1. Rainfall observations at sea
Frank Bradley
CSIRO Land and Water
Canberra, Australia
“At sea” rather than aboard ships because I shall mention moorings, radar and satellite observations
Although the point of my talk is how to do it better aboard ships, I need to put this in the context of other observational systems, on moorings, radar, satellite and sub-surface

2. Riding instructions
1. Identify current applications of research-quality in situ rainfall measurements
2. How would a network of vessels making such observations augment and expand these applications?
I’ll respond to these 2 points initially, then describe how making rain measurements is not as easy as it may seem

3. 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?

4. 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
The global hydrological cycle is a fundamental component of the earth’s climate system

5. Typical distribution of rain gauge data in the CPC daily rain gauge analysis - J.E. Janowiak et al. (2005)
When John Janowiak’s group develops products, this is the surface validation network available to them over the US – much the same for other land masses

6. TRMM coastal and island validation sites
Over the other 2/3 of the globe the network is sparse. So any regular, dependable, source of surface validation over the ocean would be an enormous benefit. But this diagram illustrates well the dissimilarity in space/time sampling that has to be overcome

7. 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
All of which contribute to our understanding of the mechanisms of heat and water transport in the coupled ocean-atmosphere system

8. Net energy and freshwater balance at the air-sea interface

9. Air-sea heat fluxes, including heat transfer by rainfall during 2-days when 150mm of rain fell
19.2 and Wm-2 for sensible and latent heat flux respectively, with 7.5 Wm-2

10. Profile measurements from towed SeaSoar in west Pacific - 4 Dec. 1992

11. TOGA-COARE Freshwater budget Ming Feng et al. (2000)

12. Optical Rain-gauge Siphon Rain-G. Skeptical scientist

13. 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

14. JOSS-WALDVOGEL Disdrometer The classic instrument for measuring rain drop size distributions

15. 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, ship- friendly disdrometers for operational applications so far unsuccessful

16. 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

17. Challenges of Marine Environment

18. Streamlines around ship (R/V Ron Brown). Courtesy Ben Moat
5 Particle total velocity magnitude (m/s) 15

19. Rain-gauges on R/V Brown (Yuter and Parker 2001)

20. R/V Ron Brown at Arica, Chile

21. R/V Ron Brown looking aft from the tower

22. IfM Kiel “ship” rain-gauge (Hasse et al. 1998)

23. Yuter and Parker results: 27 days – total accumulation (mm)
Siphon gauges
Mast 2S 2P 3S 3P 5S 5P Winch
Corrected (Yang et al. 1998)
Optical and experimental gauges, and disdometers
Hasse OD dis1 dis2 3-org 3P JW W-org

24. 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

26. Siphon and optical rain-gauges before and after correction

28. Rain-rates (mm/day) from TRMM Microwave Imager during EPIC2001 (from Wijeskara et al. 2005)
Ship at 10N,265W 40 mm/day = 560mm over 19 days The difference in sampling strategy ai apparent

29. 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)
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 ( 38
SSM/I satellite rainfall: averaged over 1.5° × 1.5° area based on 3 day averaged 0.25° × 0.25° gridded data ( 28
Climatology (GPCP [Huffman et al., 1997]); TRMM TMI and PR data for the month of September ( 10

30. 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