Title: "What We Don't Know About Snow: Overview of the NASA Global Precipitation Mission (GPM) Cold season Precipitation Experiment (GCPEx)"

NASA’s Global Precipitation Measurement (GPM) mission, which will launch in early 2014, is tasked to detect and measure precipitation to 70 degrees latitude, where satellite precipitation estimates are highly uncertain. A major push of the GPM Ground Validation (GV) program is to validate spaceborne precipitation retrievals at mid- and high latitudes to constrain and improve satellite precipitation algorithms. Physically-based precipitation retrieval algorithms require knowledge of both the atmospheric and surface contributions to radiative transfer in the microwave. Focusing on the atmospheric portion of the problem, in order to constrain retrieval algorithms, we need to understand both the microphysical properties of precipitation as well as the scattering properties of the precipitation. At high latitudes, few reliable observations exist to routinely evaluate snowfall properties. Thus our ability to solve the “inverse problem” to retrieve snowfall properties such as mass, habit, and precipitation rate remain poorly constrained.

In January and February 2012, the GPM Cold Season Precipitation Experiment (GCPEx) was conducted near Barrie, Ontario, Canada to constrain mid-latitude winter precipitation scattering and microphysical properties. Radar observations at C, X, Ku, Ka, and W band from ground based scanning and profiling radars, VHF profiling radars and multi-frequency down and uplooking radiometers, and aircraft from GCPEx, can be used to not only constrain observed reflectivites in snow as well as construct dual frequency ratios (DFRs) that can be put in context with other observed properties of snow. Data from aircraft and ground-based in-situ microphysical probes, such as 2-D and bulk aircraft probes and surface disdrometers, can be used to identify the microphysical and scattering properties of the snow throughout the column of hydrometeors.

In this presentation, microphysical property relationships and snow scattering regimes will be identified in GCPEx storm events in a multi-frequency DFR-near Rayleigh radar reflectivity phase space using matched ground-based and aircraft-based radar data. These data will be interpreted using matched in-situ disdrometer and aircraft probe microphysical data from aircraft spirals. This analysis is geared towards evaluating scattering simulations and the choice of integral particle size distributions for snow precipitation retrieval algorithms for ground and spaceborne radars at relevant wavelengths. A comparison of results for different cases with different synoptic forcing will be presented.


Speaker's website: http://www.atmos.illinois.edu/people/nesbitt.html