Technical note on in-filling of missing data for ‘Masterton’ in 2016

In April and May 2016, the Martinborough EWS (agent 21938)¹ experienced some technical difficulties, and a number of 9 am observations were missing:

• In April, 5 days of maximum and minimum temperatures were missing: 03-07 April;
• In May, there were 7 missing 9 am readings: 26-31 May and 01 June for maximum temperatures; and 25-31 May for minimum temperatures. [Note that the 9 am readings of maximum temperature over the preceding 24 hours are assumed to actually apply to the afternoon of the previous day.]

The NIWA database sets a monthly average temperature to missing if there are 5 or more consecutive days missing (or if 10 or more days in total are missing). Thus, the database shows April and May 2016 as missing for Martinborough EWS.

For the purposes of the 7-station analysis, estimates were made to in-fill the missing data.

April 2016

Temperatures for the 5 missing days at Martinborough were estimated by comparison with two other sites: Masterton Aero AWS (agent 36735), and Masterton EWS (agent 40984).

For the missing daily maximum (TMax) and minimum (TMin) temperatures, it was assumed that the average difference between Martinborough and the comparison site for the 5 missing days was the same as the average difference over the 25 non-missing days.

Using Masterton Aero AWS:

1)      For the 25 days in April 2016 not missing at Martinborough, we have

• Martinborough – Masterton Aero AWS = -0.216°C (TMax), & +1.284°C (TMin)

[Thus, Masterton Aero AWS has a larger diurnal range than Martinborough, by 1.5°C]

2)      Average over 2-6 April for Masterton Aero AWS = 19.40°C (TMax) & 9.48°C (TMin)  

3)      Estimate the full-month average by:

• Replacing missing 5 days at Martinborough with a 5-day average of 19.18°C (19.40 minus 0.22) for TMax, and 10.76°C (9.48 plus 1.28) for TMin
• Combining the 5-day average along with the 25-day average already available, to give the desired full-month (30-day) average

Monthly TMax & TMin are 19.83°C & 6.88°C, which round to 19.8°C and 6.9°C.

Using Masterton EWS:

1)      For the 25 days in April 2016 not missing at Martinborough, we have

• Martinborough – Masterton EWS = -0.728°C (TMax), & +0.736°C (TMin)

[Thus, Masterton EWS has a larger diurnal range than Martinborough, again by 1.5°C]

2)      Average over 2-6 April for Masterton EWS = 19.31°C (TMax) & 9.28°C (TMin)

3)      Apply the adjustments and weighted averaging as above, to give the desired result:

Monthly TMax & TMin are 19.82°C & 6.85°C, which also round to 19.8°C and 6.9°C.

Thus, whichever comparison site is used, the result is the same to one decimal place (the precision of NIWA’s monthly temperature tables).

Thus, the April Martinborough temperatures are taken as: 19.8°C (TMax), and 6.9°C (TMin), with a mean temperature therefore of 13.4°C.

May 2016

A similar calculation is made for May 2016, except in this case only Masterton EWS is used because Masterton Aero AWS also has missing daily data (one day, 13^th May).

The May Martinborough temperatures are calculated to be:

• 16.9°C (TMax), and 9.4°C (TMin), with a mean temperature of 13.2°C.

Note that since these temperatures used in the 7-station series are only estimates, the NIWA Climate Database will continue to show monthly temperatures for Martinborough EWS as missing in April and May 2016.

 

¹ Footnote: In NIWA’s 2010 Review of the 7-station temperature series, it was decided to use Martinborough EWS as the Masterton proxy after the closure of the long-running East Taratahi AWS (agent 2612) station. As the Review notes, Martinborough’s mean temperature must be reduced by 0.28°C to make it homogeneous with the East Taratahi record. For maximum temperature the Martinborough reading must be increased by 0.24°C, and for minimum temperature the Martinborough reading must be reduced by 0.80°C (not noted in the Review).