Lab 7-G109-1
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Indiana University, Bloomington *
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109
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Dec 6, 2023
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Lab 7: Weather Forecasting
Background Material on Model Output Statistics
Background Material on MOS
The standard four-panel progs are generated using data from the GFS and NAM dynamic
models. By "dynamic", we mean that these models solve mathematical equations in order to predict how the atmosphere will evolve with time (these equations include variables such as temperature, moisture and wind). On average, the dynamic models don't predict surface temperatures very accurately (we can say the same thing about surface dew-point temperatures and surface winds). To give you a sense for why the NAM and GFS dynamic models don't predict surface temperatures very well, we point out that physical processes near the earth's surface (such as the cooling and heating of the ground by radiation gains and losses) are just too complicated to mathematically model very accurately. Instead, meteorologists use oversimplified mathematical parameters and schemes to model complicated physical and radiative processes near the ground. Not surprisingly, these "oversimplifications" lead to forecasting errors. Why, then, are forecasts for daytime high and low temperatures generally pretty accurate? Good question. As it turns out, weather forecasters use statistical models to more accurately predict surface air temperatures, dew points and winds. Here's the scoop. Over a relatively long period of time, meteorologists kept statistics of observed surface temperatures (for example) and the corresponding data from the dynamic models that helped to predict surface temperatures. These data, called predictors, included 850-mb temperatures, wind direction, wind
speed and a couple of other variables (these are some of the parameters that govern surface temperatures). At each airport or weather station, meteorologists developed statistical equations that expressed surface temperatures as a function of these predictors. To ground these statistical equations in reality, meteorologists also incorporated climatological values into the statistical schemes. They also developed similar statistical equations for dew points, wind direction, wind speed, probability of precipitation, visibility, cloud coverage, cloud ceilings, etc. In the final analysis, forecasts based on statistical equations turned out to be a more accurate way to predict surface temperatures, surface dew points, surface winds, etc. at a given airport or weather station (compared to the raw output from the dynamic models). Collectively, all the forecasts from the statistical equations are called Model Output Statistics (MOS, for short; pronounced "moss"). For the record, the GFS MOS is run four times per day (at 00Z, 06Z, 12Z and 18Z), while the NAM MOS is run twice daily at 00Z and 12Z. Additionally, we retrieved these MOS data from the interactive
Web site at the University of Wyoming
.
To interpret the NAM MOS and GFS MOS for University Park, please carefully read the Description of the NAM MOS
and the Description of the GFS MOS
. The row labeled "N/X" G109 Weather and Climate Lab Manual
Lab 7-1
lists the mi
N
imum and ma
X
imum temperatures for the nighttime and daytime cycles during the specific forecast period (
refer to this site
). Once you feel comfortable with how to interpret NAM MOS and GFS MOS, you'll be ready to tackle Laboratory Exercise. G109 Weather and Climate Lab Manual
Lab 7-2
Name: Vivaan Vijaybhai Shah
Lab Date & Time:_______________ Exercise 9: Weather Forecasting 100points
Exercise You are given the NAM MOS (ETA MOS) and GFS MOS for St. Louis, MO (KSTL), from the 12Z runs on December 6, 2009
G109 Weather and Climate Lab Manual
Lab 7-3
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(a)
What were the high temperature forecasts from the GFS MOS and NAM MOS for December 7 and December 8? Provide a brief comparison. (5)
(b)
What were the low temperature forecasts from the GFS MOS and NAM MOS for both December 7 and December 8? Provide a brief comparison. (5)
(c)
What were the NAM MOS and GFS MOS predicted temperatures for 21Z on December 8? (5)
(d)
What were the NAM MOS and GFS MOS predicted wind direction and wind speed for 00Z on December 9? Express wind direction in degrees. Proper units on wind speed are a must! Provide a brief comparison. (10)
(e)
Give the time and date when the NAM MOS and GFS MOS predicted the lowest cloud ceilings. At what altitude were the ceilings predicted to be? Proper units are a must. (5)
(f)
Referring to part (e), did the NAM MOS and/or the GFS MOS predict any obstructions to visibility? If so, provide the specific obstruction to visibility. Please explain, incorporating your answer in part (d) where it's appropriate. (10)
a. High temperature forecasts:
•
GFS MOS December 7 = 41°C
•
GFS MOS December 8 = 43°C
•
NAM MOS December 7 = 41°C
•
NAM MOS December 8 = 42°C
b. Low temperature forecasts:
•
GFS MOS December 7 = 32°C
•
GFS MOS December 8 = 28°C
•
NAM MOS December 7 = 29°C
•
NAM MOS December 8 = 30°C
c. Predicted temperatures for 21Z on December 8
•
GFS MOS = 41°C
•
NAM MOS = 41°C
G109 Weather and Climate Lab Manual
Lab 7-4
d. Predicted wind direction and wind speed for 00Z on December 9
•
GFS MOS wind direction and speed = 120° at 16 knots
•
NAM MOS wind direction and speed = 100° at 10 knots
e. Time, date, and predicted altitude of the predicted the lowest cloud ceilings
•
GFS MOS lowest cloud ceilings = December 9 at 00:00 and 06:00 with (1) altitude of <200 feet or < 61 meters
•
NAM MOS lowest cloud ceilings = December 7 at 09:00, December 8 at 21:00, and December 9 at 00:00 with (2) altitude of 200 to 400 feet or 61 to 122 meters
f. Obstructions to visibility
•
GFS MOS = Yes. There's a mist (BR), which is a fog with visibility greater than or equal to 1000 meters. This is incorporated with wind speed of greater than 14 knots and less than 17 knots. The wind speed of December 9 at 00:00 is inside that interval.
•
NAM MOS = Yes. There's a mist (BR), haze, smoke and dust (HZ). The wind speed at the time of the forecasted obstruction of visibility doesn't correlate with the obstruction.
Go to the following link: http://weather.uwyo.edu/cities/usnorth.shtml
and select the weather station that is labeled “BMG Bloomington/Monroe, Indiana.” For consistency,
select “American Units.” G109 Weather and Climate Lab Manual
Lab 7-5
a)
Provide screenshots of your GFS MOS and NAM MOS results.(5)
G109 Weather and Climate Lab Manual
Lab 7-6
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(b) What are the high temperature forecasts from the GFS MOS and NAM MOS for the next two days? Please provide the day and time at which you accessed the models. Provide a brief comparison. (5)
GFS MOS: 31,64 degrees NAM MOS: 34,58 degrees The GFS MOS high temp forecasts are a little higher than the NAM MOS high temp forecasts.
(c) What are the low temperature forecasts from the GFS MOS and NAM MOS for the next two
days? Provide a brief comparison. (10)
GFS MOS: 31,41 degrees NAM MOS: 34,43 degrees The low temp forecasts is lower if GFS MOS than NAM MOS.
(d) What are the NAM MOS and GFS MOS predicted dewpoint temperatures for 00Z on for the next two days? (10)
GFS MOS: 22,34 degrees NAM MOS: 23,36 degrees
(e) What day and time are the dewpoint and surface temperature the closest? What does this mean for the relative humidity of the air? (10)
The time the dew point and surface temperature is usually the closest is when the temperature is at its lowest. In this case, the dew point and surface temperature is closest on Nov 3 at hour 12, where they are five units apart. The relative humidity of the air will therefore be higher.
(f)What were the NAM MOS and GFS MOS predicted wind direction and wind speed for 00Z For the next two days? Express wind direction in both degrees. Proper units on wind speed are a must! Provide a brief comparison. (10)
GFS MOS: Wind Speed = 10 mph and 7 mph, Wind Direction = 20 degrees and 22 degrees NAM MOS: Wind Speed = 8 mph and 7 mph, Wind Direction = 21 degrees and 21 degrees
(g)
Based on this information and what you have learned in class. What would be your forecast for Bloomington for the next two days? (Looking for a qualitative textual answer) (10)
Friday is going to be cooler with winds around 5-15 mph. Saturday going to be even less cool with 5-10 mph winds.
G109 Weather and Climate Lab Manual
Lab 7-7