Meteo-Blog.Net

Some of you may know libRadtran – the library for radiative transfer. It’s a great tool for all those interested in radiative transfer of the atmosphere. The library is open source and freely available on http://www.libradtran.org/.

For those users not so familiar with unix environments, it may be a little bit tricky sometimes to get those kinds of tools compiled and running. Especially if we include the dependencies the software requires for some (not all) features.

Therefore I just made some notes during the latest full installation of libradtran on my system (which is a Macbook Air running Mountain Lion [Mac OS X 10.8]).
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Robert Wood (University of Washington, Seattle, Washington) has written a very nice overview article on Stratocumulus clouds referring to their climatology, their dynamics and microphysics. The paper was published in Vol. 140 (p. 2373-2423) of Monthly Weather Review. The paper can be downloaded at here: http://www.atmos.washington.edu/~robwood/papers/reviews/MWR-D-11-00121.1.pdf

In the following i want to sum up some of the most interesting facts Robert Wood lists about Stratocumulus clouds. I reordered the information in a slightly new way. All the references can be found below.

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Using the well-known approximation to calculate the dew point temperature from the actual temperature and the relative humidity, it is very easy to write a small script that can do this calculation for us.

I choosed Python to do this job. Feel free to use this script for your own purposes:

 
import sys
import numpy as np
 
# approximation valid for
# 0 degC < T < 60 degC
# 1% < RH < 100%
# 0 degC < Td < 50 degC 
 
# constants
a = 17.271
b = 237.7 # degC
 
# sys.argv[0] is program name
T=float(sys.argv[1])
RH=float(sys.argv[2])
 
 
def dewpoint_approximation(T,RH):
 
    Td = (b * gamma(T,RH)) / (a - gamma(T,RH))
 
    return Td
 
 
def gamma(T,RH):
 
    g = (a * T / (b + T)) + np.log(RH/100.0)
 
    return g
 
 
Td = dewpoint_approximation(T,RH)
print 'T, RH',T,RH
print 'Td=',Td

The script expects two input parameters. First the temperature (in °C) and second the relative humidity (in %).

A quasi-stationary through lies west to Germany with a refilling surface low over Britain. Southwesterly wind in higher levels overlays rather weak surface winds with a convergence zone forecasted reaching from the North sea over central Germany to the Alps.

The best chances for thunderstorms in Germany today refering to WRF00Z is seen in an area between Hamburg and the North Sea as well as in the Southern part of Bavaria and Baden-Wuerttemberg.

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The NOAA Storm Prediction Center (SPC) issued a high risk for the 14th of April two days before the event took place. The overall area with moderate to high risk for severe thunderstorms and tornados already suggested that this day would become very tensing. Not only from a synoptical point of view.
With this post I just want to sum up a little bit of the material that is out there in the net. Meaning links, radar images, satellite pictures and tornado images and videos of course. Feel free to add anything that is missed in the post in the comment section.

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As mentioned in the previous blog post, I used the freely available GFS raw data from NCAR, stored in GRIB2 format, for plotting. NCAR also provides a own plotting language called NCL, which stands for NCAR Command Language. It’s a very useful tool to create both simple and very complex plots of meteorological data. It supports different data formats as e.g. GRIB2 and NetCDF to just mention the most common.
An alternative plotting tool that is commonly used to plot meteorogical data fields is GraDS.

In this post I want to share a simple plotting sample script for plotting a GFS T2m temperature field.
A wide variety of examples can be found at the official NCL page. But unfortunately there are no examples for GFS. So I hope this example can give you a good starting point.

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There are several sources for weather maps in the word wide web. Thanks to the open data policy of the United States the output of GFS (Global Forecast System) from NCAR is freely available for download in the internet. So everyone can use these data to produce own weather maps with the desired variables and in the desired style. Of course, one must have some knowledge of how to read the data and work it up to plots. During the last days i started to write some scripts that make it possible to auto-produce plots and upload these to this web project. At the moment there are only some plots available, but there is going to be a lot more soon. I’d appreciate any comments and suggestions on how to improve this service.
Notice that the GFS data is direct model output. I can not guarantee that the data is correct or that the scripts are free of errors.

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A severe winter storm will hit Germany tomorrow (2011/12/16). The last 12Z model runs differ mainly in the calculated core pressure – so there remains some uncertainty concerning the strength of the storm.
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After a period of a rather boring weather constellation over mid-europe in the past few weeks the cyclones strike back. At 2011/12/04 a wave cyclone passes Germany and will bring a lot of wind especially in the southern parts of Germany.

Let’s have a look at the synoptic setup:

A longwave through lies over the atlantic and europe. The result is a mean westerly flow with fastly travelling fronts and precipation. This will mitigate the aridity of the last few weeks when nearly no precipitation was measured.
A steering low over the Norway with a core pressure of 955 hPa influences two wave cyclones embedded in the mean westerly flow. These two waves will pass Germany throughout the day. The most noticeable effects will be the enduring rain/snow and the storm gusts.

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Introduction

Over the last weeks a very stable high pressure dominated weather situation was oberserved over Germany and Eastern Europe. While the same situation in summer would have resulted in a very warm period, the same isn’t true at autumn. One can observe relative high temperatures up to 15°C in 850hPa, but the temperature at the surface was often much cooler – known as a temperature inversion. Especially when it was foggy the whole day.
This is not an uncommon situation at autumn, but the very stable weather situation over this long time-period is definitely not oberserved all day at this region.
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