On August 19th, at around 1730 UTC, the 94 GHz Cloud Profiling Radar aboard the NASA CloudSat satellite serendipitously achieved a direct crossing over the eye of Hurricane Bill. As of this posting, Hurricane Bill is a Category 3 storm with a minimum central pressure of 958 mb. The image below is courtesy of the Naval Research Laboratory and the CloudSat Data Processing Center, and depicts the Aqua MODIS infrared brightness temperature (cloud cover) of Hurricane Bill, along with the CloudSat flight track and two dimensional “curtain” of radar reflectivity that CloudSat provides. The Aqua and CloudSat satellites are members of the NASA “Afternoon Train” or A-Train of polar orbiting, Earth observing satellites.
The value of CloudSat is apparent when considering the dramatic detail in structure not apparent in the satellite image. CloudSat is sensitive enough to detect small ice crystals, sized smaller than precipitation ice, and detects the broad region of cirrus and midlevel ice cloud occupying the layer from 10-15km. However, it is also able to penetrate below this ice cloud layer to sense lower altitude clouds with enhanced reflectivity cores, likely obtained from some of the outer rainbands and scattered convection.
Perhaps more impressive is the depiction of the hurricane eye. CloudSat depicts the column of the eye as completely cloud free, but also demonstrates the complex shape. The detection of reflectivity along the eye wall shows the outward tilt, in agreement with conceptual models. The most intense convection detected by CloudSat is immediately adjacent to the eye center, where a convective tower ascends to more than 15 km. Note that this convection is located north of the eye, based on latitudes provided by the CloudSat profiles. The convection is asymmetrically distributed north and south of the cyclone center.
Further south of the eye wall, the CloudSat cross section detects enhanced reflectivity from precipitation ice (likely graupel and snow crystals), suggesting that the northern side of the storm contained more active, deep convection than the southern side at the time of CloudSat observations. CloudSat was designed to detect cloud profiles from space and performs well in ice, however, even relatively small amounts of liquid will cause significant attenuation. Therefore, although reflectivity diminishes near the +10 isotherm, it is obviously precipitating — just that the CloudSat radar signal is attenuated and unable to detect the full precipitation profile. Downward “streaks” of greens and yellows below the +10 isotherm are likely caused by multiple scattering of the radar signal by cloud ice aloft. Even though the radar is expected to be fully attenuated, ice crystals further aloft in the column offset some of the attenuation by propagating some component of the radar signal in the forward (i.e. downward) direction, offsetting the attenuation.
In summary, this event clearly depicts the value of the NASA “A-Train” of polar orbiting satellites. The MODIS instrument aboard Aqua provides a clear depiction of cloud cover, structure and expanse of Hurricane Bill, while the same clouds are sampled only seconds later by the CloudSat radar, so that the properties of Hurricane Bill are sampled in all three dimensions. Another orbiting radar, the Tropical Rainfall Measuring Mission, undoubtedly provides additional details about tropical systems, and our ability to sample precipitation structures will lead to their improved representation within high resolution weather forecast and climate models.