Two-Dimensional Video disdrometer (2DVD) data processing at NASA Wallops Flight Facility during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACT) 2020

The field campaign conducted between January 15, 2020 (DOY 015) and February 28, 2020 (DOY 059).  At NASA Wallops Flight Facility (WFF), the Global Precipitation Measurement (GPM) ground validation program has a build in structure for in-situ and remote sensing ground-based precipitation instruments.  Among those, five 2DVD is operated at the WFF base.  The coordinates of the 2DVD are given as

37.93450 -75.47081 sn70
37.93715 -75.46622 sn25
37.92938 -75.47314 sn38
37.94432 -75.48116 sn35
37.93764 -75.45618 sn37

The 2DVD is an optical disdrometer and all the units operated during IMPACT were the third generation (Schonhuber et al. 2008, book chapter, Precipitation: Advances in Measurement, Estimation, and Prediction).  It is a research tool and requires regular maintenance.  During IMPACT 2020, all units operated but occasional failures were unavoidable.  The table below marks the days when the 2DVD was operational.  Please note that it is feasible that the 2DVD was not operational for the entire day at some instances.  The collocated or nearby instruments should be used for a comparative study.  The processing presented here assumes that all the hydrometeors are in liquid compostion.

DOY
SN25
SN35
S37
S38
S70
015

x
x

x
016
x
x
x
x
x
017
x
x
x
x

018
x
x
x
x

019
x

x
x

020





021





022





023

x



024
x
x
x
x
x
025
x
x
x
x
x
026

x
x

x
027
x
x



028
x
x
x
x
x
029
x
x
x

x
030



x

031

x



032
x
x
x
x
x
033
x
x
x
x
x
034

x

x

035
x
x
x
x
x
036
x
x
x
x
x
037
x
x

x
x
038

x

x
x
039
x
x

x
x
040

x



041
x
x

x
x
042
x
x

x
x
043

x

x
x
044
x
x

x
x
045

x

x

046

x


x
047

x


x
048
x
x

x
x
049
x
x

x

050
x
x

x
x
051
x
x

x
x
052
x
x

x
x
053



x

054




x
055

x

x
x
056

x

x
x
057
x
x

x
x
058
x
x

x
x
059
x
x

x
x

The time stamp of the 2DVD drop file includes hour, minute, second, drop equivalent diameter in mm, drop volume in mm3, drop fall speed in meters per second, maximum height in B plane in mm, maximum width in A plane in mm, maximum width in B plane in mm, minimum and maximum drop pixel number in A plane, and minimum and maximum drop pixel number in B plane.  This drop file is considered as input file for the data processing.  The data processing has 3 levels:

First level output files (pct = %):

1.1 impacts_2dvd_eachdrop_snXX_2dvd_100pct.txt

where XX is the serial number.

All drops less than 10.0 mm in diameter are included in this file.  The file has 12 columns: year, day of the year, hour, minute, second, drop equivalent diameter in mm, measured drop fall velocity in meters per second, terminal fall speed in meters per second, measured drop axis ratio, and sampling cross section in mm2. Please note that the precise measurement of oblateness and fall speed was not achieved due to severe wind conditions and the change of instrument calibration during the campaign, as compared with the 80 m fall experiment described in Thurai et al. (Journal of Atmospheric and Oceanic Technology, 2007).

1.2 impacts_eachdrop_snXX_2dvd_50pct.txt

The file screens the drops following ±50% terminal fall speed limit.  If the drop fall is outside the ±50% of its terminal fall speed, it is regarded as secondary drop and eliminated from the processing. Drops less than 10.0 mm in diameter are included in this file.  The columns are the same as in 1.1

1.3 impacts_largedrops_snXX_2dvd_50pct.txt

This file screens the drops following ±50% terminal fall speed limit.  If the drop fall is outside the ±50% of its terminal fall speed, it is regarded as a secondary drop and eliminated from the processing. Drops greater equal to 5.0 mm in diameter are included in this file.  The columns are the same as in 1.1

1.4 impacts_2dvd_diameter020.txt

This file presents the midsize diameter in mm, size bin width (0.2 mm), and terminal fall velocity in m sec-1 following Beard (1976, Journal of Atmospheric Sciences) for 50 size bins from 0 to 10.0 mm.  Please note that the terminal fall speeds are interpolated for the drops between 6.0 and 8.0 mm in diameter using measured fall speed increments just under 6.0 mm diameter, and assumed constant for the drops larger than 8.0 mm in diameter.
	
Second level output files:

2.1 impacts_2dvd_snXX_dropcounts_50pct.txt

This file uses 1.2 as input and outputs 50 size bin drop counts for 0.2 mm size width from 0 to 10.0 mm in diameter.  The output file is generated for each minute where at least 10 drops were observed and a minimum rain rate of 0.01 mm h-1 are considered an event.  The year, day of the year, hour, and minute are the first 4 columns prior to the 50 column number of drops.

2.2 impacts_2dvd_snXX_raindsd_50pct.txt

This file uses 1.2 as input and outputs 50 size bin raindrop size distribution in number of drops per cubic meter of air per mm for 0.2 mm size width from 0 to 10.0 mm in diameter.  The output file is generated for each minute where at least 10 drops were observed and a minimum rain rate of 0.01 mm h-1 are considered an event.  The year, day of the year, hour, and minute are the first 4 columns prior to the 50 column size spectra.  The size distribution calculation is based on observed fall speed.  

2.3 impacts_2dvd_snXX_raindsd_ter.txt

This file uses 1.2 as input and outputs 50 size bin raindrop size distribution in number of drops per cubic meter of air per mm for 0.2 mm size width from 0 to 10.0 mm in diameter. The output file is generated for the same number of drops and rain rate criteria as in 2.1.  The columns in the output file are the same as in 2.1. The size distribution calculation is based on terminal fall speed.  

2.4 impacts_2dvd_snXX_rainparameter_50pct.txt

This file uses 1.2 as input and the output is the integral rain parameters based on measured fall velocities at 1-minute integration.  The file consists of 12 columns:  year, day of the year, hour, minute, total number of drops, total concentration (drops m-3 of air), liquid water content (g m-3), rain rate (mm h-1), reflectivity in Rayleigh regime (dBZ), mass-weighted drop diameter (mm), maximum drop diameter (mm), minimum drop diameter (mm), and standard deviation of the mass-weighted drop diameter following Ulbrich and Atlas (1998, Journal of Applied Meteorology).  It should be noted that four of these rain parameters, total concentration, liquid water content, reflectivity in Rayleigh regime, and mass-weighted drop diameter require fall speed information in their formulations.  More information on the disdrometer-based calculation of integral rain parameters can be found in Tokay et al. (2001, Journal of Applied Meteorology).

2.5 impacts_2dvd_snXX_rainparameter_ter_50pct.txt

The file uses 1.2 as input and the output is the integral rain parameters based on terminal fall velocities at 1-minute integration.  The columns in the output file are the same as in 2.4.


Third level output files:

3.1 impacts_2dvd_snXX_raintotalhour.txt

The file uses 2.4 as input and provides the rain event summaries.  The events are separated by 1-hour-or-more rain-free periods in rain rate time series.  Events that are less than 3 minute in duration or with rain totals less than 0.1 mm are not included.  The file has eight columns: year, event start day of the year, event start hour and minute, event end day of the year, event end hour and minute, event rainy minutes, event maximum rain rate (mm h-1), event rain total (mm), and event maximum drop diameter (mm).