70% of the Earth’s surface is covered by oceans. It is the only inner planet where all three phases of water (liquid, ice, and vapor) coexist. The movement of water in its different forms, and the perpetual water phase changes are essential ingredients of the planetary water cycle (also known as the hydrological cycle). Precipitation is a major component of the water cycle, and is responsible for most of the fresh water on the planet. It occurs when water vapor levels in the atmosphere reach saturation upon which water vapor condenses or deposits on small particles called condensation/ice nuclei to form clouds consisting of suspended liquid or ice particles or a mixture of both. Under appropriate conditions larger liquid and ice particles form that fall to the surface as precipitation due to gravity. Precipitation is associated with a vast range of weather events: tropical cyclones, thunderstorms, frontal systems, drizzle, snowfall, etc.
The driving force of precipitation and the water cycle in general is the solar energy from the Sun. Earth maintains a delicate balance of radiative energy by reflecting approximately one third of the incoming solar radiation, and emitting the remaining two-thirds that are absorbed as infrared radiation back to space. At the Earth’s surface and within the atmosphere, the energy balance is more complex than for the planet as a whole. In fact, neither the surface nor atmosphere can achieve radiative energy balance by themselves without the critical involvement of water. The surface absorbs more solar radiation than is lost by net emission of infrared radiation, with the excess energy transferred to the atmosphere mostly in the form of latent heat – the energy required to evaporate surface water and then released to the atmosphere when cloud formation and precipitation occurs. The water in the atmosphere, whether in vapor, liquid or ice form, further affects atmospheric radiation and heating or cooling. Thus, the cycling of water between its different phases, and its transport across the globe (i.e., between the surface and atmosphere, the ocean and land, and from the tropics to the poles and back), are all intricately connected and responsible for the water cycle of the Earth.
The physical processes governing the water and energy cycles are extremely complicated, involving scales ranging from the planetary to the microscopic. Any alterations in atmospheric gaseous composition (water vapor, carbon dioxide, ozone, etc.), particulates (desert dust, smoke, urban smog, etc), or clouds (coverage and brightness) can disturb the radiative heat balance and result in chain reactions in the hydrological cycle. It is very important for the climate community to not only closely monitor the regional and global water budget, but to also understand changes in frequency of occurrence and strength of individual weather events. This is especially true of extreme weather events, which have great societal and economic impacts. Whether we will have more or more intense tropical storms, mega-snow events, or dust-bowls in the near or far future climate is one of the key focus areas of climate research.
Scientists in the Climate and Radiation Lab make synergistic use of satellite and ground based observations of precipitation and clouds to understand the characteristics and interactions of various components of the water cycle and to detect possible trends and variability that may be linked to climate forcing. Recent efforts along these lines include studies of tropical rainfall variability from TRMM, weekly cycle of precipitation and storm activity due to modulation by pollution aerosols, and of recent trends in North Pacific and Atlantic precipitation from tropical cyclones. Numerical simulations from high-resolution cloud resolving models, medium range weather research forecast models and fully-coupled land-ocean-atmosphere climate models are used in conjunction with observations to understand physical processes that modulate weather, climate and extreme events and their future projections. For example, CRL investigators have recently analyzed the precipitation projections of coupled global models used for the next IPCC report under increased carbon dioxide emission scenarios and found circulation and moisture variability changes large enough to induce more frequent drought and flood episodes in certain regions of the planet. Lab scientists are also involved in satellite-based remote sensing of precipitation which is expected to make a big leap forward with the Global Precipitation Measurement (GPM) mission, an international constellation of satellites that provide the next-generation global observations of rain and snow. The centerpiece of the mission, the GPM Core Observatory expected to launch in 2014 carries two advanced space-borne sensors, a microwave imager and a precipitation radar which are capable of providing more complete insight into the nature of precipitation processes.