Mobile Mesonets – A Comprehensive Overview

Instrument specifications for each mobile mesonet. (N/A: not applicable)

SENSOROBSERVATIONRANGE, ACCURACYRESPONSE TIME
R. M. YOUNG 05103VWIND SPEED0 – 100 m s–1, ±0.3 m s–12.7 m [distance constant]
R. M. YOUNG 05103VWIND DIRECTION0 – 360°, ±3.0°1.3 m [distance constant]
KVH INDUSTRIES C100MAGNETIC HEADING0 – 360°, ±0.5°N/A
VAISALA HMP35CSLOW AIR TEMPERATURE–35 – +55°C, ±0.9°C63% in +15 s
VAISALA HMP35CRELATIVE HUMIDITY0 – 100%, ±3.0%63% in 15 s
APOGEE INST. ST-110FAST AIR TEMPERATURE–50 – +70°C, ±0.15°C63% in 7 s
R. M. YOUNG 61402VAGL PRESSURE500 – 1100 hPa, ±0.3 hPaN/A
GARMIN GPS16-HVSLATITUTDE / LONGITUDEN/A, > 3 m WAAS N/A
GARMIN GPS16-HVSALTITUDEN/A, ±120 mN/A
GARMIN GPS16-HVSVEHICLE SPEED0 – 999 kts, 0.1 ktsN/A
GARMIN GPS16-HVSVEHICLE COURSE0 – 360°, N/AN/A

Wind Speed / Direction

The anemometer we are employing for each rack is an R. M. Young 05103V Wind Monitor. This propeller-vane style instrument measures wind speed and direction simultaneously. Propeller rotation induces an AC sine wave signal with frequency proportional to wind speed. Internal circuitry converts the raw signal to linear voltage output. Vane position is sensed by a 10K ohm precision conductive plastic potentiometer. This signal is also converted to voltage output (WIND MONITOR WITH VOLTAGE OUTPUTS MODEL 05103V, R. M. Young Company, https://centralchasers.org/wp-content/uploads/2022/10/R.M.-YOUNG-MODEL-05103V-WIND-MONITOR.pdf).

The Wind Monitor is located approximately 1.8 meters from the top of the vehicle and 0.75 meters in front of where the windshield meets the roof. The data logger reads the sensor via single-ended voltage input. The 0 – 5V output of the wind monitor is linearly proportional to both wind speed and direction.

A Garmin GPS16-HVS GPS receiver is used to measure vehicle speed and heading. This sensor outputs serial RS-232 strings in NMEA 0183 format. $GPRMC and $GPGGA strings are read and parsed by the data data logger. The GPS receiver also provides a 100MS PPS signal of which the data logger utilizes for program execution. The GGA string provides time, longitude, latitude, and elevation data. The RMC string provides date, speed, and heading data.

Because the observation platform is in mobile, wind data is only relative to the vehicle on which it is mounted. To correct this, simple vector math is used in real time to derive estimated ambient wind. We can accomplish this through the method described below:

Variables:

AWS: Apparent Wind Speed (relative to vehicle, 05103)

AWD: Apparent Wind Direction (relative to vehicle, 05103)

SOG: Speed Over Ground (vehicle speed, GPS16-HVS)

COG: Course Over Ground (vehicle direction, GPS16-HVS)

TWS: True Wind Speed (estimated ambient, derived)

TWD: True Wind Direction (estimated ambient, derived)

Creating X Y components of each vector:

XV = (SOG)*cos(COG) (Subscript V denoting “vehicle”.)

XW = (AWS)*cos(AWD) (Subscript W denoting “wind”.)

YV = (SOG)*sin(COG)

YW = (AWS)*sin(AWD)

Subtracting Vehicle vector from Wind vector:

X = XV – XW

Y = YV – YW

Now with each vector component computed, we can calculate estimated ambient wind.

Calculating “True” Wind Speed:

TWS = SQRT((X*X)+(Y*Y))

Calculating “True” Wind Direction:

Within the data logger program, IF ELSE statements are utilized to correctly compute wind direction. When both vector components X Y meet the condition of a given statement, the corresponding quadrant formula is used to calculate “True” Wind Direction.

TWD = θ


QUADRANT I

X > 0

Y > 0

θ = tan-1(Y/X)

QUADRANT II

X < 0

Y > 0

θ = 180 + tan-1(Y/X)

QUADRANT III

X < 0

Y < 0

θ = 180 + tan-1(Y/X)

QUADRANT IV

X > 0

Y < 0

θ = 360 + tan-1(Y/X)


Moving vs. Stationary:

For while the vehicle is in motion, GPS heading is used to derive ambient wind direction. While stationary, a KVH Industries C100 fluxgate compass is utilized for heading correction. The compass outputs serial RS-232 strings easily read by the data logger. The use of two forms of directional data are required as GPS heading becomes erroneous while stationary.

The logger program uses an IF ELSE statement to switch between GPS and fluxgate data depending on whether or not automobile speed is less than 0.5 knots. If speeds are greater than 0.5 knots, GPS heading and vector calculation are used. If speeds are less than 0.5 knots, fluxgate compass direction is used to correct raw 05103 data. GPS heading data is unused.

Temperature / Relative Humidity / Dew Point (Coming soon…)

Sensor

Schematic

Recalculating Relative Humidity

While we can derive dew point directly via the Vaisala HMP35C, the same cannot be said about relative humidity. This is because relative humidity is temperature dependent. Recalculation is required because of a phenomenon known as gradient dampening. The term “gradient dampening” was coined by Sherman Fredrickson and Sean Waugh, NSSL. The phrase describes lagging temperature response induced by a membrane filter commonly found on most if not all temperature/relative humidity probes.

The filter protects the sensing elements from dust and debris while allowing water vapor to traverse freely. This is not the case with temperature. The membrane filter severely “dampens” the effective response time rendering the measurement useless. To mitigate this problem, one simply recalculates relative humidity entirely with the use of an external temperature sensor. This sensor is most commonly a thermistor as response times for these devices are typically on the order of a few seconds.

Then using the derived dew point measurement from the T/RH probe and fast response temperature measurement of the thermistor, we can recalculate relative humidity. (Information work in progress…)

Radiation Shield Apparatuses

U-tube

RMY 43408

Barometric Pressure (Coming soon…)

Barometric pressure sensor

Sensor schematic

GPS Receiver

Static pressure port

Hypsometric equitation

Data Logging and Acquisition (Coming soon…)

Data logger

Software suite

Wiring diagram

Program

Apologies in advance – Work In Progress…