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Archive for the ‘CloudSat’ Category

Hurricane Ida (now a tropical storm) was located in the eastern Gulf of Mexico at 0745 UTC on November 11, 2009.  A descending orbit of the NASA A-Train observed the tropical cyclone.  Here, rain rates estimated from the passive microwave data of AMSR-E are shown (inset), with widespread heavy rains located throughout the northern half of the circulation center.  Further north, estimated precipitation rates decreased with distance from the strongest convection.  Passive microwave brightness temperatures and retrieved rain rates provide additional detail over the traditional infrared appearance, where the structure of the cyclone rain bands is masked by dense cirrus overhead.

In addition to the AMSR-E aboard Aqua, the CloudSat radar passed just to the west of the circulation center.  Radar reflectivity indicates a steady decrease in (detectable) cloud top height moving from 25 to 28N latitude, or decrease in cloud top altitude with distance from the circulation center.  Neither the AMSR-E or infrared data are able to depict the 5 km variability in cloud top height.  The radar also provides value by highlighting the presence of individual convective cores, where reflectivity is likely enhanced by the presence of graupel.  Near the 4-5 km level, the thin band of enhanced reflectivity suggests the presence of the melting level.  This feature does not seem to appear underneath the higher cloud tops at 25 N, however, the disappearance is likely a result of attenuation of the radar signal through the deeper convective cores.  The CloudSat signal is best suited for small particles and ice, and attenuates rapidly in heavy liquid precipitation.  Therefore, returns are limited below the melting level, given the high rain rates suggested by AMSR-E data.

The multiple, yet nearly simultaneous perspectives provided by the A-Train allow for a more complete depiction of precipitation structures, especially for storms that are offshore and out of the range of ground based radars.

Composite of satellite imagery related to Hurricane Ida: rain rates provided by AMSR-E passive microwave retrieval (inset), and CloudSat 94 GHz radar reflectivity cross section (below), with geostationary infrared imagery provided as a background. Infrared image and CloudSat cross section provided by the Naval Research Laboratory.

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Due to a convenient alignment between the A-Train orbital trajectory and the path of Hurricane Bill along the eastern coastline of North America, the CloudSat radar and AMSR-E radiometer was able to take a second look at the structure of the cyclone and passed very near to, or perhaps directly across, the cyclone center.  This overpass occurred around 18 UTC on August 22, 2009 when Hurricane Bill was classified as a Category 1 storm, with tropical storm warnings issued for Nova Scotia and Bermuda by their respective meteorological agencies.

Passive microwave brightness temperatures depict the convective towers associated with the eye wall, as well as the cyclonic shape of the symmetric rain bands located to the north and south of the storm.  The most intense activity appears to be in the southeast quadrant, while northwest of the storm, activity may have been deterred as the cyclone was impacted by wind shear associated with an upper level shortwave departing the northeastern United States.

The AMSR-E 89 GHz passive microwave radiometer depicts the brightness temperatures associated with Hurricane Bill at 18 UTC, 22 August 2009 as it departs the eastern coast of the United States.

The CloudSat radar follows a similar track to the Aqua satellite, the platform supporting the AMSR-E instrument.  Therefore, once again their observations are nearly coincident in time.  It is unclear whether CloudSat directly sampled across the eye, however, with the center located near the 36th parallel, it is likely that CloudSat is depicting the structure of convection to the north and south of the storm.  The relative lack of convection near 36 degrees may be associated with the eye of the storm, with some shallow convection present.  The outermost rain bands, approaching the coastlines of New Jersey, New York and Massachussetts, are present in the cross section near the 40th parallel.   Numerous, isolated convective towers are evident in the CloudSat cross section, which is again attenuated severely just below the melting level due to the large amount of liquid precipitation.

Assuming that CloudSat sampled the eye, it is interesting to note that it depicts a layer of relatively thick, high altitude cloud which may have obscured the location of the center in visible or infrared satellite imagery.  CloudSat indicates that some shallower convection may be occurring near the eye center, while AMSR-E is able to use passive microwave brightness temperatures to see through these nonprecipitating ice clouds for a clearer shot at the cyclone center.

CloudSat observes the precipitation and cloud structures of Hurricane Bill, then a category 1 storm, as it travels along the eastern coast of the United States.

Finally, the CALIPSO instrument provides imagery of high altitude ice clouds associated with the tropical cyclone.  The highest cloud tops are located near the cyclone center at 36 degrees, and maintain a cloud top altitude of 15 km as in the previous sampling.  Meanwhile, the outer band approaching the United States appears weaker with tops ranging from 8 to 12 km.

The CALIPSO satellite indicates significant lidar backscatter obtained from the high altitude ice clouds associated with Hurricane Bill as it passes along the east coast of the United States.

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In a previous post featuring CloudSat observations of Hurricane Bill, the southern side of the storm appears to have colder cloud tops and a more uniform appearance than the northern half of the cyclone.  However, CloudSat radar reflectivity suggested that clouds observed on the southern side of the storm were thick, midlevel ice clouds with scattered, low topped convection underneath.  This demonstrates the value of the two-dimensional (X,Z) radar profiles in characterizing precipitation structures “underneath” more traditional satellite observations.  The northern side of the storm is less uniform in infrared appearance, but CloudSat detected active, deep convection along the majority of the flight track.

The infrared image can be supplemented by 89 GHz, passive microwave brightness temperatures from the AMSR-E instrument aboard Aqua.  These brightness temperatures are obtained simultaneously with infrared brightness temperatures from MODIS.  The passive microwave brightness temperatures are sensitive to the precipitation mass (ice) within the vertical column, and clearly depict the structure of the hurricane eye wall and outer rain bands.

Passive microwave brightness temperatures (89 GHz) from the AMSR-E instrument aboard Aqua, observing Hurricane Bill on August 19, 2009. The flight track of CloudSat and 94 GHz Cloud Profiling Radar reflectivity are provided in a cross section below.

The SPoRT program provides AMSR-E brightness temperatures to partners in the National Weather Service for a variety of applications, including offshore rain rates, convective percentages, and soil moisture.  Images courtesy of the Naval Research Laboratory and the CloudSat Data Processing Center.

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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.

The NASA A-Train of polar-orbiting, Earth observing instruments sampled the eye of Hurricane Bill directly: infrared brightness temperatures (cloud cover) is shown from Aqua MODIS, and precipitation structures are clearly evident thanks to the CloudSat 94 GHz Cloud Profiling Radar, which sampled across the eye wall a few seconds after the MODIS image was obtained.

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.

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Keeping with the pattern of the past few days, thunderstorms and occasional severe weather are occurring across the Tennessee Valley, generated along and in the vicinity of a stubborn stationary front that has wavered back and forth across the area.  Severe thunderstorm and tornado watch boxes were issued across Arkansas, Mississippi and Alabama through the early morning hours and are likely to continue.  This area of active weather was sampled by CloudSat, the first orbiting94 GHz Cloud Profiling Radar, and member of the NASA “Afternoon Train” or “A-Train” of polar-orbiting, Earth observing satellites.  Several SPoRT products are developed from MODIS and AIRS data made available by the Aqua satellite, which flies in line with CloudSat, CALIPSO and other instruments, providing a broader view of cloud characteristics versus traditional two dimensional cloud mapping.

The image below is provided by the Naval Research Laboratory, showing the CloudSat orbit track (red) during a descending pass around 08 UTC.  Although the two image segments break up the block of profiles, CloudSat basically sampled both the trailing stratiform and convective line associated with the morning MCS across eastern Arkansas and western Mississippi.  Since CloudSat samples vertical profiles and has a greater sensitivity to small cloud ice and water droplets, it provides a better estimate of cloud top versus traditional ground based radars.  However, CloudSat attenuates significantly in regions of light to moderate liquid precipitation.  Therefore, in many cases the radar reflectivity decreases at low altitude — although precipitation was widespread at the surface.  The freezing level is estimated by model forecasts (overlaid contours), and reflectivity decreases, then increases again in the vicinity of this layer, which has been cited by researchers as a “dim band” that occurs with this type of radar.  Meanwhile, since the bulk of the attenuation is caused by precipitation, methods are being developed to exploit the 94 GHz attenuation to estimate precipitation for global cloud systems, which will broaden our knowledge of global climate and weather processes.  Attenuation is particularly strong at about 33-34 degrees latitude, where strong echos are detected aloft.  This is likely the “overshooting top” or vigorous updrafts of the convective line, coincident with the highest altitude cloud tops at 15km (~50kft).  Large hydrometeors (supercooled rain, hail, graupel) extinguish much of the signal below 5 km AGL.

CloudSat 94 GHz Cloud Profiling Radar reflectivity (below) for cloud profiles sampled along the orbit track (above).

Traditional NEXRAD Weather Service reflectivity across the continental United States, valid at 0800 UTC. Note the area of organized, active thunderstorms in Arkansas, Tennessee and Mississippi that were sampled by the CloudSat radar.

Although CloudSat attenuates significantly in liquid precipitation, it is ideal for studying snow processes, including those that contribute to the widespread stratiform precipitation that occurs with mesoscale convective systems.  Within SPoRT, CloudSat is being used to investigate forecast model parameterizations used in forecasts for winter storms, using the CloudSat radar to check that simulated cloud reflectivity reasonably approximates that observed from space.  These parameterization improvements may in turn benefit a variety of forecast precipitation processes.

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