EPIC Stratocumulus Integrated Dataset

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Examine the Fields
zi_advect, jdate, z_b, z_i, cloudfrac, SHF, LHF, surfwind, surfwinddir, w_s850, w_e, Tadvect, qadvect, LWP, drop_conc, surfswdn, surflwdn, SST, surfpres, O3, Precip, hts, hts_fill, Temp, Temp_fill, u_wind, u_fill, v_wind, v_fill, Qv, Qv_fill, Qt_fill

This page explains and provides access to a set of observational data compiled from 6 days (Oct. 16th-22nd) of the EPIC 2001 stratocumulus field campaign.  During this time, the NOAA vessel Ronald H. Brown (RHB) was stationed at  20° S, 85° W, where it served as a platform for 3-hrly radiosonde launches and as a host for a wide array of other meteorological instruments.  More information about the stratocumulus segment of the EPIC campaign is available from the link on the left. 

The dataset presented in this page was compiled from a combination of measurements made aboard the Brown during this time, concomitant output from European Center for Medium-Range Weather Forecasting (ECMWF) and National Center for Environmental Prediction (NCEP) forecast models, data derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, and idealized tropical profiles from McClatchey et al., 1971 (references available at link on left).  The purpose of this data set is to provide modelers with easy access to the EPIC data in order to encourage its use.  As such, variables with missing data are provided in two formats: "raw" (with missing values left in) and "filled" (with interpolation used to fill in holes).  The "raw" data is best for comparing with model output, while "filled" data is necessary as model input.

The data may be downloaded in netcdf format from the link on the left.  In addition to the data, this file contains a .source field for each variable which explains how the variable was computed from the observations and, where applicable, a .warning field which points out potentially uncertain data. For convenience, this information is repeated below, along with a plot of each variable.

zi_advect (=0.00049m/s)

Advection of cloud-top height, signed such that positive values imply that advection makes the BL shallower.  Diagnosed from MODIS satellite, see Caldwell et al., 2005.  Note that this value is quite uncertain, but its magnitude is so small that exact diagnosis is probably not important.

jdate (48 values)

Times corresponding to each radiosonde release.  Radiosondes were launched every 3 hrs from 2UTC on Oct 16th to 23UTC on Oct 21st.  All other data was averaged to give representative values at each of these times.  Times are presented in Julian date =day of year + hour/24 (where hour is measured in UTC).

z_b (cloud base) and z_i (cloud top)

Cloud base was diagnosed from ceilometer, cloud top from millimeter cloud radar (MMCR).  See Caldwell et al. 2005 for more detail.

cloudfrac (cloud fraction)

Fraction of each 3hrly period in which the ceilometer returned a cloud base.

SHF (surface sensible heat flux)

Derived from temperature and humidity measurements taken on the RHB instrumented tower (10m altitude) using the bulk flux algorithm described in Fairall et al. 1996.

LHF (surface latent heat flux)

Computed similarly to SHF.

surfwind (10m wind speed)

From the RHB 10m instrumented tower.

surfwinddir (10m wind direction)

Given in degrees from North (increasing clockwise).  Computed similarly to surfwind.

[E,N]w_s850 (850mb subsidence rate)

Subsidence rate at the 850mb model level from ECMWF and NCEP analyses.  The ECMWF data is probably better.

w_e (entrainment rate)

Entrainment rate from ECMWF mass budget as derived in Caldwell et al. 2005.  The uncertainty in this measurement at any one time is quite large; w_e should only be used an an averaged sense.

[E,N]Tadvect (Temperature advection)

Low level temperature advection, an output field from ECMWF and NCEP analyses.  Note that the NCEP data was only available at 12hr resolution and is linearly interpolated to 3hrly (so ECMWF is probably better).  Note that this data is signed so positive values COOL the BL.

[E,N]qadvect (specific humidity advection)

Similar to Tadvect, but for specific humidity.  Note that positive values mean that advection is DRYING the BL.

LWP (liquid water path)

LWP was diagnosed from microwave radiometer following the procedure of Hogg et al. 1983.  (Corrected 7/27/05)

drop_conc (droplet concentration)

Nighttime values are extrapolated from daytime values, which are derived following Dong and Mace 2003 using simultaneous measurement of cloud thickness, LWP, and optical depth.

surfswdn (surface shortwave downwelling flux)

From shipboard radiometer.

surflwdn (surface longwave downwelling flux)

From shipboard radiometer.

SST (Sea Surface Temperature)

Surface skin temperature (at 0.5m depth) from RHB.

surfpres (Surface pressure)

Surface pressure calculated by linearly extrapolating 10 and 20mb pressures downward. Due to discord between pressure-measuring devices and uncertainty in the height at which pressures were measured, there could be a mean bias of  up to ~2mb in this field.

O3 (Ozone mixing ratio)

Ozone mixing ratio from McClatchey sounding. Note that this variable is included solely to provide a complete set of model inputs - no ozone measurements were taken during EPIC.

Pres (atmospheric pressure)

This is the height coordinate to which all of the 2D datasets below have been interpolated to.  It has 1mb resolution and a lowest level of 1020mb.  Soundings with surface pressure lower than 1020mb are accommodated by setting below-surface levels to NaN.

hts (height profile)

The data is gridded to 1mb resolution, so height (altitude) varies slightly with time.  The lowest altitude could be off by ~6m, an effect that would make all heights in the profile 6m lower than stated here (although the given heights are probably only accurate to ~10m anyways).

hts_fill (filled height profile)

As in hts, but missing values at altitudes less than 15km are filled in by interpolating neighboring profiles.  If a missing value doesn't have both a left and a right neighbor, it is copied from the neighbor it does have.  Above 15km, pressure levels are taken from the McClatchey tropical sounding (McClatchey et al. 1972).

Temp (temperature profiles)

Temperature profiles from radiosonde ascents.  Note that data in the 300m above cloudtop seems to be corrupted.  See Caldwell et al. 2005 for a discussion of this point.

Temp_fill (filled temperature profiles)

Temp profiles filled using the same process described for hts_fill.

u_wind (zonal wind)

Zonal wind component from the radiosondes.

u_fill (filled zonal wind)

radiosonde-measured u_wind filled by interpolation of neighboring profiles to fill missing values. McClatchey data is NOT used in this case because none exists for winds.  Because of this, data at altitudes above 300mb are based on very few soundings and should be used with caution.

v_wind (meridional wind)

Meridional wind component from the radiosondes.

v_fill (filled meridional wind)

Meridional wind filled in the same manner as u_fill.

Qv (water vapor mixing ratio profiles)

water vapor mixing ratio profiles from radiosonde.  The problem above cloudtop mentioned in the Temp description does NOT apply to this field.

Qv_fill (filled water vapor mixing ratio profiles)

Qv profiles filled using the same process described in hts_fill.

Qt_fill (filled total water mixing ratio)

Qt_fill is Qv_fill plus a liquid water mixing ratio (Ql) profile calculated by assuming Ql increases linearly from 0 at z_b to the value at z_i that makes the integral of density times ql over the BL equal to the LWP.

Precip (precipitation rate)

Synthesized from MMCR, C-band, and sedimentation/evaporation model results.  See Comstock et al. 2005 for details.  Note that this quantity is quite uncertain. In this field, in-cloud precipitation is taken to be the cloud-base value as discussed in Caldwell et al. 2005.


Bretherton, C.S. and 7 coauthors: 2004, The EPIC 2001 stratocumulus study. BAMS, 85, 967-977.

Caldwell, P.M., C.S. Bretherton, and R. Wood: 2005, Mixed-layer budget analysis of the diurnal cycle of entrainment in SE Pacific stratocumulus. Accepted, J. Atmos. Sci.

Comstock, K.R., R. Wood, S.E. Yuter, and C.S. Bretherton: 2004, Reflectivity and rain rate in and below drizzling stratocumulus. Quart. J. Royal Meteor. Soc., 130, 2891-2819.

Fairall, C.W. and 4 coauthors: 1996, Bulk parameterization of air-sea fluxes for tropical ocean global atmosphere coupled ocean atmosphere response experiment. J. Geophys. Res. - Oceans, 101, 3747-3764.

Hogg, D.C. and 4 coauthors: 1983, A steerable dual-channel microwave radiometer for the measurement of water vapor and liquid in the troposphere. J. Clim./Appl. Meteorol., 5, 789-806.