Prather, Michael

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WORKSHOP OS EARLY DETECTION OF STRATOSPHERIC CHANGES
March 5-7, 1986 Boulder, CO
POSITION PAPER: Sampling Frequency & Required Network Size
Michael Prather
NASA/Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025
What should be the location, size and sampling frequency for a network of
stations designed to give an early warning of stratospheric changes? In lieu
of providing immediate answers to this question, let us examine some of the
problems which might obscure the detection of trends in the stratosphere and,
therefore, some of the sampling strategies which may allow extraction of trends
from the natural atmospheric variance. I will try to point to necessary
research—both theoretical & observational--which is needed to answer better
this primary question. I shall focus on the middle to upper stratosphere (above
30 km) where chemistry plays an active role on dynamical time scales, and where
the photochemical perturbations to ozone are predicted to be greatest
(Antarctica aside).
1. What atmospheric conditions should we expect in 10-20 years?
Chemical composition:
Extrapolation of current trends is too simple a model for atmospheric changes,
but it may suffice for the next decade or so. The table below is my rough
estimate of the changing atmospheric composition (loosely derived from Chapter
3, NASA Assessment). A look at the RAND assessment of halocarbon growth over
the next two decades gives a similar estimate of Clx increases. Verification
of these trends in the long-lived trace gases will come about mainly through
tropospheric monitoring; trends unique to the stratosphere (HC1 and C2H5 are
discussed by Sze et al) represent important confirmation of the atmospheric
models.
Observed trends (Chapter 3) Rand scenario (1986)
year Clx Brx CK^ ^0 C02 Clx
1980 2.77 28.1 1.62 296 343 2.8
3.2-3.2
3.6-3.9
4.1-4.7
4.5-5.7
5.0-6.8
Early Detection of Stratospheric Change (Prather, March 1986)
1985
1990
1995
2000
2005
3.23
3.85
4.70
5.87
7.51
29.2
31.2
35.2
42.7
57.2
1.70
1.79
1.88
1.97
2.07
300
304
308
311
315
350
357
364
372
379
https://ntrs.nasa.gov/search.jsp?R=19910073965 2020-03-17T14:53:06+00:00Z
Climate:
We expect a detectable change in global climate within the next 2 decades (due
mainly to increases in C02) . The direct stratospheric effects of CC>2 increases
are small (cooling of less than 0.5°C), but the tropospheric climate during
this period is expected to undergo substantial warming ( > 0.5°C). The climate
change is likely to result in significant reductions of long-wave eddy kinetic
energy as the pole -to -equator gradient in temperature is reduced. These
planetary waves have a major impact on the forcing of the mean stratospheric
circulation. This impact is a topic of current research and its effect on
stratospheric monitoring strategies is unknown.
2. What should we try to measure: 63, T, CIO, NO, ILgO, ...?
This topic is discussed by others, and I include only the following comments:
Simultaneous observation of several variables would definitely aid in the
interpretation of trends. Furthermore, the observed correlations of ozone with
temperature ( etc.) would allow some of the atmospheric "noise" to be removed
(e.g., ozone values corrected to a standard temperature), perhaps even the
interannual variability & solar variability. Current photochemical models are
capable of predicting such correlations .
Systematic errors or signals specific to a C1K perturbation:
Do the correlations of ozone with temperature change with Clx? Does the
altitude dependence of this correlation change at high levels of Clx? (Can we
sample so as to eliminate this atmospheric noise?)
Will elevated levels of Clx and CH4 affect the diurnal cycles and therefore
possibly corrupt or disguise the signal of a trend? (In 30-50 km range: no)
Can we adequately interpret solar occultation data today? (Systematic errors in
the inferred abundance of NO-N02 occurs below 35 km and for 03 above 55 km.)
Can we remove the effect of atmospheric tides which have a large diurnal
amplitude in the tropics? (Systematic differences of at least 10% in ozone
concentrations are predicted for the tropical stratopause . )
3. How frequently should ve sample, over what spatial scales?
High frequency variability at a fixed location is directly related to the
spatial scales of inhomogeneous air parcels. We see spatial variability in the
stratosphere on vertical scales of 100 m and (as inferred from ascent and
descent data several hours apart) on horizontal scales of 100 km. Does a
"stratospheric-trends" network need to resolve and individually average over
these lamina? Or can we effectively average over them by integrating the
"signal" over 1 km in altitude and half the day? This problem is unresolved.
Early Detection of Stratospheric Change (Prather, March 1986) *
Ve need to develop new types of theoretical models which are able to infer the
source of Lhc iliflerent composiLion of these layers and the impact of this
small scale structure on average ozone concentrations (at the 5% level). Is
there any hope of modelling these vertical scales in current or projected 2/3-D
models? (probably not) Are the 100m lamina essential to the chemistry, or can
we calculate the photochemical response of the system on the average of
conditions, or will they be mixed with their environs in relatively short time
scales? (probably not)
(Example: In the lower troposphere we need "to resolve the spatial/temporal
scales of the event/parcels which we are measuring. For example, at Cape Grim,
Tasmania 'polluted' air parcels from the mainland of Australia sweep over the
observing station in less than 24 hours; but at Adrigole, Ireland, events with
elevated levels of CFCs typically last from 3 to 6 days. This difference is
predicted by 3-D tracer models and can be identified with the relatively small
extent of the Australian source region (Melbourne) as compared with that of
Europe.)
Is there a measurement problem in averaging over these layers? Is a "mean"
value correct? For example, when we integrate thermal emissions over a large
volume, is it homogeneously weighted as in temporal averaging? Or does it
disproportionately weight the hotter parcels? (It is likely that temperature
will be correlated with some concentrations.)
A. Where should we locate ground stations?
If we are limited in spatial sampling to a few fixed locations (i.e., ground
stations) then the choice should be made so as (1) to enable effective spatial
sampling through a time series and (2) to eliminate noise associated with
natural atmospheric variability.
(1) The location should not preferentially sample air with respect to
longitude. If all air parcels in a given latitude/altitude range are equally
likely to pass over the station then a good time series of observations gives
effective spatial sampling. (This is clearly a problem in the troposphere where
we have strong longitudinal gradients associated with continental vs. oceanic
sources.) At mid-latitudes the large planetary waves in the troposphere might
create standing waves in the middle stratosphere, but this seems less a problem
in .cummer than winter. A more serious problem may occur in the tropics where
there is major forcing at the tropopause due to deep convection over the
continents. Depending upon the zonal winds (which change seasonally and
quasi-biennially), this signal may propagate through the stratosphere and
create a systematic, standing disturbance in temperature which would bias the
observations over one station.
(2) The location should avoid strong gradients in chemical concentrations
because it would be difficult to extract chemical trends from atmospheric
variability in dynamical transports. Thus 40°N is a less desirable location
because of the strong, observed latitudinal gradients.
Early Detection of Stratospheric Change (Prather, March 1986) 3
(Example: The tropospheric monitoring station at Cape Meares, Oregon sits at
the edge of a large gradient in anthropogenic gases between the North American
continent and the Pacific Ocean. Khali1 and Rasmussen (Science, 1984) reported
large seasonal variations in CO and a trend based on 3 years of nearly
continuous monitoring. The large rate of increase first reported (+6%/yr) has
not been confirmed by two subsequent years of observation. Although many
interpretations are possible, one candidate could be the interannual variations
in transport of CO near the vicinity of these large gradients.)
(added note)
5. Correlations of trace gases in the lover stratosphere:
Following on a point emphasized by Sze, the concentration of C^ H^  in the lower
stratosphere reflects the concentration of active chlorine: atomic Cl. The
profile of this species is expected to show a more rapid fall-off with height
as total Clx increases in the next decades. The actual change in the mean
profile may be difficult to observe accurately because of the small scale
height. However the effect of an increase in Clx would be evident in the
changing correlation of C£Hg with other gases which are mainly photolyzed
(CFC13) or destroyed by reaction with OH (C^ CC^ , CH^ . It should be
emphasized again that C^ ffg responds not just to the total chlorine in the
stratosphere, but particularly to the active form (C1/C10) which we predict
will become increasingly important in the destruction of ozone in the lower
stratosphere. If these correlative measurements are made with grab samples
then it would be important also to measure ^0 simultaneously!
Early Detection of Stratospheric Change (Prather, March 1986)


Sampling frequency and required network size

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