Simulations of the Early Cretaceous (120,000,000 years before the present day: 120 Ma) and the Last Cretaceous (65 Ma) have been performed using an atmospheric general circulation model (AGCM) coupled with a 1.5-layer reduced-gravity ocean model. After the initial spin-up period, both the runs are integrated for approximately 70 years. The simulation results confirm the occurrence of first-order changes in tropical atmospheric circulation in response to changes in the land/sea distribution. The simulation results show that the continental drift during the Cretaceous strongly affects the Walker and Hadley circulations. The birth of the Atlantic resulting from the breakup of the Gondwana continent causes splitting of a Walker circulation cell into two, and this in turn reduces the zonal gradient of the equatorial SST over the Pacific. The resultant SST warming in the equatorial Pacific enhances the Hadley circulation. The northward drift of the Indian continent causes significant SST warming in the Indian Ocean and intensifies the monsoon precipitation over Asia. It is also shown that the seasonal variations in the Asian monsoon are much stronger in the 65-Ma run than in the 120-Ma run. Interestingly, continental breakups cause the mega-monsoon system to split into distinct monsoon systems such as the Indian, South American, and African monsoon systems.
We have investigated the seasonal and inter-annual variations of the difference in partial pressure of CO2 between surface seawater (pCO2sea) and overlying air (pCO2air) and the air-sea CO2 flux in the mid-latitudes of the western North Pacific (WNP; 25-40°N, 140-170°E) and eastern North Pacific (ENP; 25-40°N, 120-150°W) from 1999 to 2006 using the latest voluntary observation ship data. In the WNP and ENP, the area-averaged ΔpCO2 (pCO2air-pCO2sea) was at its minimum in late summer (-4.6 to 6.7 µatm in the WNP and -32.5 to -20.5 µatm in the ENP) and at its maximum in late winter (51.0 to 59.8 µatm in the WNP and 35.1 to 46.2 µatm in the ENP). The WNP acts as a moderate sink for atmospheric CO2 (4.1 to 5.5 mmol m-2d-1), while the ENP acts as a weak sink (1.1 to 1.9 mmol m-2d-1). Because ΔpCO2 is mainly controlled by pCO2sea, we have evaluated the effect of the factors controlling pCO2sea: sea surface temperature (SST), salinity (SSS), dissolved inorganic carbon (TCO2), and total alkalinity (AT). In the WNP, not only SST but also TCO2 plays an important role in the seasonal pCO2sea variation, while the SST could only explain most of the pCO2sea variation in the ENP. From 1999 to 2006, pCO2sea increased at a significantly lower rate (0.53 ± 0.11 µatm yr-1) than pCO2air (1.81 ± 0.01 µatm yr-1) in the WNP, and at a slightly lower rate in the ENP (1.32 ± 0.16 µatm yr-1). The air-sea CO2 flux increased at a rate of 0.19 ± 0.05 mmol m-2d-1 yr-1 in the WNP and 0.09 ± 0.03 mmol m-2d-1 yr-1 in the ENP, suggesting that the WNP is a stronger sink for atmospheric CO2.
The relationship of lightning with rainfall and temperature is investigated on the basis of a four-year (2004-2007) data set over the Pune region. It is found that the annual variations of stroke count and rainfall both are bimodal with the first peak value of rainfall showing a one month time lag from the first peak value of stroke count. This is attributed to the prime period of the onset phase of summer monsoon rainfall over the Pune region. Lightning is found to be highly correlated with rainfall (r = 0.792, significant at 0.4%), but by excluding the data for the month of July. The annual variation in the monthly mean surface wet bulb temperature and stroke count for the period under study over the Pune region shows similar peaks in June and September. These two parameters are well-correlated and show a positive correlation coefficient of 0.59, which is significant at the 5% level. A massive reduction in the lightning stroke count during July and August is attributed to low ground temperature that gives rise to lower updraft velocities and shallower cloud depth. There is good parallelism in the variation of surface air temperature and Point Discharge Current (PDC). The correlation coefficient between these two parameters is 0.934, and it is highly significant.
Multiscale structures near the line-shaped precipitation systems observed around Osaka Bay on July 2 and 5 2006, were analyzed using observational data and a numerical model. In both cases, a cold front extending from a meso-α-scale cyclone in the Sea of Japan moved eastward over central Japan, and just before its passage a meso-β-scale low (named “Tokushima small low”) was formed over the eastern part of Shikoku Island in the warm sector of the meso-α-scale cyclone. On the eastern side of Tokushima small low, the southwesterly below 900hPa level was intensified (∼15 m s-1) in the warm sector, and it converged with westerly on the western (cold) side of the cold front. Clockwise rotating vertical shear was produced between this southwesterly and the Baiu jet (20-30 m s-1) around 700-hPa level. The stability over Osaka Bay decreased in warm-moist air transported by the southwesterly (equivalent potential temperature > 345 K at 950-hPa level and < 335 K at 600-hPa level). In addition, meso-γ-scale lee waves were generated by the westerly on the western side of the cold front flowing over the mountains (Awaji Island and Rokko Mountains) surrounding Osaka Bay, and they triggered the development of the line-shaped precipitation system around Osaka Bay. A Tokushima small low was generated in four cases from among 15 cases of meso-α-scale cold fronts that passed in July during 2003 to 2007. An intense precipitation system related to Tokushima small low was observed only in the two cases presented in this paper.
It is well known that rainfall rate is enhanced over mountains due to orographic uplifting. It would be beneficial to rainfall nowcasting if the intensity of the orographic enhancement could be estimated using simple parameters. In the present study, we found a clear relationship between orographic enhancement of the rainfall rate and the movement speed of radar echoes in a case study of rainfall over mountains in the southwestern area of the Kanto District in Japan during Typhoon 0709, by using rainfall data derived from X-band polarimetric radar. The increasing rate of rainfall rate per unit altitude (ΔR/ΔH) showed a positive correlation (r = 0:95) with the movement speed of radar echoes (V) when V > 10 m s-1. Such a correlation suggests that V is an effective parameter for quickly estimating the orographic enhancement of rainfall, however, more case studies are required before it can be used in practical applications.