption that the vertical heating profile has the shape of the first baroclinic mode, and that the analytical expression for vertical velocity has two modes, representing shallow and deep convection. The thermal assumption of the model is given through the convective inhibition closure, i.e. negative convective inhibition results in increased precipitation. These modeled modes are the free Kelvin waves and the convectively coupled Kelvin waves. We find the latter mode to be unstable, with maximum growth rate at wavelengths of 6 000 kilometers. The model successfully captures the observed nature of the Kelvin waves and shows that convective inhibition closure is sufficient to trigger the observed destabilization of the convectively coupled Kelvin mode.Termodinamičke pretpostavke modela uključuju vertikalni profil grijanja koji ima oblik prvog baroklinog moda. Pretpostavljeni oblik vertikalne brzine sastoji se od dva dijela koji odgovaraju plitkoj i dubokoj konvekciji. Za parametrizaciju oborine uzeta je negativna konvektivna inhibicija (CIN). Dobiveni modovi su slobodni Kelvinovi valovi i Kelvinov val povezan s konvekcijom. Kelvinov val povezan s konvekcijom je nestabilan, a najveću nestabilnost pokazuje pri valnoj duljini od 6 000 km. Ovim modelom uspješno je reproducirana opažena priroda Kelvinovih valova, a iz modela se vidi da je konvektivna inhibicija dovoljna za modeliranje opažene nestabilnosti Kelvinovih valova povezanih s konvekcijom
Prostorna razdioba glavnih tipova šuma u Hrvatskoj kao funkcija makroklime
The model of spatial distribution of major forest types in Croatia was developed as a function of macroclimatic variables (monthly mean temperature, monthly precipitation, monthly mean global solar irradiation and monthly potential evapotranspiration) and variables derived from digital elevation model (terrain aspect and slope). Neural networks were used as modelling tool. The model was developed within the frame of a raster geographic information system with a spatial resolution of 300 × 300 m, and it was based on a forest vegetation map (in scale of 1 : 500000) and interpolation macroclimatic models. The agreement between modelled and mapped forest types was very good, which suggests a strong correlation between macroclimate and the main forest types in Croatia and high model reliability. The model was applied to the entire area of Croatia, aiming at the construction of the potential spatial distribution of major forest types. The model could be useful for reforestation planning and for prediction of vegetation succession under assumed climatic changes.Model prostorne razdiobe glavnih tipova šuma u Hrvatskoj razvijen je kao funkcija makroklimatskih varijabli (srednja mjesečna temperatura, mjesečna oborina, srednje mjesečno Sunčevo ozračenje i mjesečna potencijalna evapotranspiracija) te varijabli izvedenih iz digitalnog elevacijskog modela (orijentacija i nagib terena). Neuralne mreže korištene su kao oruđe za modeliranje. Model je razvijen u okviru rasterskog geografskog informacijskog sustava, uz prostornu razlučivost od 300 x 300 m i baziran je na karti šumske vegetacije (mjerila 1:500000) i interpolacijskih makroklimatskih modela. Podudaranje između modeliranih i kartiranih tipova šuma vrlo je dobro, što upućuje na jaku korelaciju između makroklime i glavnih tipova šuma u Hrvatskoj, te na visoku pouzdanost modela. Model je primjenjen na cijeli hrvatski teritorij s ciljem konstruiranja potencijalne prostorne razdiobe glavnih šumskih tipova. Model bi mogao biti koristan u planiranju pošumljavanja, kao i u predviđanju vegetacijske sukcesije pod utjecajem pretpostavljenih klimatskih promjena
MODELLING OF CLIMATIC VARIABLES AS PART OF A DENDROECOLOGICAL STUDY IN THE REPAŠ FOREST, PREKODRAVLJE, CROATIA
In this paper the preparation of climatic data for the purpose of a dendroecological study in the Repaš forest (Prekodravlje, Croatia) near Đurđevac is presented. The study was part of the research work done for future hydroelectric power plant \u27Novo Virje\u27 on the river Drava. As the necessary climatic indicators were not measured in the Repaš forest, data from the nearest Đurđevac meteorological station for the period 1960-1997 were used (air temperature and precipitation were directly measured, and potential evapotranspiration and solar irradiation were modelled). These data were extrapolated backward to the year 1899 by the method of general linear modelling and using the secular
data series of the Zagreb-Grič and Osijek meteorological stations. Additionally, the precipitation data were corrected on the basis of data (1987-1991) from the Gola station, which is the nearest to the Repaš forest. A dendroecological analysis of the correlation between the radial increment of a selected pedunculate oak tree in the Repaš forest and the described climatic variables is presented, using multiple linear regression. The climatic variables significantly
influenced the radial increment of the observed tree (correlation coefficient 0.664) in the period 1899-1997
Poboljšanje sheme vertikalne difuzije u ARPÈGE/ALADIN modelu
In atmospheric numerical weather prediction (NWP) models, the use of
long time-steps as allowed by efficient numerical/dynamical schemes can
lead to spurious oscillations due to the parameterized physical part. Typical
examples of this are the oscillations associated with simplified parameterization
schemes for vertical diffusion or shallow-convection, such as usually
used for NWP.
The oscillations generated by K-type vertical-diffusion schemes are well
documented, and being called fibrillations; they are characterized by high
temporal and vertical frequencies. Since they are linked to high vertical
resolution, these spurious oscillations are generally found in the low-levels
of model’s domain.
In ARPÈGE, the MÉTÉO-FRANCE NWP global model, and in ALADIN,
its limited-area model (LAM) version developed in cooperation with
Eastern European countries, and also used for operational NWP purpose,
some oscillations still remained in the evolution of the forecast fields, despite
the fact that a first anti-fibrillation scheme (AFS) had been included.
This study was made to examine the possible sources of these oscillations,
through the 1-D (vertical) version of these models.
First, the parameterization of shallow-convection (which is in fact part
of the vertical diffusion scheme) was found to be an important source of oscillations,
and some solutions for eliminating this problem are proposed.
Second, the original AFS is shown not to completely prevent the generation
of fibrillations, and a more efficient formulation is derived.
All AFSs basically consist in a temporal first-order decentering of the
diffusion equation, keeping an explicit form for the exchange coefficient itself.
The AFS correction thus always improves the stability at the expense of
the accuracy in some way. In the new AFS proposed here, the number of
grid-points which need a correction is lessen from almost 90% to some 5%,
resulting in a more accurate scheme. Unlike AFSs proposed in the literature,
the correction has now to be applied not only for grid points of atmospheric
stable conditions (i.e. Richardson number, Ri > 0) but also for atmospheric
slightly unstable conditions (Ri < 0).Učinkovite numeričko-dinamičke sheme u atmosferskim numeričkim prognostičkim modelima dozvoljavaju uporabu duljeg vremenskog koraka prilikom numeričke integracije, ali često dolazi do neželjenih oscilacija uzrokovanih parametrizacijom fizikalnog dijela. Tipičan primjer je pojava oscilacija povezanih s pojednostavljenim parametrizacijskim shemama za vertikalnu difuziju ili plitku konvekciju, koje se uobičajeno koriste u numeričkoj prognozi vremena.
Oscilacije koje se generiraju shemama vertikalne difuzije K-tipa do sada su detaljno razmatrane, i nazvane su fibrilacijama. Visoke prostorne i vremenske frekvencije karakteristike su fibrilacija. Zbog njihove povezanosti s velikom vertikalnom rezolucijom,
ove lažne oscilacije se općenito uočavaju na nižim nivoima domene modela.
U prognostičkim poljima ARPÈGE-a (numeričkom prognostičkom globalom modelu u MÉTÉO-FRANCE) i ALADIN-a (njegovoj verziji modela za ograničeno područje razvijenom u suradnji sa zemljama istočne Europe) neke oscilacije su se i dalje zadržale unatoč činjenici da je uključena prva anti-fibrilacijska shema (AFS). Ovaj rad prikazuje provedena ispitivanja mogućih izvora fibrilacija pomoću 1-D (vertikalne) verzije ovih modela.
Ispitivanja su dala sljedeće rezultate: 1. znatan izvor oscilacija je pronađen u parametrizaciji plitke konvekcije (što je u stvari dio sheme vertikalne difuzije) te su predložena neka riješenja za njihovo uklanjanje, 2. pokazalo se da početna AFS ne može u potpunosti spriječiti generiranje fibrilacija te je izvedena uspješnija formulacija.
Jednadžbe vertikalne difuzije prikazane u numeričkom vremenskom raspisu prvog reda, zadržavajući eksplicitnu formu samog koeficijenta izmjene, u osnovi sadrže AFS. Stoga, korekcija AFS-om uvijek poboljšava stabilnost na račun točnosti.
S novom anti-fibrilacijskom shemom, broj točaka mreže u kojima je potrebno izvršiti korekciju smanjen je s gotovo 90% na približno 5%, čime je shema dobila na točnosti.
Za razliku od predloženih AFS iz literature, korekcija se mora primjenjivati ne samo na točke mreže sa stabilnim atmosferskim uvijetima (Richardsonov broj, Ri > 0) već i na one točke u kojima je atmosfera blago nestabilna (Ri < 0)