Detection of a climatological short break in the polar night jet in early winter and its relation to cooling over Siberia

The polar night jet (PNJ) is a strong stratospheric westerly circumpolar wind at around 65∘ N in winter, and the strength of the climatological PNJ is widely recognized to increase from October through late December. Remarkably, the climatological PNJ temporarily stops increasing during late November. We examined this “short break” in terms of the atmospheric dynamical balance and the climatological seasonal march. We found that it results from an increase in the upward propagation of climatological planetary waves from the troposphere to the stratosphere in late November, which coincides with a maximum of the climatological Eliassen–Palm (EP) flux convergence in the lower stratosphere. The upward propagation of planetary waves at 100 hPa, which is strongest over Siberia, is related to the climatological strengthening of the tropospheric trough over Siberia. We suggest that longitudinally asymmetric forcing by land–sea heating contrasts caused by their different heat capacities can account for the strengthening of the trough.

Improving Seasonal Prediction of East Asian Summer Rainfall: Experiments with NESM3.0

It has been an outstanding challenge for global climate models to simulate and predict East Asia (EA) summer monsoon (EASM) rainfall. This study evaluates the dynamical hindcast skills with the newly developed Nanjing University of Information Science and Technology Earth System Model version 3.0 (NESM3.0). To improve the poor prediction of an earlier version of NESM3.0, we have modified convective parameterization schemes to suppress excessive deep convection and enhance insufficient shallow and stratiform clouds. The new version of NESM3.0 with modified parameterizations (MOD hereafter) yields significantly improved rainfall prediction in the northern and southern China but not over the Yangtze River Valley. The improved prediction is primarily attributed to the improvements in the predicted climatological summer mean rainfall and circulations, seasonal march of the subtropical rain belt, Nino 3.4 SST anomaly, and the rainfall anomalies associated with the development and decay of El Nino events. However, the MOD still has notable biases in the predicted leading mode of interannual variability of precipitation. The leading mode captures the dry (wet) anomalies over the South China Sea (northern EA) but misplaced precipitation anomalies over the Yangtze River Valley. The model can capture the interannual variation of the circulation indices very well, but the bias in the circulation-rainfall connection caused predicted rainfall errors. The results here suggest that over EA land regions, the skillful rainfall prediction relies on not only model’s capability in predicting better summer mean and seasonal march of rainfall and ENSO teleconnection with EASM, but also accurate prediction of the leading modes of interannual variability.

Are CMIP5 Models Better than CMIP3 Models in Simulating Precipitation over East Asia?

The performance of climate models participating in phases 5 and 3 of the Coupled Model Intercomparison Project (CMIP5 and CMIP3, respectively) is evaluated and compared with respect to precipitation over East Asia (20°–50°N, 110°–150°E). The target period covers the 20 years from 1981 through 2000. The CMIP5 and CMIP3 models underestimate precipitation amounts over East Asia in the warmer season (May–September), while they overestimate precipitation amounts in the colder season (October–April). Both sets of models have some difficulty in simulating the seasonal march of the rainy season over China, the Korean Peninsula, and Japan, and they also underestimate the precipitation intensity over East Asia. Nevertheless, the CMIP5 models show a higher reproducibility of precipitation over East Asia than the CMIP3 models with respect to the geographical distribution of precipitation throughout the year, seasonal march of the rainy season, and extreme precipitation events. Models with a higher reproducibility of annual precipitation tend to show a higher reproducibility of precipitation intensity for both the CMIP5 and CMIP3 models. Correlation analysis using all of the CMIP5 and CMIP3 models reveals that models with higher horizontal resolution tend to perform better than those with a lower resolution. The advantage of the CMIP5 models over the CMIP3 models in the simulation of the East Asian climate can be partly attributed to the improved representation of the west Pacific subtropical high in the CMIP5 models, especially during the summer.

Seasonal changes in diurnal rainfall cycle over and around the Indochina Peninsula observed by TRMM-PR

This study used TRMM-PR data to examine seasonal changes in rainfall characteristics over the Indochina Peninsula, with a focus on the diurnal rainfall cycle. No distinct seasonal changes in the phases of diurnal variations of rainfall were found, even though low-level wind fields changed largely with the seasonal march. Regions with an afternoon maximum received large amounts of rainfall during the pre-monsoon season, whereas regions with a nocturnal or morning rainfall maximum received little rainfall during the pre-monsoon season. This result suggests that the difference in diurnal rainfall variations may be associated with the regional differences in seasonal march of monsoon rainfall over and around the Indochina Peninsula, through the different mechanism between evening and morning rainfalls.