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CRAPTER 2

GATE OBSERVATIOEAL STUTEGY: A LOOK 110 RETROSPECT

by Joachim P. Kuettner

(National Center for Atmospheric Research, Boulder, Colorado, U.S.A.)

2.i INTRODUCTION

The strategy for a large field experiment such as GATE begins with the first day of the Experiment Design. It continues throughout the field operations but it receives its last judgement only in retrospect, that is, in and after the scientific analysis phase. Strategy implies a counter-player, normally a human being, friend or enemy. In scientific research the main counter-player is nature, but technology and economy are second players in the cast. ,

2.2 OBSERVATIONAL STRATFGY IN THE PLANEING PHASE

The experiment design for GATE was a two-year full-time effort with a large team of outstanding consultants. It addressed the question of how and where best to observe the mechanism and related scale interactions of the tropical heat engine.

The first decision was to use a composite -observing system that would have *nested* resolution from the cumulus scale (D scale) over the meso and cloud cluster scales (C and B scales) to the synoptic-planetary scale (A scale) and to place its centre into the Eastern Atlantic. The next decision was to select a geo- graphical longitude far enough from Africa to suppress continental influences but close enough to make it accessible to long-range aircraft from the operations centre in Dakar. The latitude was chosen so as to intercept the Intertropical Convergence Zone (ITCZ) and the cloud clusters developing and travelling in it in an optimum fashion. Based on all data available since 1887, including the sea-surface tempera- tures, the selected centre point was placed at 22.5'W and 8.5'N. The A, B and C scale ship array was constructed around this point. We can say today that this centre point was in a nearly ideal position and that the tracks of the cloud clusters in the ITCZ during the three phases of GATE (late June to mid-September) were euc- ceesfully intercepted (Figures 2.1 and 2.2).

It is also interesting to compare the originally-estimated long-term sea-surface temperature field (Figure 2.3) with the one actually measured during GATE (Figure 2.4, after Krishnamurti and Pasch, Chapter 3, this volume; see Wing et al., 1988). One can see that the selected centre point of the array was placed near the 27 C isotherm as intended.

2.2.1 Configuration of the Observing S;Estem ----- ____________________--- __ --_-

A difficult question facing the experiment design was how to configure the B scale (cloud cluster scale) ship array for upper-air soundings. On one hand, it should intercept as many cloud clusters as possible travelling west from the African coast; this would favour a large north-south extension. On the other hand, one wanted to get an insight into the life cycle of the cloud clusters and their heat budget. This would require a good overlap of quantitative radar ranges and favour strong east-west extension.

As it happened, only four of the nine available radar ships had digital 5cm radars suitable for quantitative precipitation measurements. Furthermore, the need for upper-air soundings on the A/B scale suggested a well-spaced distribution of the wind-finding ehips. The actually selected double hexagon configuration (Fig- ure 2.5) turned out to be a good compromise satisfying both the grid point and the precipitation requirements. With four strongly overlapping radars (Figure 2.6) un- precedented quantitative information on the convective precipitation systems over the tropical oceans was obtained (see Chapter 9, this volume).

5N 3ow 25W 2ow 15w

Figure 2.1 - Interception of cloud clusters by the ehip array. The arrow8 show the mean path of the convective eysteme during each phaee ae determined from eatellite IR pictures.

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Figure 2.6 - Positions of ships carrying quantitative (digital) radars during Phase 3 of GATE.

2.2.2 @optic network and WWW - -___________________--

Another problem of the experiment design was the optimum coverage of the synoptic/planetary scale (A scale) by upper-air stations over land and over the ocean within the GATE area. On land, one obviously had to rely on the World Weather Watch but past records had shown that critical gaps existed in the WWW observing network and the Global Telecommunication System (GTS), especially over Africa. Over water, the competing ship requirements of the various scales presented some problems. In spite of the fact that eventually 39 ships with upper-air capability became available. Numerical studies on optimum ship distribution were conducted during the experiment design phase by the U.S.S.R* and the U.S.A. and an A scale ship array of 12 ships reaching as far north as 20 I was foreseen (Figure 2.7).

Life, however, has a way of disregarding the best laid plans. In spite of a lead time of nearly three years, shortcomings of the WWW-GTS in some parts of Africa persisted. Most serious was the critical gap in the upper-air network along the west coast of Africa. Just upstream of the A/B ship array, there were no data between Dhahran and Abidjan, since Freetown and Monrovia did not operate. (As we will describe later, one way to correct the situation in the field was to redistri- bute the ships thereby diluting the A scale distribution, see Figure 2.5.)

Purthermore, the geostationary satellite placed over the GATE area was to cover any gape by cloud vector winde at two or even three levels. This proved to be extraordinarily useful, though not perfect.

Besides the missing WW stations, it was expected that some of the upper- air stations which operated as planned would have communication difficulties via the GTS. It was planned to at least partly correct this by a simple paper-tape collection. As it happened, this increased the synoptic coverage by about 20$.

2.2.3 Option plan - ----- ---

In order to reduce the critical decision time during the actual operations, most of the conceivable options were anticipated in the planning stage, to the degree possible, and the various alternatives were predetermined.

This was especially important for the aircraft planning. Decisions and options were tried and tested in the planning stage making for a rather smooth opera- tion of the aircraft programme in the field phase.

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0 W/U upper air mtationm reported to make at lurt on. observation per day.

0 A-scale ship poBition* B-scale eroa,

EZJ Altern~tiro A r - -, ‘_ _ 2 Alternatiro B

Figure 2.7 - Originally reoommended layout of ship array

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2.2.4 Subprogrammes and central programme --- -- -----------------_- -- --_--

Due to the complexity of the scientific objectives it was decided to form five subprogrammes which were held together by the "Central Programme". In this way it was possible to attract the top talents of the international scientific community in the fields of boundary layer physics, convection, synoptic-scale dynamics, radiation and oceanography. The Central Programme, in turn, made sure that sight was not lost of the primary objectives, namely the scale interaction and the needs of numerical modelling, such as the parameterization, initialization and verification requirements. The strategy adapted early in the CARP experiments was to combine the numerical objectives with phenomenological and physical research. In this way full scientific use could be made of the enormous resources marshalled by GATE. The vast scientific literature that emerged from this experiment teeti- fies to the stimulation and advance of practically all fields of atmospheric and oceanographic sciences in the tropics by GATE.

2.3 STRATEGY DURIBG FIELD OPERATIONS

Strategical decisions during the field phase are rendered difficult by two circumstances: the time limitations and the undesirable consequences of any change. Nearly all decisions in the field are urgent. Endless argumentations are entirely useless. The pre-developed option plan was therefore very important - but not all situations and not all consequences could be anticipated. It is good principle to comply, to the extent possible, with the original plans. Attempts to *improve" things during the field phase are dangerous. Usually a chain reaction of technical and logistics difficulties follows such changes. A "better" project often becomes a worse project. Changes should only be made if one of three circumstances arises, namely (in order of priority):

(1) the safety of the participants is endangered (2) the primary scientific objectives are jeopardized

(3) unexpected natural phenomena demand adjustment (one may call this serendipity).

All three happened during GATE.

2.3.1 Safet;Z! considerations ----- ----------__-__

The safety of the aircraft crews demanded certain adjustments with the ship programme due to hazards with the tethered balloons. Also, intercomparison flights between aircraft of different propulsion systems led to some hazardous situa- tions rendered worse by language difficulties. Cases of sickness among ship personnel made it necessary to move ships from their assigned positions. Aircraft were grounded when fuel contamination was suspected. Safety always took priority over science.

2.3.2 Maintaining the scientific obJectives ---------- -------------_---- ______-

tives, The second problem, a possible jeopardy of the primary scientific cbjec-

also had to be faced. Two events led to this situation. It was technically and cost-wise impossible to equip all 39 ships with stabilized wind-finding radar; only the U.S.S.R. ships had fully-tested and reliable radar wind equipment. The rest of the ships carried the so-called navigation aid wind-finding system ("AavaidV'). This system was newly developed and not sufficiently tested. When it became doubtful that the "navaid" data were reliable, the question arose whether or not the problem of scale interaction between the A and B scale could be solved. There was a pos- sibility that one of the prime objectives of GATE was in jeopardy.

The common human approach to priority problems is to "rob Peter to pay Paul". This was done in GATE too. Peter was the A scale ship array. It was a difficult decision and it was complicated by the second circumstance mentioned earlier, a critical gap in the WWW along the African coast, upstream of the A/B scale area. Therefore, after the first phase of GATE a double adjustment was neces- sary: some of the U.S.S.R. ships had to be pulled closer to the A/B scale hexagon and other ships farther north had to be moved close to the African coast to cover the upstream conditions (Figure 2.5).

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The resulting dilution of the A-scale network in the northern and western Atlantic was considered carefully. It was accepted as the lesser of two evils since a more perfect synoptic observing grid for numerical models would probably be avail- able at some future time, but the chance to repeat the multi-scale central ship array for scale interaction studies was practically nil. Also, the operational forecasts for GATE demanded this decision.

As expected, the shortcomings were felt later, for example in difficulties to track and determine the identity of westward-travelling easterly waves from Africa to the Caribbean Sea (sge Burpee and Reed, Chapter 4, this volume). These waves can best be tracked near 15 Ii at the 700 mb level. The missing ship soundings at this latitude and in the western Atlantic could not be fully replaced by satellite cloud vector winds because these usually refer to levels lower or higher than 700 mb.

All in all, the synoptic subprogramme seems to have suffered under the given situation more than the other subprogrammes.

2.3.3 Unexpected behaviour of nature ---- ____________________-----

The aircraft programme, with its many types of flight tracks, had been designed primarily to explore the nature and life-cycle of the cloud clusters, a phenomenon that appeared rather obscure prior to GATE. Once its nature became more familiar during the GATE field phase the programme had to be swiftly adjusted. The main factor in this adjustment was the unpredictability of the location and the almost explosive growth of the cloud clusters in their initial phase. Mephisto said to Goethe's Faust: "Grey, my dear friend, is all theory". Our well-laid plans of how to disect the cloud clusters were not adequate. The idea to make consecutive flight sorties to follow their development and life-cycle had to be given up. The elusive nature of the phenomenon required decision-making almost on the spot and the practical considerations of crew rest and equipment readiness limited our flexibility. As a consequence, another strategy had to be developed.

The role of the "airborne mission scientist" on the command aircraft was

strengthened as compared to the ground mission scientist and the precise flight tracks were determined during the flight itself, often with the help of real-time ship radar and satellite data. This required great-skill and excellent communica- tions. Night flights and the so-called "butterfly" pattern (Figure 2.8) had to be given up after a few attempts. This pattern, while very useful in the exploration of quasi-stable vortices (for example, during MONEY) cannot follow the fast changes of the cloud cluster core. The "box pattern" turned out to give better data, also for energy budget calculations. There is little doubt now that this strategy change contributed greatly to the better understanding we now have of the mechanism of tropical convection and the subsystems making up the cloud cluster (see Betts and Houze, Chapter 9, this volume).

On the other hand, the plan to vertically "stack" the research aircraft was not as effective as we had hoped - partly because of failure or inconsistencies of the various on-board systems - partly because of performance problems, especially under icing and turbulence conditions. Data from more levels would have been desir- able in many cases.

By combining the high-resolution aircraft data with the multi-scale ship data, it has been possible to establish the interconnection between the larger scale divergence fields, often determined by easterly waves, and the mass fluxes in the cloud cluster system themselves.

2.4 STRATEGY IR RETROSPECT

We said in the beginning that the "best judgementw is only possible in retrospect, that is, when the plans can be compared with the actual accomplishments. For the observational strategy this is easier done than for the scientific strategy. An example of the first type is 'the aircraft programme.

In Table 2.1 the apportionment of the available flights (322 sorties) to the various subprogrammes is shown: first, as it was originally planned; second, as the specific GATE Aircraft Plan visualized it; third, as it was actually flown. To come that close to the planned percentages required three things: careful strategy in the field, co-operation of nature and - luck.

TIMNSIT DISTANCE 528 n.m.

CUCUIT DISTANCE 350 n.m.

EACH WAY

Figure 2.8 - The "butterflyw flight pattern for the exploration of cloud alusters was given up because of unexpected behaviour of nature.

Table 2.1: Apportionment of total flight missions planned for various scientific tasks se compared to actually flown missions

TPDe of mission General Goal GATE Aircraft Actually

Plan fLOWn

Basic GATE Missiorls plus convection flights 60 62

i 57

Special Boundary Layer missions

20 18 18

Special Radiation missions 15 18 21

Special Oceanographic missions I

5 2 4

In regard to the scientific strategy the results available today indicate that it was successful in producing the needed data sets.

The B scale observations provided much improved information on the verti- cal distribution of heating, moisture and precipitation in the tropical belt enabling first tests of different parameterization schemes for tropical convection. We do not yet know if certain details of these schemes such as entrainment, mass flux, evaporation can be verified. Progress is already being made in q esoscale parameteriza- tion of convective systems but the verification and operational use of advanced cumulus parameterization schemes in large-scale prediction models is still outstanding.

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The first systematic use of satellite data in a major field project provided excellent coverage by cloud vector winds, useful, for example, in the determination of the large-scale mean state (see Krishnamurti and Pasch, Chapter 3, this volume). In addition, it resulted in. the first "Satellite-Derived Precipitation Atlas for GATE" (1980), including satellite-ship radar comparisons.

2.5 SOME LESSONS LEARNED

GATE's unique operational and scientific experience was useful to the planning of later large-scale field projects such as MONEX and ALPEX. It also pro- vided several strategic lessons, of which only three shall be mentioned here:

(1) Advances in technology do not in themselves justify the introduction of new observing systems. -~ 4

(2) The experiment design must be "frozen" sufficiently ahead of the experiment and should be altered only for exceptional reasons.

(3) Research should be started as early as possible and should not wait for "perfect" data. In other words, a comprehensive early "Quick- Look Data Set" is of overriding importance.

.:

Finally, we should all remember that the human factor is more important than technology in international field research of such magnitude and complexity. If human relations between the scientists of many nations are as good as they were in GATE, success is almost assured. Without such relationship the best technology will be useless.

REFERENCES AND OTHER RELEVAKT PIJDLICATIOHS

DUing, W., F. Ostapoff and J. Merle, 1980: "Physical Oceanography of the Tropical .,. * : ).

Atlantic during GATE" (Atlas), University of Miami. : :.* f

Kuettner, J. P. and D. E. Parker, 1976: 'GATE: Report on the Field Phase". Bull. Am. Meteorol. sot. x, 11-27.

"Satellite-Derived Precipitation Atlas for GATE", 1980: U.S. Dept. of Commerce, KOAA.