The Haunts of Cloud Climatologies; Diurnal cycles, Sensor scan angles, FOV size and,

The Haunts of Cloud Climatologies; Diurnal cycles, Sensor scan angles, FOV size and,Sensor Calibrations

Donald WylieSpace Science and Engineering Center

University of Wisconsin-MadisonMadison, WI 53706

Average Diurnal Cycle of Low Clouds

Average Diurnal Cycle of High Clouds

From B. Cairns, 1995, Atm. Res. 37, 133-146, Fig. 2

Local Noon

Local Midnight

SunsetSunrise

Average Diurnal Cycle of Low Clouds

Average Diurnal Cycle of High Clouds

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The Cairns study shows very late afternoon and past sunset maxima in the high cloud diurnal cycle. Low clouds follow a more consistant pattern of local noon max over land and pre-sunrise maxima over oceans.

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Cairns also used ISCCP data.

From J.W. Bergman and M. L. Salby, 1996, J. Climate, 9, 2802-2820

High clouds lag low clouds

Largest cycle magnitude (0.30 range)

Smallest cycle magnitude (0.08 range)

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This depiction of the diurnal cycle shows the lag between low and high clouds over land. Low cloud maxima being just afternoon local noon while high clouds following around sunset. The low cloud maxima is pre-dawn consistant with other studies. Also of interest is that high clouds over oceans have an afternoon maxima.

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Bergmann and Salby used ISCCP data similar to the two previous studies.

Diurnal Cycle in NOAA HIRS Data

Boreal summer 1986-97(June, July and August)

Boreal winter (Austral summer) 1986-97(December, January and February)

From the data use in Wylie, Jackson, Menzel, and Bates, 2005, J. Climate, 18, 3021-3031

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The HIRS data show diurnal cycles consistant with the other studies. The strongest cycles are over sub-tropical land and over marine stratus clouds especially in the Austral summer.

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The data set is a subset of that used in Wylie, Jackson, Menzel and Bates. The period of 1986-97 had consistant coverage in both the 8 am/pm and 2 am/pm orbits for 11 years. Outside of this period there are large gaps in the 8 am/pm coverage so the diurnal cycle study was limited to the years of consistant coverage.

Sunrise Sunset

Cloud Cover Over Southern Tropical Oceans (Equator - 30 South)in Austral Summer From 1986 - 96

20.0

30.0

40.0

50.0

60.0

70.0

80.0

0 3 6 9 12 15 18 21 24

Local Standard Time

Clo

ud

Co

ve

r (%

) All Clouds (10% Range)

High Clouds (3% Range)

NOAA 9,11 and 14

NOAA 10and 12

NOAA 9,11 and 14

NOAA 10 and 12

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The southern tropical oceans have a strong diurnal cycle in general cloud cover of 10% in the HIRS data. But there is little diurnal cycle in high clouds.

Sunrise Sunset

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The northern sub-tropics has a strong diurnal cycle over land while it is weaker over oceans. Here, high clouds exhibit a strong diurnal cycle of nearly 10%.

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Diurnal cycles are slightly weaker in the tropics than the previous two examples. Oceans show a slightly stronger diurnal cycle than land for all clouds, but for high clouds the diurnal cycle is mainly over land.

High Cloud Diurnal Cycle from GOES VAS Data

Local Noon

Local Midnight

From D. P. Wylie and H. M. Woolf, 2002, Monthly Wea. Rev., 130, 171-179

10% range

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The GOES-VAS data offered an opportunity to study the diurnal cycles of high clouds using the CO2 slicing method with ~8 sample/day. The diurnal cycles were strong over the Rocky Mountains and the coast of the Gulf of Mexico. These are areas of strong cumulus convection in the summer. Over oceans, diurnal cycles were very weak.

Similar shape

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This is an example of one region with a strong diurnal cycle. Note that the ISCCP IR Only data follow the shape of the diurnal cycle reported in the GOES-VAS data even though less clouds are reported in the ISCCP IR Only analysis.

Diurnal Cycle SummaryCloud studies that use visible data will over report cloud

cover over land and under report cloud cover over oceans from their diurnal cycles. Magnitude of the error is around 2-10%.Averaging of the ascending and descending orbits with an analysis technique that does not depend on solar reflection measurements, will greatly reduce the diurnal cycle errors.

Diurnal cycles in high clouds mainly occur where there are strong diurnal cycles in cumulus clouds – sub-tropical mountain slopes and coastal land areas. The high cloud maxima lag the low cloud maxima by 4-6 hours.

Over oceans high cloud diurnal cycles are weak and absent in many areas.

In winter diurnal cycles in high clouds are usually absent.

Change of Cloud Frequencies with Scan Angle (Tropics)

45°

TropicsScan Angle

Category Nadir > 41DegAll Clouds over Land 0.70 0.10 14%All Clouds over Oceans 0.72 0.17 24%Light Ci over Land 0.23 0.03 13%Light Ci overOceans 0.27 0.06 22%Dense Ci over Land 0.29 0.02 7%Dense Ci over Oceans 0.19 0.02 11%High Clouds over Land 0.48 0.05 10%High Clouds over Oceans 0.40 0.07 18%

Cloud Cover

The Increase in Reported Cloud Cover from Nadir to the Highest Scan Angle

Reasons why the reported cloud cover increases with scan angle

1. Slanted path through atmosphere increases cloud optical depth by 141% or by a factor of 1.41. This could increase reported cloud cover by 0.01-0.02.

2. FOV size increases by a factor of 2. Using pairs of HIRS FOVs this will increase cloud cover reported by 0.06.

vs.

Count pixels individually

Counting a pair as cloudy if either is cloudy.

vs.

HIRS will recognize the cloud

HIRS will not find the cloud

2% of clouds

4% ofclouds

GLAS

There are many small clouds but it’s theIt’s the large ones that account for the coverage.

All Clouds

High Clouds

Sensor calibrationUse AIRS hyper-spectral measurements to estimate HIRS

Preliminary HIRS-AIRS Comparison

Ch 4 Ch 5 Ch 6 Ch 7 Ch 8

Mean Difference N16-AIRS (K)(From one day of HIRS-AIRS data 380 points.) 0.33 0.66 0.34 0.28 0.10

St-dev of Difference (K) 0.18 0.27 0.43 0.66 1.02

Correction for part of spectra not sampled by AIRS

0.04 0.24 -0.61 0.24 -0.33

Final difference (K) 0.29 0.43 0.95 0.04 0.43

N16 – Radiance calculated from Sounding. (K) -1.50 -1.80 -1.45 -0.50

Unexplained measured differences betweenHIRS-2 and HIRS-3 (K) -2.5 -2.2 0.7 -0.6 0.40

Summary

1. Diurnal cycles can cause errors of 2% to 10% in reported cloud cover. The largest error is when only the day time part of the 2 pm NOAA orbit is used.

2. Scan angle - changes reported cloud cover by 7-24%, but averaging all FOVs from all angles changes together biases cloud cover by

only 3%.

3. Holes and clouds below the 20 km FOV size of HIRS cause over reporting of cloud cover where the clouds are cold, but under reporting of clouds that are very small and optically thin at all altitudes. The second error cancels part of the first.

4. Sensor calibration problems – a preliminary comparison of HIRS-3 to AIRS shows +0.3 to +0.9 K possible biases. We need to compare

HIRS-2 on NOAA 14 to AIRS to see if there is a bigger bias or if there is a bias of the opposite sign.

Documents

The Haunts of Cloud Climatologies; Diurnal cycles, Sensor scan angles, FOV size and,