Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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MESAN Mesoscale Analysis of Total Cloud Cover
Günther Haase and Tomas Landelius
Swedish Meteorological and Hydrological Institute (SMHI), S – 60176 Norrköping, Sweden, e-mail: gunther.haase@smhi.se
1. Introduction
The real-time Mesoscale Analysis System (MESAN) has
been designed to provide objective and spatially continuous
fields of parameters relevant in meteorology and hydrology:
precipitation, 2 m temperature and humidity, wind at 10 m
level, visibility, and clouds (Häggmark et al. 2000). This is
conducted hourly at a spatial resolution of 0.2° covering the
BALTEX (Baltic Sea Experiment) area (Fig. 1).
Figure 1. MESAN domain.
The basis of the analysis system is the optimal interpolation
technique. Input data to MESAN are surface and METAR
observations, weather radar and satellite imageries,
numerical weather prediction (NWP) model fields,
physiographic data, and climate-related information. Users
of MESAN information are e.g. forecast meteorologists, a
hydrological model, and commercial interests. This paper
aims to present the most recent developments of total cloud
cover analysis within MESAN.
2. Total Cloud Cover Analysis
Cloud cover information is received from different sources:
manned and automatic stations, METAR observations,
geostationary (METEOSAT) and polar (NOAA) satellites.
As first guess, HIRLAM (High Resolution Limited Area
Model; Undén et al. 2002) forecasts with the same spatial
resolution as MESAN are used.
a) current status
Beside direct measurements, total cloud cover is estimated
from the IR-channel on METEOSAT and NOAA,
respectively. When the difference between MESAN’s 2 m
temperature and the brightness temperature (effective
radiation temperature of a black body) measured by the
satellite is greater than two standard deviations from the
mean difference between the two temperatures during cloud
free situations, it is assumed that clouds are present
(Häggmark et al. 2000). Inversions are treated separately.
Additionally, total cloud cover estimates are derived from
the operational cloud classification scheme (CCS)
SCANDIA (Karlsson 1996, 2003). This is a multi-spectral
scheme based on NOAA AVHRR (Advanced Very High
Resolution Radiometer) data which utilizes the horizontal
structure of clouds to distinguish between different cloud
types. Since SCANDIA has problems for low sun
elevations it is not used when the sun angle is between 2
and 6 degrees and in cases with very skew observation
angles.
b) recent developments
In June 2003 a new project called MESAN-Y was initiated
at SMHI in order to improve the precipitation and cloud
analysis. This paper will only address the latter.
A novel CCS has been developed within EUMETSAT’s
SAFNWC (Satellite Application Facility to support
Nowcasting and Very Short Range Forecasting) project
(Dybbroe and Thoss 2003; Dybbroe et al. 2003a, 2003b).
The scheme, which became recently operational, is based
on AVHRR data as received from the current NOAA (15
and onwards) and future satellites. The main output of the
cloud type product is summarized in Tab. 1.
Table 1. Cloud types as defined within the
EUMETSAT SAFNWC project.
0 Non-processed
1 Cloud free land
2 Cloud free sea
3 Land contaminated by snow
4 Sea contaminated by snow/ice
5 Very low stratiform clouds (include fog)
6 Very low cumuliform clouds
7 Low stratiform clouds
8 Low cumuliform clouds
9 Medium level stratiform clouds
10 Medium level cumuliform clouds
11 High stratiform clouds
12 High cumuliform clouds
13 Very high stratiform clouds
14 Very high cumuliform clouds
15 High semitransparent very thin cirrus
16 High semitransparent thin cirrus
17 High semitransparent thick cirrus
18 High semitransparent cirrus above low or
medium level clouds
19 Fractional clouds (sub-pixel water clouds)
20 Undefined
Concerning total cloud cover estimates for MESAN, there
are three major benefits for replacing the SCANDIA CCS
by the new SAFNWC products: i) the spatial resolution of
the new CCS (1 km) is much higher, ii) the new CCS
includes so called quality flags (Tab. 2), which indicate for
each pixel its reliability and iii) the SAFNWC products
will be available for future satellite generations whereas