Estimates of Cloud Top Pressure (CTP), Cloud Top Temperature (CTT), and Effective Cloud Amount (ECA) from the GOES sounder are used to infer temperature near the level of nondivergence in clouds where saturated ascent exists through a deep layer.  The purpose is to estimate locations of high precipitation efficiency where dendritic ice crystal growth is taking place. Studies have identified a relatively narrow temperature range (centered near -15 C) at which dendritic ice crystal growth by deposition and efficient snow production occurs (e.g., Auer and White, 1982).  An operational technique has been proposed which evaluates areas where strong forcing for ascent coincides with regions of sufficient moisture and temperatures favorable for maximum depositional growth (Wetzel and Martin, 2002).  The analysis technique outlined here, together with analyses from numerical models may be of use in assessment of such regions.

The technique used here is as follows:

Sounder products are obtained hourly from the Cooperative Instutute of Meteorological Satellite Studies (CIMSS), University of Wisconsin-Madison.  They are McIDAS image areas from the GOES-12 and GOES-10 sounders representing cloud information.  The ECA is used to screen out thin or broken cloud layers. Currently, a minimum threshold of 98% is used to process cloud information.  Next the CTP and CTT are used to compute the equivalent potential temperature (EPT) at cloudy points (assuming saturation).  For points where CTP is less than 600 mb, the temperature at the level of nondivergence is computed from the EPT (again assuming saturation).  Currently, the level of nondivergence is assumed to be near 600 mb, typical for significant snow events (Auer and White, 1982).  An image is made which highlights the in-cloud temperature near -15 C at 600 mb (see T(600 mb) in Table 1). 

In addition, another image is created which shows the pressure at which in-cloud temperatures are near -15 C (see P(-15 C) in Table 1).  This is computed from cloud top pressure and temperature (for clouds with top temperature less than -15 C).  From this image, variable levels can be identified where dendritic growth may be important (not just 600 mb).  A refinement to be added in the future is to utilize a mesoscale forecast model (such as the RUC) to determine the upward motion at each of these pressure levels.  With the help of a model analysis, it might also be possible to identify isolated cloud layers which are above a continous layer of moist ascent.  Such layers can give a erroneous result (too warm) and can obscure the relevant cloud tops below.  It is important to identify such situations.  

 

CTT (IR window: 11 microns, deg C)

CTP (mb)

ECA (per cent)

T(600mb) deg C

P(-15C) mb

Radar Reflectivity

Auer, A.H., J.M. White, 1982: The combined role of kinematics, thermodynamics and cloud physics associated with heavy snow episodes. J. Meteor. Soc. Japan, 60, 500-507.

Wetzel, S.W., J.E. Martin, 2001: An operational ingredients-based methodology for forecasting midlatitude winter season precipiation. Wea. Forecasting, 16, 156-167.