Transcript BradleyRainfallatsea - Center for Ocean

Rainfall observations at sea
Frank Bradley
CSIRO Land and Water
Canberra, Australia
Riding instructions
1. Identify current applications of researchquality in situ rainfall measurements
2. How would a network of vessels making such
observations augment and expand these
applications?
Outline
• Why precipitation measurements are important
• How rainfall is measured aboard ships and
moorings
• Problems encountered which degrade accuracy
• How can these problems be overcome and
accuracy improved?
Why precipitation measurements are
important at sea
• “Understanding the full cycle of evaporation,
cloud formation, and precipitation is the
highest priority for predicting climate change
and is the goal of GEWEX”
• Assemble datasets and develop global and
regional models
• Reliance on satellite observations – TRMM etc
• Require surface validation
Typical distribution of rain gauge data in the CPC daily rain
gauge analysis - J.E. Janowiak et al. (2005)
TRMM coastal and island validation sites
Other applications requiring accurate
measurements of rainfall
• Surface heat fluxes
• Models of ocean mixed layer dynamics
• Ocean heat and freshwater budgets
These studies contribute to knowledge of
the processes of water transport in the
coupled ocean-atmosphere system on
various scales
Net energy and freshwater balance at the air-sea
interface
Air-sea heat fluxes, including heat transfer by rainfall
during 2-days when 150mm of rain fell
400
Latent
Sensible
Rainflux
EPIC2001
23-24 September
Turbulent fluxes W/m2
300
200
100
0
266
266.2
266.4
266.6
266.8
267
267.2
Year day 2001
267.4
267.6
267.8
268
Profile measurements from towed SeaSoar in west
Pacific - 4 Dec. 1992
Salinity psu
SST °C
0
2
2
2
4
4
4
6
6
6
8
Depth m
30
31
32
33
14:31
14:31
17:11
17:11
19:50
8
19:50
34.2
14:31
8
20:28
20:28
10
34
34.4 20
0
33.8
0
29
10
Density sigma-t
20.5
21
17:11
19:50
20:28
10
21.5
TOGA-COARE Freshwater budget
Ming Feng et al. (2000)
Optical Rain-gauge
Siphon Rain-G. Skeptical scientist
Rainfall measuring instruments used aboard
ships and moorings
• Siphon rain-gauge
+ Volumetric – direct calibration
- Distorts wind flow
- Funnel can clog with debris or guano
- Misses catch when siphoning
- Evaporation loss at low rain-rates
- Affected by ship motion
• Optical rain-gauge (ORG)
+ Open path, less wind distortion
+ Sensitive to low rain-rates
- Requires calibration
- Uncertain directional response
JOSS-WALDVOGEL Disdrometer
The classic instrument for measuring rain drop
size distributions
Rainfall measuring instruments used aboard
ships and moorings
• Optical rain-gauge (ORG)
• Siphon rain-gauge
• Disdrometer (acoustic and optical)
- J-W subject to ship vibration
- Systematically underestimates
- Expensive
- Attempts to develop inexpensive, shipfriendly disdrometers for operational
applications so far unsuccessful
Rainfall measuring instruments used
aboard ships and moorings
• Siphon rain-gauge
• Optical rain-gauge (ORG)
• Acoustic disdrometer
Also:
• “Hasse” funnel gauge
• IfM optical disdrometer
• C-band radar, profilers
• “Nystuen” submerged acoustic system
Challenges of Marine Environment
Streamlines around ship (R/V Ron Brown).
Courtesy Ben Moat
5 Particle total velocity magnitude (m/s) 15
Rain-gauges on R/V Brown (Yuter and Parker 2001)
R/V Ron Brown at Arica, Chile
R/V Ron Brown looking aft from the tower
IfM Kiel “ship” rain-gauge (Hasse et al. 1998)
Yuter and Parker results:
27 days – total accumulation (mm)
Siphon gauges
Mast
2S
2P
3S
3P
5S
5P
Winch
288
326
257
281
212
200
212
279
Corrected (Yang et al. 1998)
349
250
Optical and experimental gauges, and disdometers
Hasse
OD
dis1
dis2
3-org 3P
324
429
126
1592 332
212
JW
W-org
199
453
Conclusions of Yuter and Parker (2001)
• No one perfect location
• Use multiple locations – P, S and centre
• Locate where flow distortion is locally
minimized
• Use low location for lower relative wind
• Deploy baseline instrument
• Apply appropriate wind correction (negligible
for U< 3 m/s)
• Windward gauge catches less than leeward
8
Rain-gauge corrections
Ratio Reference/Funnel
7
Koschmeider (1934)
Yang et al. (1998)
Airport data
6
5
4
3
2
1
0
0
5
10
15
Wind speed m/s
20
25
Siphon and optical rain-gauges before and after
correction
70
240
50
40
200
160
120
30
80
20
40
10
0
0
-40
263
264
265
266
Year day 2001
267
268
269
Rainfall mm
60
Relative wind m/s
EPIC2001
ORG#2
Stbd2
Port3
Relative wind
ORG#2corr
Stbd.2corr
Port3corr
Hasse
Port5
500
TOGA-COARE Rainfall
December WWB
Accumulated rainfall (mm)
400
Wave ORG
300
Wecoma sips
Wecoma ORG
radar_imet
200
100
0
350
355
360
365
Decimal day
370
375
380
Rain-rates (mm/day) from TRMM Microwave Imager
during EPIC2001 (from Wijeskara et al. 2005)
Wijsekera et al. 2005
Mm/day
Freshwater budget (averaged over a 146 × 146 km domain)
29
ORG:R/V New Horizon (averaged along the butterfly)
29
R/V Ron Brown (cruise-averaged near the center of the butterfly) [Hare et al.,
2002; Hare et al., submitted manuscript, 2005]
25
C-band Doppler radar [Hare et al., 2002]
Averaged over a circle of radius 10 km
16
Averaged over a circle of radius 100 km
11
TRMM TMI satellite rainfall: averaged over 1.5° × 1.5° area based on 3 day
averaged, 0.25° × 0.25° gridded data (http://www.remss.com)
38
SSM/I satellite rainfall: averaged over 1.5° × 1.5° area based on 3 day averaged
0.25° × 0.25° gridded data (http://www.remss.com)
28
Climatology (GPCP [Huffman et al., 1997]); TRMM TMI and PR data for the
month of September (http://www.trmm.gsfc.nasa.gov)
10
Recommendations for best results
measuring precipitation – EFB and CWF
• Use a single location, if possible elevated to avoid
severe updrafts
• Deploy both a siphon gauge and an ORG
• Have an anemometer at the same location for correction
• Pre-cruise, operate the gauges at a land site, preferably
alongside a tipping bucket instrument
• Continue to collect rain data in dock to inter-compare the
ORG and siphon under more favourable conditions
• The Hasse gauge shows promise, but is not yet an
operational instrument