Comparison of visual observations and automated ceilometer cloud reports at Blindern, Oslo. Anette Lauen Borg Remote sensing MET-Norway

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Kimberly Lawson
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1 Comparison of visual observations and automated ceilometer cloud reports at Blindern, Oslo Anette Lauen Borg Remote sensing MET-Norway

2 A test of our ceilometer data To fully exploit our new ceilometer we want to know more about its abilities and limitations. At the Blindern site, there is both a test ceilometer and a long record of manual observations. A comparison of one year of data has been carried out and we need to interpret the results.

At the Blindern site, there is both a test ceilometer and a long record of

3 A ceilometer comprises a laser and a backscatter receiver The laser sends a light pulse through the atmosphere. The light is scattered by particles. Some of the scattered light hits the ceilometer and is measured as a signal. The time it takes for the scattered light to travel from sender to receiver is used to make a vertical profile.

Some of the scattered light hits the ceilometer and is measured as a signal.

4 The new ceilometer provides the entire vertical profile up to ~15km Clouds cause stronger signals than aerosols and precipitation. (Logarithmic colour scale, clouds in black, noise removed/reduced.)

5 The new ceilometer provides the entire vertical profile up to ~15km Aerosols up to 5 km. Black lines are ceilometer's attempt at identifying aerosol layers. (Logarithmic colour scale, noise somewhat reduced.)

6 Ceilometer: Advantages and disadvantages A ceilometer runs continuously at high time resolution. Provides a remote view of the sky conditions at site. The cloud altitude measurements are more accurate than those of a human observer A ceilometer provides a point measurement. It can not penetrate a heavy layer of clouds. May give wrong readings during heavy fog/rain?

The cloud altitude measurements are more accurate than those of a human observer.

7 The ceilometer at Blindern, Oslo, was installed in March 2013 The ceilometer is situated on the building roof and the manual observations are taken from the same building. The time resolution is 15s and the vertical resolution 15m. Parameters used in this comparison: Cloud base height, cloud cover, sky condition (rain/fog/snow/clear).

building. The time resolution is 15s and the vertical resolution 15m.

8 Manual observations at Blindern are taken every 3 hours There is also a pluviometer, or rain gauge, measuring precipitation at 1 min resolution. The parameters used in this comparison: Weather (code), precipitation (resolution 1min/1h), cloud height, cloud cover, visibility.

9 Comparison of data parameters in the interval March June 2014 Ceilometer Cloud base height (CBH) Total cloud cover Sky condition index Sky condition index Sky condition index Observations Cloud height, base layer Cloud cover Weather at observation Rain measurements Visibility

index Sky condition index Sky condition index Observations Cloud

10 The manual observations were used as the starting point Example: When rain is manually observed, how often does the ceilometer measure clear sky/rain/fog/snow? Ceilometer data at ±10 min around the observation time was included in the test.

ceilometer measure clear sky/rain/fog/snow?

11 Cloud base height (m) compared Heights: <50, , , , , , , , , >2500 Height categories Same height ±1 category ±2 categories No obs data Per cent (%)

>2500 Height categories 10 9 8 7 6 5 4 3 2 1 Same height ±1

12 Cloud base height compared: High altitude and clear sky discrepancies Total data distribution: Same height 39%, within ±1 category 67%, within ±2 categories 76%. The ceilometer is doing a point measurement to find the base cloud height. The observer will look at the entire sky for clouds and report the altitude of the base. Also: How accurate are the observer estimates at high altitudes?

The ceilometer is doing a point measurement to find the base cloud height.

13 Is ceilometer cloud base height measurements influenced by rain? The ceilometer identifies clouds by looking for signal thresholds. Could the signal returned by heavy precipitation trick the ceilometer into reporting a cloud?

14 10 Cloud base height (m) during rain 9 8 Heights: <50, , , , , , , , , >2500 Height categories Same height ±1 category ±2 categories No obs data Per cent (%)

2000-2500, >2500 Height categories 7 6 5 4 3 2 1 Same height ±1

15 Cloud base heights Cloud base heights during rain

16 The ceilometer provides an estimate of the total cloud cover Point measurements over time are combined to provide an estimated cloud cover. This estimate is compared to the manually observed cloud cover at one point in time.

17 Total cloud cover compared Octa Octa Same cover ±1 octa ±2 octa Per cent(%)

18 Multiple scattering is used by the ceilometer to identify precip./fog High particle density produce a stronger signal than usual close to the instrument. In cases of low density precipitation or a clear area around the instrument during fog, this method will not work.

19 Ceilometer sky condition index vs weather at observation time (WW) Clear Rain Fog Snow Clear 98% 1% 0% 0% Rain 22% 74% 2% 0% Fog 29% 15% 53% 1% Snow 23% 18% 0% 57% Example: When snow is observed, the ceilometer will indicate snow 57%, rain 18% and no precip 23% of the time. However, the visual WW code used by observers can be difficult to compare to the automatic sky index.

observed, the ceilometer will indicate snow 57%, rain 18% and no precip 23% of the time.

20 Ceilometer sky index vs pluviometer (rain gauge) measurements 1min resolution pluviometer data Clear Rain Fog Snow No precip. 88% 6% 3% 2% Precipitation 5% 89% 1% 2% 1h resolution pluviometer data Clear Rain Fog Snow No precip. 99% 0% 1% 0% Precipitation 5% 86% 0% 9%

88% 6% 3% 2% Precipitation 5% 89% 1% 2% 1h 99% 0% 1% 0%

21 Ceilometer sky index and observed visibility compared Clear Rain Fog Snow Visibility > 1km 87% 8% 2% 2% Visibility < 1km 2% 10% 59% 26% Visibility can be low due to other reasons than fog (rain, snow, aerosols).

22 The agreement between ceilometer and observer is mostly good The ceilometer should not be used as the only source of information but adds valuable data. It provides high time resolution data 24/7. It provides aerosol information!

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