Población de referencia y hogar tipo en el rediseño de la CFB

T h o m a s [1986] and Laxon [1989] both provide excellent overview s o f the rem ote sen sing techn iq ues w hich can be u tilised in sea ice-co v ered reg io n s, and the m easu rem en ts w hich they provide. In this section, w e in troduce the range o f spacebom e instrum ents which have been or are currently used to m onitor sea ice, in particular ice extent, concentration and motion. Table 1.3 lists som e o f the satellites w hich carry or have carried instrum ents relevant to the rem ote sensing o f sea ice, along w ith their associated periods of operation.

Satellite NO A A series Nim bus 5 N im bus 7 D M SP series ERS-1 ERS-2

Period of operation R elevant instrum ents

1970- 1973-76 1978-87 1987- 1991-96 1995- A V H RR ESM R SM M R SSM /I SAR, A T S R -1 ,R A SAR, A TSR-2, RA

Instrum ent abbreviations:

A TSR Along-Track Scanning Radiom eter

A VH RR Advanced Very High Resolution Radiom eter

ESM R Electrically Scanning M icrow ave Radiom eter

RA Radar Altimeter

SAR Synthetic Aperture Radar

SM M R Scanning M ultichannel M icrow ave R adiom eter

SSM /I Special Sensor M icrow ave/Im ager

Table 1.3 Instrum ents used in the remote sensing o f sea ice.

W e now discuss in detail some of the applications o f rem ote sensing data aim ed at m easuring the clim atologically im portant param eters in the A rctic O cean specified in table 1.2. The extent to w hich they m eet the accuracy and resolution requirem ents specified in this table will also be discussed where appropriate.

1.6.3.1 Ice extent and concentration

Passive m icrow ave observations o f sea ice have been used for a num ber o f years in the rem ote sensing o f sea ice, most notably in the m easurem ent o f sea ice extent and co n ce n tratio n . T hese instrum ents provide o b serv atio n s o f su rface b rig h tn ess

tem perature during both night and day, and in the presence o f cloud cover. The SM M R provided com plete coverage every 2 days at a resolution o f 30 km (to 84°N). In contrast, the SSM /I instrum ents provide daily coverage to 87°N at a resolution o f 25 km , alm ost m eeting the m inim um observational requirem ents for ice extent and concentration specified in table 1.2.

Ice extent m apping from passive m icrowave instrum ents relies on the high contrast betw een ice and open w ater in both the visible and m icrow ave frequency bands. Several authors have reported decreases in the areal extent o f sea ice in the A rctic

over the last tw o decades from passive m icrow ave observations (e.g., G loersen and

Cam pbell [1991]; Johannessen [1995]). M ost recently, a decrease in extent o f 2.9 ±

0.4% was observed betw een N ovem ber 1978 and D ecem ber 1996 by C avalieri et al.

[1997]. U sing the sam e observational technique, P a r k in s o n [1992] o b serv ed a

shortening in the sea ice season in much of the eastern A rctic betw een 1979 and 1986.

C avalieri et al. [1984] used data from the d u al-p o larised m u ltisp ectral SM M R instrum ent to estim ate ice concentration with a precision o f 5-9% , the low er end o f w hich satisfies the accuracy requirem ents specified in table 1.2. D iscrim in ation betw een first-year and m ultiyear ice was also shown to be possible, and the precision o f the m ultiyear ice fraction was estimated to be in the range 13-25%.

Infra-red observations from either the AVHRR or A TSR instrum ents can also be used to study ice extent and concentration at a much higher resolution (1 km), but w ith the restrictions im posed by cloud cover. SAR can provide ice extent inform ation at an even higher resolution (30 m) even in the presence o f cloud cover, but at the expense o f a m uch greater data volum e and limited coverage. R adar altim etry has also been

used to m ap ice extent, both in the A ntarctic using G eosat [Laxon, 1990] and the

A rctic using ERS-1 [Laxon, 1994b].

1.6.3.2 Ice motion

E stim ates o f ice m otion are possibie from satellite im agery, using sop histicated algorithm s to track the m ovem ent of distinguishable features in the ice cover over a sequence o f im ages. Passive m icrowave, SAR and A V H R R observations can all be used for this purpose, and the results assessed by com parison w ith the ice m otion

derived from buoys. K w ok et al. [1998] used passive m icrow ave observations to track

ice every 3 days in the A rctic Ocean and daily in the Fram Strait and B affin Bay betw een O ctober 1992 and M ay 1993. The results w ere com pared w ith buoy- and

S A R -d eriv ed ice m otions, to give an estim ate o f the u ncertainties in the m ean displacem ent vectors o f between 5 and 10 km.

1.6.3.3 Ice thickness

T he concept o f using spacebom e passive and active m icrow ave instm m ents to infer ice thickness from the surface texture and scattering characteristics is a relativ ely

recent developm ent. Comiso et al. [1991] show ed that a positive correlation exists

betw een SA R backscatter and ice elevation, suggesting that such observations could

be used to infer ice thickness. As indicated by W adham s [1995], further advances in

this technique requires extensive validation. By com bining surface tem peratures from

A V H R R w ith a therm odynam ic ice growth m odel, Yu and R othrock [1996] show ed

that the thickness o f thin ice (less than 1 m thick) could be estim ated, w ith an uncertainty o f roughly 50%. The uncertainty in the cum ulative thickness distribution was found to be considerably lower (3% for ice less than 20 cm thick, and 9% for ice around 1 m thick).

C om iso et al. [1991] used airborne laser profilom etry to m easure ice freeboard, and com bined the m easurem ents with coincident subm arine observations o f ice draft to deduce a relationship betw een ice draft and freeboard. This dem onstrated that ice thickness could be estim ated from airborne or spacebom e m easurem ents o f freeboard. O ne o f the earliest attem pts at using spacebom e altim etry to estim ate ice freeboard

w as perform ed by Stanley et al. [1979]. They used data from the Geos-3 altim eter to

estim ate w hat they thought was ice freeboard. H ow ever, for a num ber o f technical reasons, their results are now known to be fallacious. It may also be possible to use satellite laser altim etry from future m issions to m easure ice freeb oard, although technical lim itations and the presence of cloud cover could restrict its use.

1.6.3.4 Ice albedo and surface melting

Lindsay and Rothrock [1994] used AVHRR data to estim ate the albedos o f 145 square cells o f side 200 km in the Arctic. They observed a reduction in the m onthly averages o f albedo in the central A rctic from 0.76 in A pril to 0.47 in A ugust, equivalent to a decrease o f about 30%. M onthly averages o f the standard deviations o f albedo w ithin each cell in the same region were found to be 0.04 in April and 0.06 in Septem ber.

U sing passive m icrow ave im agery from the SM M R and SSM /I instrum ents. S m ith

[1998] observed an increase in the length o f the m elt season o f perennial A rctic sea ice betw een 1979 and 1996, equal to 5.3 days (8%) per decade in the num ber o f m elt

days per summer. Use of these instm ments for the detection o f surface m elting allow s both the spatial and temporal resolution requirem ents o f ICEX to be met.