**Panel 1. What kind of atmospheric temperature variations can the
current and recent observing systems detect?**

**[Definition of space-time domain.]{.underline}**

We take the time domain to be mainly later than 1958 (IGY-Period).

We take the spatial domain to be global and regional, with some emphasis
on the tropics.

We are talking about atmospheric temperature (troposphere and lower to
mid stratosphere), surface air temperature and sea surface temperature.

Add cartoon

**[Main observing systems recently available.]{.underline}**

1\. Surface land and marine *in situ* observations (sea surface
temperature & surface air temperature).

2\. Radiosondes (Surface air temperature and atmospheric temperature)

3\. Satellite measurements (Passive microwave/infrared)

**[Skill in detection of phenomena/ signals.]{.underline}**

**I. Diurnal Variations:**

1.  Surface air diurnal variations over land: climatology well defined
    but long term changes questionable.

2.  Sea surface diurnal variations: not well defined but known to be
    small.

3.  Atmospheric diurnal variations over land and ocean: poorly
    understood and not well measured.

**II. Synoptic Scale Variations:**

Very well measured in general.

**III. Intraseasonal (30-60 days) Oscillations in Tropics:**

Well measured except in some oceanic areas aloft. Satellites (OLR)
particularly useful.

**IV. Annual Cycle:**

Very good-shape in measuring these features.

**V. Interannual Variations (eg ENSO):**

Well monitored at the surface and upper air over land and ocean.
Satellites play an important role over oceans where *in situ* network is
sparse.

**VI. Quasi-Biennial Oscillation:**

Adequately monitored using upper air temperature and winds in the
equatorial regions. Depends critically on a few key stations.

**VII. Episodic Variations:**

Volcanic events: well observed in low stratosphere but vertical
resolution inadequate. Time-lagged surface and tropospheric response
less well monitored.

**VIII. Abrupt Variations (Decadal Scale):**

e.g. Pacific Decadal Oscillation

If large, well observed retrospectively (not immediately).

If small, difficult to distinguish from spurious data artifacts

IX\. Gradual Trends

Inappropriate to focus solely on linear trend

Surface: regional, seasonal and global trends well characterized

Upper air: trends differ from different systems, not robust

Better (x,y) coverage from satellites

Better vertical coverage from radiosondes

Poorer representation of stratosphere than troposphere in raobs

**[Passive Microwave Instrument]{.underline}**

The radiation measured by microwave temperature sounders are ideal for
obtaining the extremely small climatic changes of atmospheric
temperature. This stems from their following unique characteristics;

1.  The measurements are directly proportional to atmospheric
    temperature

2.  The measurements are unaffected by non-precipitating clouds.

3.  The measurements are only weakly dependent on surface features.

> 4\. The measurements are very stable over the satellite lifetime.

Passive microwave radiometers contain different channels to measure the
tropospheric and stratospheric temperature in broad layers defined by
Gaussian-shaped weighting functions. The first operational instrument
was the Microwave Sounding Unit (MSU). It was first launched on the NOAA
series of polar orbiting satellites in 1979 and contained four channels.
Two of the channels were used to monitor the tropospheric temperature
around 500 mb and the stratospheric temperature around 70 mb. The
measurements have been used to separately observe the expected value and
anomalies of the globally-averaged temperature. Anomalies are due to
weather-dependent short time fluctuations and non-periodic episodes such
as$\text{El} - \text{Ni \{}\overset{\sim}{n}o$events and the weather
changes induced by volcanic eruptions. All of these anomalies have been
observed from the current MSU measurements. Of greater importance is the
expected value, which contains the periodic diurnal and seasonal
variations in addition to the long-term climatic trend. Again, all of
these parameters were observed from the time series of MSU measurements
from 1979 until 2002.

Beginning in 1998, a new instrument called the Advanced Microwave
Sounding Unit was launched by NOAA to continue the MSU time series. This
new instrument contains twenty channels, so that temperature
measurements can now be obtained with higher vertical resolution and
greater accuracy than that of the MSU throughout the atmosphere. The
improved accuracy and vertical resolution is needed to better profile
the changes in temperature trend as a function of altitude. This feature
should enable one to better reconcile the differences between the
climatic trend observed at the surface and atmosphere. The AMSU also has
much improved thermal stability than MSU, thereby reducing the nonlinear
instrument temperature dependence. Furthermore, the AMSU measurements
have already made a major impact on the accuracy of numerical weather
prediction by the National Weather Service of NOAA so that one should
expect similar impacts in monitoring climatic change.