observed and modelled long-term water cloud statistics for the murg valley kerstin ebell, susanne...
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Observed and modelled long-term water cloud statistics for the Murg Valley
Kerstin Ebell, Susanne Crewell, Ulrich Löhnert
Institute for Geophysics and Meteorology, University of Cologne
7th COPS Workshop, 27 – 29 October 2008, Strasbourg
2 7th COPS Workshop, 27 – 29 October 2008, Strasbourg 27th COPS Workshop, 27 – 29 October 2008, Strasbourg
Motivation 30 July 2007, Murg Valley
IR sky temperature + LWP
N W S E N
UTC
14
13
12
11
• LWC is important
- as prognostic parameter in NWP/climate models- in describing cloud radiative effects- for predicting attenuation in radio propagation problems
due to scarsity of observations simple cloud models were developed to diagnose LWC profiles from radiosonde measurements
• Clouds are complex in - time/space- microphysics
• Observations need to captureall characteristics see Poster by Kneifel et al. for spatio-temporal structure
• Liquid water content (LWC)can not be observed by a single instrument Integrated Profiling Technique
3 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Motivation
How are water clouds characterized in the Murg valley during April – December 2007?
„observations“
How are they represented by models?
Comparison to• simple cloud models• (NWP model COSMO-DE)
Cloudnet Target Classification(provided by Ewan O‘Connor)
Integrated ProfilingTechnique
4 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
provides continuous vertical profiles of temperature, humidity and liquid water content (LWC); first attempts to derive droplet number concentration
data set of retrieved profiles available for April – December 2007!
Integrated Profiling Technique (IPT) – Overview
Microwave radiometers Cloud radar A priori information
(radiosondes)
CloudNet Target Categorization: Each pixel is categorized in terms of
the presence of liquid droplets, ice, insects or aerosol.
Discrimination between precipitating and non-precipitating clouds.
Integration of measurements in the framework of optimal estimation:
inversion method including - instrument characteristics (theory, error) - forward model, i.e. radiative transfer, Z- LWC-relationship - background information
Retrieval
5 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Cloud models - overview
Simple cloud models: diagnosis of clouds from radiosonde profiles Modified adiabatic approach (Karstens et al., 1994) Decker model (Decker et al., 1978) modified Salonen model (Mattioli et al., 2006)
General principle:
1) Localization of cloud base and cloud top height
2) Calculation of LWC: depending on height over cloud base (Mod.ad., Salonen) depending on cloud thickness (Decker), Min 0.05 gm-3, Max 0.2 gm-3
Assumption: no water clouds below -20 °C
RH sondeRhcrit modad / DeckerRhcrit Salonen 06
6 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Results – Critical humidity thresholds
Distribution of relative humidity (radiosonde) in cloudypixels (April – December 2007);
cloudy pixels: Cloudnet classification detects cloud droplets
detection of clouds from relative humidity profiles problematic
7 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Results – Cloudnet and cloud models
Number of clouds
Cloud occurence (%)
Mod. Ad. 69.4
Decker 69.4
Salonen 75.3
Cloudnet 55.5
April-December 2007
Cloudnet classification profiles within 30 min. aftera RS ascent are taken into account for the comparison
8 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Results – Cloud base height (lowest cloud)
April-December 2007
9 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Results – Cloud thickness distribution
April-December 2007
most frequent cloud thickness is 100-200 m
only ~17 % of clouds with thicknesses > 500 m
only ~10 % of clouds with thicknesses > 1000 m
10 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Results - LWP
Cloud freq. (%)
Mean LWP (gm-2)
Mod. Ad. 56.3 41.2
Decker 56.3 36.3
Salonen 65.8 14.6
IPT 34.1 15.9
COSMO-DE
21.4 20.8
For COSMO-DE evaluation more time steps and inclusion of fractional clouds needed!
11 7th COPS Workshop, 27 – 29 October 2008, Strasbourg
Results - LWC
12 7th COPS Workshop, 27 – 29 October 2008, Strasbourg 127th COPS Workshop, 27 – 29 October 2008, Strasbourg
Summary
• Derivation of liquid water, temperature and humidity profiles for nearly the whole AMF deployment (21.4.-31.12.2007) using the Integrated Profiling Technique 88110 profiles (cloudy: 33168) corresponding to 2706 hours (5 min sampling)
• Most clouds are single layer, 100-200 m thick with low LWP
• Simple cloud models based on RS profiles - overestimate the occurence of water clouds by ~25-35%- overestimate thin clouds (< 300 m)- overestimate of multilayer clouds and LWP
• COSMO-Model evaluation is limited due to missing information on fractional cloudiness
Outlook• Investigation of cloud radiative effects using IPT/Cloudnet data and single
column version of the radiation code of the COSMO model
• sensitivity studies, e.g. additional retrieval of number concentration