Rainfall observations at sea

Frank BradleyCSIRO Land and Water

Canberra, Australia

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?

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 rainfallduring 2-days when 150mm of rain fell

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Year day 2001

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

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, ship-friendly disdrometers for operational applications so far unsuccessful

Rainfall measuring instruments used aboard ships and moorings

• Siphon rain-gauge• Optical rain-gauge (ORG)• Acoustic disdrometerAlso:• “Hasse” funnel gauge• IfM optical disdrometer• C-band radar, profilers• “Nystuen” submerged acoustic system

Challenges of Marine Environment

5 Particle total velocity magnitude (m/s) 15

Streamlines around ship (R/V Ron Brown). Courtesy Ben Moat

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 JW W-org

324 429 126 1592 332 212 199 453

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

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Wind speed m/s

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l Koschmeider (1934)Yang et al. (1998)Airport data

Rain-gauge corrections

Siphon and optical rain-gauges before and after correction

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TOGA-COARE RainfallDecember WWB

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Wecoma sips

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Rain-rates (mm/day) from TRMM Microwave Imager during EPIC2001 (from Wijeskara et al. 2005)

Wijsekera et al. 2005

Mm/dayFreshwater 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]

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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)

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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)

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Climatology (GPCP [Huffman et al., 1997]); TRMM TMI and PR data for the month of September (http://www.trmm.gsfc.nasa.gov)

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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