Utjecaj leda na površini mora na sezonske klimatske promjene

The sensitivity of the atmospheric circulation to a different specification of sea-ice temperature and its seasonal cycle is analysed from the 50-year long integrations with SPEEDY, an intermediate complexity atmospheric general circulation model (AGCM). This impact is inferred from the difference between model atmospheric states obtained with and without the inclusion of the thermodynamic effects of sea ice. The two experiments with sea ice were made – the first one used climatological monthly mean temperatures for sea ice (derived from ERA–15 data), whereas in the second experiment the sea-ice temperature was determined by a thermodynamic model embedded into the SPEEDY code. It is shown that the thermodynamic model tends to amplify the seasonal cycle of temperature. In the boreal winter, the inclusion of the thermodynamic model for sea-ice temperature leads to a general cooling of the model atmosphere at high latitudes (when compared with the experiment with climatological sea ice), associated with the reduction in geopotential heights and the strengthening of zonal winds. It also reduces the extent and amount of cloud cover in the mid- and high latitudes. Atmospheric cooling could be directly linked to the increased sea-ice seasonal cycle, because the increased albedo over sea ice reduces incoming solar radiation and further stabilises already cold air. Some of the changes induced by sea ice extend throughout the whole depth of the model atmosphere and could be linked directly to strong meridional differential temperature gradients. In addition, some seasonally varying symmetry between the Northern and the Southern Hemisphere is also found. In summer when the receding sea ice is included in model integration, the opposite effects are seen: differential temperature gradients are of the opposite sign, the atmosphere is warmed thus effecting a reduction in zonal winds and an increase in cloudiness. These effects are stronger in amplitude than those associated with the maximum winter extent of sea ice, suggesting that ocean heat flux from the ice-free water together with increased solar radiation and convection bear a strong mark on the model atmosphere.Utjecaj leda na površini mora na opću cirkulaciju atmosfere razmatran je pomoću relativno jednostavnog atmosferskog globalnog cirkulacijskog modela (nazvanog Speedy). Posebna je pažnja posvećena sezonskim promjenama raspodjele leda te njegovom termodinamičkom utjecaju. U tu su svrhu definirana dva eksperimenta: prvi koristi klimatološke mjesečne temperature leda na površini mora (dobivenih pomoću ERA–15 podataka), dok drugi eksperiment uključuje termodinamički model za dobivanje temperatura leda. U oba eksperimenta je integracija modelom izvedena u trajanju od 50 godina. Rezultati pokazuju da termodinamički model pojačava sezonski ciklus temperature. Tako tijekom zime uključivanje termodinamičkog modela uzrokuje dodatno hlađenje atmosfere viših geografskih širina u odnosu na temperature dobivene integracijom modela s klimatološkim vrijednostima temperature leda na površini mora. Takva temperaturna raspodjela praćena je smanjenjem visina geopotencijalnih ploha te jačanjem zonalnog vjetra. Također je modelom dobivena i smanjena naoblaka u području srednjih i viših geografskih širina. Hlađenje atmosfere može se izravno povezati sa sezonskim ciklusom jer led na površini mora povećava albedo te na taj način smanjuje upadno sunčevo zračenje, a time dodatno stabilizira ionako hladni zrak. Neke od promjena uzrokovane ledom na površini mora se protežu kroz cijelu modeliranu atmosferu. Takvo ponašanje može se izravno povezati s jakim meridionalnim gradijentim temperature. Nadalje, uočena je određena sezonska simetričnost između Sjeverne i Južne Hemisfere. Tijekom ljeta kad je smanjen ledeni pokrov, model daje suprotne rezultate od onih dobivenih za zimsku sezonu: atmosfera je toplija uz slabljenje zonalnog vjetra i povećanu naoblaku. Ove promjene imaju veću amplitudu od onih povezanih s maksimalnom količinom ledenog pokrova tijekom zime. Taj rezultat upućuje na to da toplinski tokovi sa slobodne površine mora zajedno s povećanim sunčevim zračenjem i konvekcijom imaju značajan utjecaj na modeliranu atmosferu

Utjecaj leda na površini mora na sezonske klimatske promjene

The sensitivity of the atmospheric circulation to a different specification of sea-ice temperature and its seasonal cycle is analysed from the 50-year long integrations with SPEEDY, an intermediate complexity atmospheric general circulation model (AGCM). This impact is inferred from the difference between model atmospheric states obtained with and without the inclusion of the thermodynamic effects of sea ice. The two experiments with sea ice were made – the first one used climatological monthly mean temperatures for sea ice (derived from ERA–15 data), whereas in the second experiment the sea-ice temperature was determined by a thermodynamic model embedded into the SPEEDY code. It is shown that the thermodynamic model tends to amplify the seasonal cycle of temperature. In the boreal winter, the inclusion of the thermodynamic model for sea-ice temperature leads to a general cooling of the model atmosphere at high latitudes (when compared with the experiment with climatological sea ice), associated with the reduction in geopotential heights and the strengthening of zonal winds. It also reduces the extent and amount of cloud cover in the mid- and high latitudes. Atmospheric cooling could be directly linked to the increased sea-ice seasonal cycle, because the increased albedo over sea ice reduces incoming solar radiation and further stabilises already cold air. Some of the changes induced by sea ice extend throughout the whole depth of the model atmosphere and could be linked directly to strong meridional differential temperature gradients. In addition, some seasonally varying symmetry between the Northern and the Southern Hemisphere is also found. In summer when the receding sea ice is included in model integration, the opposite effects are seen: differential temperature gradients are of the opposite sign, the atmosphere is warmed thus effecting a reduction in zonal winds and an increase in cloudiness. These effects are stronger in amplitude than those associated with the maximum winter extent of sea ice, suggesting that ocean heat flux from the ice-free water together with increased solar radiation and convection bear a strong mark on the model atmosphere.Utjecaj leda na površini mora na opću cirkulaciju atmosfere razmatran je pomoću relativno jednostavnog atmosferskog globalnog cirkulacijskog modela (nazvanog Speedy). Posebna je pažnja posvećena sezonskim promjenama raspodjele leda te njegovom termodinamičkom utjecaju. U tu su svrhu definirana dva eksperimenta: prvi koristi klimatološke mjesečne temperature leda na površini mora (dobivenih pomoću ERA–15 podataka), dok drugi eksperiment uključuje termodinamički model za dobivanje temperatura leda. U oba eksperimenta je integracija modelom izvedena u trajanju od 50 godina. Rezultati pokazuju da termodinamički model pojačava sezonski ciklus temperature. Tako tijekom zime uključivanje termodinamičkog modela uzrokuje dodatno hlađenje atmosfere viših geografskih širina u odnosu na temperature dobivene integracijom modela s klimatološkim vrijednostima temperature leda na površini mora. Takva temperaturna raspodjela praćena je smanjenjem visina geopotencijalnih ploha te jačanjem zonalnog vjetra. Također je modelom dobivena i smanjena naoblaka u području srednjih i viših geografskih širina. Hlađenje atmosfere može se izravno povezati sa sezonskim ciklusom jer led na površini mora povećava albedo te na taj način smanjuje upadno sunčevo zračenje, a time dodatno stabilizira ionako hladni zrak. Neke od promjena uzrokovane ledom na površini mora se protežu kroz cijelu modeliranu atmosferu. Takvo ponašanje može se izravno povezati s jakim meridionalnim gradijentim temperature. Nadalje, uočena je određena sezonska simetričnost između Sjeverne i Južne Hemisfere. Tijekom ljeta kad je smanjen ledeni pokrov, model daje suprotne rezultate od onih dobivenih za zimsku sezonu: atmosfera je toplija uz slabljenje zonalnog vjetra i povećanu naoblaku. Ove promjene imaju veću amplitudu od onih povezanih s maksimalnom količinom ledenog pokrova tijekom zime. Taj rezultat upućuje na to da toplinski tokovi sa slobodne površine mora zajedno s povećanim sunčevim zračenjem i konvekcijom imaju značajan utjecaj na modeliranu atmosferu

SEASONAL DYNAMICAL DOWNSCALING WITH ERA-4O DATA: A SENSITIVITY STUDY

The Regional Climate Model (RegCM) with 50 km horizontal resolution was used for seasonal dynamical downscaling of ECMWF ERA-40 data over central and southern Europe and the northern Mediterranean region for one winter and one summer season. Various configurations in initial conditions (ICs) and lateral boundary conditions (LBCs) as well as in the model vertical domain were tested. First, the horizontal resolution of ICs and LBCs was increased from T42 to T159 and then a gradual "degradation" was conducted: the frequency of the LBCs update was reduced from 6-hourly to 12-hourly intervals, the model top was lowered from 100 to 200 hPa and the number of vertical levels was reduced from 18 to 14. The latest, most "degraded", configuration is the closest to that available in the ECMWF seasonal forecast archive. The limited number of experiments (a single experiment per configuration and per season) did not allow a thorough statistical assessment of model responses; however, it can be concluded that, though the differences between various configurations and resolutions are generally small, they are far from being negligible. The increase in the ICs and LBCs horizontal resolution yields a reduction in geopotential, in both the upper-air and surface temperature (cooling) and a reduction in the summer convective precipitation. The reduced, 12-hourly, frequency of the LBCs update mostly renders the opposite result, i.e. an increase in geopotential and temperature. However, it was not possible to fully establish how actually detrimental this reduced frequency in LBCs is, because the input ERA-40 upper-air data "osciliate" between consecutive 6 hours due to the difference in the processing of temperatures from satellites and from radiosondes within the ECMWF 3D-Var assimilation system. The model upper troposphere winds are strengthened when the model top is lowered; however, this effect is partly offset when the number of model levels is reduced. Changes in the model vertical configuration cause on average much weaker effects on surface fields than changes in LBCs. The Arakawa-Schubert ciosure in the parameterisation of convection reduces the amount of summer convective precipitation over the northern European lowlands when compared with the Fritsch,Chappel closure. However, in a large portion of the integration domain precipitation is still too high in respect of the CRU observational data