| The GPM Microwave Imager (GMI) will now include four high frequency channels that will be used to measure precipitation in the form of falling snow and light rain. GMI, which will fly on the GPM Core Spacecraft, is being built by Ball Aerospace and Technologies Corporation. Due to great interest shown by the science community in the observations these high frequency channels will enable, NASA Headquarters elected to include the channels on GMI through a contract modification. The new high frequency channels—known as channels 10-13—will augment the original nine baseline channels on GMI. Channels 10 and 11 will be centered around 165.5 GHz, with a bandwidth of 3 GHz. Channel 10 will be vertically polarized and channel 11 horizontally polarized. Channels 12 and 13 will be vertically polarized and centered about the water vapor absorption line at 183.31 GHz. Channel 12 will receive energy on wings offset by 3.0 GHz on both sides of the central frequency. Similarly, channel 13 will receive energy on wings separated by 9.0 GHz from the central frequency. The bandwidths of channels 12 and 13 are 7.0 GHz and 9.0 GHz respectively. NASA selected 183.31 GHz as the central frequency for channels 12 and 13 specifically because the atmosphere is opaque at this frequency. Observing at frequencies surrounding an atmospheric gas resonance (in this case, the water vapor resonance) is often termed "sounding," which implies probing to different depths in the atmosphere. Furthermore, the frequencies surrounding 183.31 GHz are inherently sensitive to the ice particles in clouds because ice particle sizes are on the order of the central channel wavelength (about 0.16 cm). In addition, at these high frequencies, certain radiative properties (such as scattering) differ for liquid and solid water particles, making it easier to conclusively detect cloud-borne ice. This ability to resolve ice particles in clouds will allow scientists to obtain more accurate estimates of light rain and falling snow. GMI channels 10-13 will work in conjunction with channels 8 and 9 (which are centered at 89.0 GHz) for the retrieval of snow. For this reason, channels 10-13 have the same spatial resolution as channels 8 and 9 (i.e., all four channels have "instantaneous fields of view" of approximately 4 km by 7 km and identical sampling intervals). Although instruments observing in these high frequencies have flown on spacecraft before, this will be the first time an instrument employing these frequencies has been designed to detect falling snow and light rain from space. Remote measurements of these types of precipitation are typically difficult to make because it is challenging to discriminate between ground features and falling precipitation, especially in places where the Earth’s surface and temperature are not very uniform, as is the case over land. The opaque nature of the 183.31 GHz channels will allow the surface features to be screened, resulting in more accurate precipitation values. To assist in resolving GPM's snow and light rain measurements, scientists will utilize observations made by the Dual-frequency Precipitation Radar (DPR), also carried on the GPM Core Spacecraft. DPR will operate at both 13.6 GHz (Ku-band) and 35.55 GHz (Ka-band). The higher frequency Ka-band radar will provide good sensitivity to snow and light rain, furnishing a critical reference for GMI's high frequency channel observations. The science community is excited about GPM's unique combination of radar and high frequency channel measurements because it will mark the first time this measurement complement is used to quantify light rain and snow. Other spaceborne radiometers carry high frequency channels, but none carry a radar instrument as well. |