Water balance of extratropical cyclones

Abstract

THE WATER BUDGET EQUATION FOR A VOLUME ENCLOSING AN AREA OF PRECIPITATION MAY BE WR ITTEN

_j (W + L) + R (} T (t )

WHERE w AND L REFER TO THE WATER VAPOR AND LIQUID WATER CONTENTS, R AND E ARE THE RATES OF PRECIPITATION AND EVAPORATION FROM THE UNDERLYING SURFACE, Fw AND FL ARE THE NET HORIZONTAL INFLUXES OF WATER VAPOR AND LIQUID WATER, AND Fp IS THE NET INFLUX OF WATER SUBSTANCE THROUGH THE TOP.

NOW IF (t ) IS AVERAGED OVER THE ENTIRE ATMOSPHERE AND OVER A LONG PERIOD OF TIME, WE HAVE APPROXIMATELY R = E WHERE THE BAR DENOTES MEAN CONDITIONS. THAT IS, ON THE AVERAGE, PRECIPITATION AND EVAPORATION ARE EQUAL (1 .E .1 NO NET TRANSPORT AT THE EARTH•S SURFACE) • HOWEVER , EVAPORATION IS OCCURRING MORE OR LESS CONTINUOUSLY OVER A LARGE PORTION OF THE EARTH'S SURFACE WHEREAS PRECIPITATION IS FALLING ON ONLY A SMALL FRACTION (PERHAPS TWO PER CENT) OF THE TOTAL AREA AT A GIVEN TIME . IF ATTENTION IS RESTRICTED ONLY TO SHORT TIME PERIODS IN AREAS OF PRECIPITATION, E SHOULD BE A SMALL TERM COMPARED TO R • THIS IS SUPPORTED BY CALCULATIONS OF E BY PETTERSSEN, ET AL (196Z) OVER THE NORTH ATLANTI C IN DEVELOPING CYCLONES. MAXIMUM VALUES OF E WERE ONLY ABOUT 0 .75 MM HR-1 WHI CH IS SMALLER THAN TYPICAL VALUES OF R . OVER LAND, EVAPORATION RATES WOULD, OF COURSE, BE EVEN LESS THAN OVER THE SEA .