Evaluating the Variability of Mass-Dimension Parameters Within Ice Clouds

Mass-dimension (m-D) relationships that determine bulk microphysical properties such as total water content (TWC) and radar reflectivity factor (Z) from particle size distributions are used in both numerical models and remote sensing retrievals. A common way to estimate a-b coefficients used in m=aD^b relationships is to minimize the chi-squared difference between the TWC or Z derived from measured number distribution functions and that directly measured by a bulk water probe or radar. These a and b values, however, can vary significantly based on meteorological conditions, particle habit, definition of particle maximum dimension, probes used to obtain the data, techniques used to process the cloud probe data, and for other unknown reasons.

Microphysical data collected by two-dimensional optical array probes (OAPs) installed on the University of North Dakota Citation aircraft during the Mid-latitude Continental Convective Clouds Experiment (MC3E) and the Olympic Mountain Experiment (OLYMPEX) are used here in conjunction with TWC data from the Nevzorov probe and ground-based radar data at S-band to test a novel approach that determines m-D relationships for a variety of environments and TWC regimes. A surface of equally realizable a and b coefficients in (a,b) phase space is determined using a technique that minimizes the chi-squared difference between TWC or Z derived from the OAPs and that directly measured by a TWC probe or radar, accepting as valid all coefficients within a specified tolerance as equally realizable solutions. The surfaces of solutions for different cases are examined as functions of temperature and TWC to understand how environmental parameters influence both the parameters themselves and their variability. It is shown that using fixed a-b coefficients in select numerical modeling and remote retrieval schemes cannot adequately represent the ensemble-retrieved particle mass-dimension variability of observed cloud conditions.