Recent advancements in the measurement and modeling of heavy stable water isotopes (i.e., H218O and HDO), especially in situ and remote sensing spectroscopic vapor isotope measurements and isotope-incorporated general circulation and regional models, have rapidly improved our understanding of the behavior of water isotopes on Earth. These advancements have significantly increased the recognition of the usefulness of information on stable water isotopes in the geoscience community. This study reviews these recent advancements and their applications in climatological, meteorological, and hydrological sciences. It also explores two emerging directions. The first is the direct validation of climate models using independent isotopic proxy data. I particularly discuss comparisons of the 20th Century Isotope Reanalysis dataset, which covers 1871-2008, to ice core δ18O, tree cellulose δ18O, and coral δ18O. The second is the use of isotopic data as constraints in various climatological, meteorological, and hydrological models. Ideally, vapor isotope data could be used to improve weather forecasting.
The atmospheric component of the Zebiak-Cane (ZC) coupled model is a simple elegant framework that plays an instrumental role in the investigation of the fundamental coupled dynamics of the tropical ocean-atmosphere interaction and the El Niño/Southern Oscillation (ENSO) phenomenon. We attempt two simple thermal and dynamical modifications to reduce apparently spurious wind activities simulated by this atmospheric model in the eastern tropical Pacific under ENSO sea surface temperature (SST) anomaly forcing. First, motivated by established observational evidences, we manipulate the anomalous convective heating to be dependent on the sum of mean SST and SST anomalies and the low-level convergence. Second, we add an ad hoc parameterization for the effect of convective momentum transport (CMT) into the zonal momentum balance. By adding a background-dependent efficiency factor into convective heating and further adjusting the convective feedback parameter used in the original model, we are able to substantially reduce the wind bias in the tropical Pacific, especially in the eastern tropical Pacific. Meanwhile, with a simple CMT parameterization, it compensates an over reduction in the central Pacific wind due to the first modification. By comparing the simulated wind anomalies in our modified version with that from the original version and with the observation, we find that the equatorial wind response to observation SST anomaly forcing for the period 1982-2010 has been improved in terms of amplitudes and spatial patterns. We also show that these improvements are carried over to ZC coupled model as well when we add these modifications to coupled simulations.
The Tokai region in central Japan often receives heavy rainfall because of typhoons. Furthermore, because of global warming, the intensity of heavy rainfall events is expected to increase in the future. Therefore, assessment of possible differences in such events between the present and future is important. In this study, a record heavy rainfall event in the Tokai region on 11 September 2000, the so-called Tokai Heavy Rain (THR), was numerically simulated by weather research and forecasting model with triple nesting grid system of 50-, 10-, and 2-km horizontal resolution. Simulated results present characteristics of rainfall and atmospheric conditions similar to the actual event. Thus, the simulation is considered valid for reproducing rainfall processes of the THR. In addition, variations of heavy rainfall events in future climate scenarios are investigated using numerical simulations based on pseudo global warming (PGW) conditions, constructed using third-phase results of Coupled Model Intercomparison Project multi-model global warming experiments. Under certain future climate scenarios, the Tokai region may experience heavy rainfall events in which maximum hourly rainfall and extent of heavy rainfall areas increases. Such variations are mainly attributed to increased specific humidity in the lower troposphere. In some PGW runs, there was no significant rainfall around the Tokai region. There was increased specific humidity in these runs, and the horizontal distribution of lower atmospheric air temperature was favorable for the formation of a mesoscale convergence zone, as seen in PGW runs with heavy rainfall. However, vertical profiles of equivalent potential temperature and saturated equivalent potential temperature showed unsaturated and stable atmospheric stratifications that are unfavorable for convective activity. Even in cases with increased atmospheric temperature and specific humidity caused by global warming, differences in their spatial distributions and vertical profiles could lead to contrasting effects of global warming on a specific extreme weather event.
We investigate future changes in winter precipitation around Japan and their uncertainties using the downscalings of a non-hydrostatic regional climate model (NHRCM) with 20-km grid spacing according to global climate projections. The global climate projections were conducted by the atmospheric general circulation model with three patterns of sea surface temperature changes in the Coupled Model Intercomparison Project Phase 5 under the Representative Concentration Pathway 8.5. Moreover, three cumulus convective parameterizations were applied in the present and future climate experiments. The ensemble mean of nine future NHRCM experiments shows decreases in the winter precipitation on the coast of the Sea of Japan and over the Pacific Ocean in the south of the Japanese archipelago. The former decrease in precipitation results from a weakened winter monsoon. The latter corresponds to changes in extratropical cyclone number around Japan, which have a large uncertainty. On the other hand, winter precipitation increases over the northernmost part of Japan (Hokkaido) and the northeastern Asian continent. The strengthened northwesterly around Hokkaido, which results from the reduction of sea ice in the Sea of Okhotsk, causes increased precipitation in the inland area of Hokkaido. In addition, moistening due to global warming relates to increased precipitation in extremely cold regions. These signals are common to most experiments.
Climate monitoring in urban areas is important because climate change in densely populated areas has a strong influence on society. The rate of long-term temperature increase in high-latitude snowy urban areas is relatively large due to global warming and urban heat islands. However, the influence of snow cover on urban heat islands is unclear. The purpose of this study is to assess the effect of snow cover in urban canopy layer on winter heat islands using a mesoscale atmospheric model coupled with an urban canopy model. Numerical experiments indicate that snow cover in urban areas serves to decrease surface air temperature, with a stronger decrease in daily maximum temperatures (0.4-0.6°C) than daily minimum temperatures (0.1-0.3°C). The increase in surface albedo is primarily responsible for the decrease in net shortwave radiation and sensible heat flux. In addition, increased evaporation causes a weakened sensible heat flux. The estimated snow cover effect during the day is comparable with the typical magnitude of anthropogenic heat release. In urban canopy layer, snow cover on roofs plays a significant role in reducing surface air temperature. Snow clearing on roads tends to increase nocturnal surface air temperature, especially in suburban areas because decreased snow depth increases ground heat transfer. These results indicate that snow cover in urban canopy layer reduces surface air temperature, resulting in weakened urban heat islands.