Profiling radar observations of coastal orographic feeder cloud structure during CALJET, PACJET and HMT-West

The most commonly documented conceptual model of orographic precipitation enhancement is associated with the seeder-feeder process (Bergeron 1965).  In this process, water drops in the orographically forced feeder cloud are accreted or collected by larger ice or liquid hydrometeors, respectively, emanating from seeder clouds aloft.  The seeder cloud may not be entirely distinct from the feeder cloud and often originates from the same extratropical frontal system that forces the orographic feeder cloud.  Orographic precipitation enhancement has also been attributed to a process that includes a feeder cloud but lacks a significant seeder cloud as manifested by the lack of a radar brightband (White et al. 2003).  In both cases, the orographic feeder cloud plays a critical role in the precipitation development process.

The signature of an orographic feeder cloud is increasing total water content (ice plus liquid) with decreasing altitude, which signals the growth of precipitation-sized hydrometeors as they fall in the lowest few kilometers above the terrain.  Radar has often been used to document orographic feeder cloud structure.  White et al. (2003) employed profiling Doppler radar data to examine the structure of rain-producing storms impacting the coastal mountains of northern California during a complete winter season.  They focused on contrasting the vertical structures of profiles with and without a radar bright band.  The White et al. composite nonbrightband (NBB) profile was characterized by weaker radar reflectivity compared to their composite brightband (BB) profile.  Radar reflectivity of the composite NBB profile increased with decreasing height in the lowest 2 km above the terrain, a structure that is consistent with the low-level growth of raindrops and the signature of an orographic feeder cloud.  In contrast, radar reflectivity was approximately constant below the brightband for the composite BB profile.  However, this composite profile was composed of two distinct populations.  Approximately 70% of the individual BB profiles were characterized by increasing radar reflectivity below the brightband with slope less than -0.1 dBZ_e km^-1.  These profiles  indicate the presence of both a seeder cloud and an orographic feeder cloud.  The remaining ~30% of BB profiles show evidence of decreasing radar reflectivity below the brightband, suggestive of evaporation and the presence of a seeder cloud in the absence of an orographic feeder cloud.

The major limitation of the White et al. (2003) study regarding orographic feeder cloud structure was that the degree of enhancement, as inferred by the low-level slope of radar reflectivity, was not comprehensively documented.  Quantitative information about low-level reflectivity slope was not provided for NBB profiles and only an arbitrary and relatively small slope threshold (-0.1 dBZ_e km^-1) was employed to characterize BB profiles.  As a result, the variability of low-level precipitation growth in orographic feeder clouds was not characterized, which prompts several questions.  For example, what is the statistical variation of precipitation growth in orographic feeder clouds as inferred by the low-level slopes of radar reflectivity?  How do these statistics differ, if at all, for orographic feeder clouds associated with and without a radar brightband?  What is the relationship between these statistics and rainwater content and upslope wind speed? 

This study addresses these issues with profiling radar data collected in the coastal mountains of northern California over 15 winter seasons during the CALJET, PACJET and HMT-West field campaigns operated by NOAA.  More than 2600 30-minute average profiles of reflectivity from an S-band precipitation profiler at Cazadero (475 m MSL) were examined.  Profiles of rainwater content were calculated from the reflectivity profiles using reflectivity-rainwater content relations derived from collocated raindrop disdrometer data.  New findings to date from this effort include:

·       The mode of the reflectivity (rainwater content) slope distribution is -3.3 dBZ_e km^-1 (-59.3 mg m^-3 km^-1) in the lowest 1 km above the surface.

·       Reflectivity and rainwater content slope is steeper (i.e., more negative) for NBB profiles compared to BB profiles.

·       Reflectivity slope is steeper for lower values of first-good-gate reflectivity whereas rainwater content slope is steeper for higher values of first-good-gate rainwater content.

·       Reflectivity slope is steeper for lower values of upslope wind speed whereas rainwater content slope is steeper for higher values of upslope wind speed.

Bergeron, T., 1965: On the low-level redistribution of atmospheric water caused by orography.  Suppl. Proc. Int. Conf. Cloud Phys., Tokyo, pp 96-100.

White, A. B., P. J. Neiman, F. M. Ralph, D. E. Kingsmill, and P. O. G. Persson, 2003: Coastal orographic rainfall processes observed by radar during the California Land-Falling Jets Experiment.  J. Hydrometeor., 4, 264-282.