The Need for Interdisciplinary Research in Ocean Science

of the Fujiwara Medal of the Meteorological Society of Japan, is an expert on lightning, clouds, and rain. A professor at Obirin University, Japan, Takahashi visited the IPRC during May and June 2003 to collaborate with ShangPing Xie, co-leader of the IPRC Indo-Pacific Ocean Climate Team, on studies of tropical rain and its structure using in situ and satellite observations as well as modeling experiments. Takahashi is intrigued by recent results from the Lightning Imaging Sensor of the Tropical Rainfall Measuring Mission (TRMM) satellite: On the annual average, there are 100 times more lightning flashes over land and along coastal regions than over the open ocean, while precipitation shows little difference (Figure 7). Why is this? Having researched lightning clouds for nearly 40 years, Takahashi is well-suited to address this question. “I was pushed into studying lightning by my professor,” he recalls. “In the 1960s, when I did my dissertation at Hokkaido University, there were nearly as many theories about the charge-separation process in thunderstorms as there were researchers.” Brook’s group at the New Mexico Institute of Mining and Technology was then at the forefront of this research. They proposed that a large electrical charge is created during collision between graupel and ice crystals in the cloud, a process called “riming electrification.” In the laboratory, they noted that ice crystals had a positive and graupel a negative charge—ice crystals are the light and pristine particles in clouds and graupel the heavier ice particles formed when ice crystals capture supercooled drops. Takahashi decided to study riming electrification in natural clouds on top of Mt. Teine in Japan. “I found just the opposite: Ice crystals have a negative and graupel a positive charge. My professor couldn’t believe the results and came up on skis to check the findings of his student!” Why is there this difference in electric charges? Takahashi explored the mystery in the laboratory by systematically changing temperature and cloud water. He discovered that the charges of the particles depended upon temperature: Below -10oC ice crystals are positively charged and graupel negatively; at or above this temperature, the reverse tends to be the case. In other words, the two particles reverse chargesigns with temperature. The next question then for Takahashi was whether this charge reversal actually happens in natural clouds. He developed a unique videosonde system to study the life of particles and their electric charges in thunderstorm clouds. This sonde sucks in precipitation particles, an induction ring then measures their electric charge, and a video camera captures their image. With this tool, Takahashi could now look at the size and shape of the particles, and found that at –10oC the graupel charge changes sign. The sonde also succeeded in measuring the basic tripole electric structure of lightning storms. The top, coldest layer, has a relatively high concentration of ice crystals and is positively charged; the middle, warmer layer, has both negatively charged graupel and ice crystals; the bottom, warmest layer, consists mainly of positively charged graupel together with frozen drops and raindrops. Back in the laboratory, Takahashi studied the physical reason for the sign change at –10oC. As graupel warms above –10oC, it forms a liquid coating. This allows ice crystals to “steal” negatively charged hydroxyl radicals (OH) from the surface of the graupel, making the ice crystals negative and the graupel positive. At temperatures lower than –10o, the graupel surface becomes solid. When ice crystals bump into the solid graupel, branches break off the ice crystal and free hydrogen ions are formed that move from the warmer graupel side to the colder ice crystal side, giving the ice crystals now a positive charge and leaving the graupel negatively charged. These different riming processes also explain why around –10oC there is an unusually large negative charge in the tripole charge distribution: Around this temperature the negatively charged ice crystals, which have stolen the negative charge from the graupel in the lower level and have been drafted upward, meet the negatively charged graupel falling from above. To explore and confirm his findings, Takahashi developed a numerical thunderstorm model, the first of its kind. With this model he was able to simulate lightning and the importance of the tripole charge structure. The model also showed that a certain threshold concentration of ice crystals and graupel was necessary for lightning.