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Cloud Droplet Probe / Fogg Monitor Postprocessing functions

Code related to the CDP-2 (Cloud Droplet Probe 2) of DMT (Droplet Measurement Technologies) that (should) also work for the FM (Fog Monitor).

Basic starting point is either direct sampling of CDP/FM when the DMT software is not available, for example when a laptop would need too much power. The data therefore is the initial count of droplets per bin. This can also work with reprocessing of original data when e.g. the pitot tube on the fog monitor broke

This repository is linked to the publication Droplet size distribution, liquid water content and water input of the seasonally variable, nocturnal fog in the Central Namib Desert by Spirig et al. 2021.

This repository also includes functions for rebinning based on the two publications:

Requirements and extra

  • numpy

The functions were used in conjunction with pandas time series of the bins (1 bin -> 1 column) as the numpy functions do broadcasting/expect rather 2D than 1D input as timeseries x binsizes

Function names

MVD is the mean volume diameter ED calculates the effective diameter LWC calculates the liquid water content ConcPerCCM calculates the concentration per cubic centimeter

internal dependencies

Dependencies between functions: MVD depends on LWC (either calculated before or during use) LWC depends on ConcPerCCM (either calculated before or during use) meaning MVD depends also on ConcPerCCM ConcPerCCM depends on nothing ED depends on nothing

Basic usage and examples

As above the starting point are the bincounts (for the CDP per default 30 bins). The first decision is whether rebinning should take place via Gonser2011 or Spiegel2012 as rebinning affects all other steps. Note that both Gonser2011 and Spiegel2012 have a keyword to return the bin sizes they create when rebinning without making a rebinning as this is required for all other functions (LWC, ED, MVD) except ConcPerCCM.

# dependencies
import pandas as pd
import numpy as np

# load functions from all.py
from all import *

# these variables you need to get from somewhere else
# windspeed
ws = 10 # in m s-1, will be converted in ConcPerCCM to cm s-1
freq = 10 # in Hz, 1 would be one sample per second, this is needed as the volume that passes by with the wind depends on how long we measured

# example loading csv file
data = pd.read_csv(FILENAME, index_col=0)

# rebin data according to Gonser2011
data, binsizes = Gonser2011(data)

# combined = False means ConcPerCCM is given for each bin instead of the sum of bins per sampling volume
# also needed for calculating lwd
conc = ConcPerCCM(data, windspeed=ws, samplefrequency=freq, combined=False)

# the factor 10**6 is to convert the # cm-3 to # m-3
lwc = LWC(conc * 10**6, combined=False, binsizes=binsizes) # the keywords windspeed and samplefrequency are ignored when data_is_conc = True (the default)
total_lwc = LWC(conc * 10**6, combined=True, binsizes=binsizes) # the keywords windspeed and samplefrequency are ignored when data_is_conc = True (the default)
# note also that LWC takes binsizes as a keyword in case Gonser2011 or Spiegel2011 have been applied; without these the default bin sizes are taken

# mvd is the diameter where half of the particle size distribution is reached
mvd = MVD(lwc, binsizes=binsizes)

Processing pipeline

The order of calling should be as follows: 0. Load the bincounts from your file somehow, depending on how they are saved

  1. first get the concentration by conc = ConcPerCCM(bincounts) in per cm^3 for a timestep
  2. get the liquid water concentration (per bin or total) with either
    • lwc = LWC(bindata, data_is_conc=False)
    • lwc = LWC(bindata, data_is_conc=False, combined=False)
    • lwc = LWC(ConcPerCCM(bincounts,combined=False))
  3. then get the median volume diameter by one of the following
    • MVD(LWC(ConcPerCCM(bindata, combined=False),combined=False))
    • MVD(LWC(concentration,combined=False))
    • MVD(lwc)
  4. Finally ED(bincounts) can be called at any time since it does not depend on anything

Notes

Note 1

For MVD and LWC the windspeed, samplearea and frequency are optional and default to 1, 0.298, 0.1 Hz as given in ConcPerCCM as these are forwarded to ConcPerCCM in case the concentration needs calculation

Note 2

The windspeed has no bearing on the MVD as the windspeed will be the same for one record and while the lwc calculation depends on it, the formula used for MVD uses relative values and a mulitiplication of all values does not impact this