Module 2 Elements of Basic Meteorology and Oceanography 2.1 Understanding weather The atmospheric general circulation and its variance – produced by the embedded movements of moisture laden air masses, constant solar heating, inhomogeneity in the earth’s surface characteristics due to oceans, land, solid water mass distribution (snow and ice pack), forest and deserts – have been thoroughly studied by the scientists and the savants of different epochs who have advanced our knowledge of weather, climate and their changes at various time scales. Clouds, rain and thunderstorms, clear skies, low pressure centres, cyclones in the oceans produce weather conditions that impact our daily life, agriculture, power and industry. In simple terms, weather is a manifestation of changes in air motions and distribution of rains forming from the interaction of water vapour with solar radiation. Rightly, the logo of the India Meteorological Department (IMD) is emblazoned with the Sanskrit Sloka saying “Ädityäya jäyatā vrishtē (Rains emerge from The Sun)”. The humankind has thus always craved for (and successfully accomplished) a deep understanding of the exchange processes (or of the physical laws) that are responsible for such weather events mainly driven by solar heating and dynamical exchanges, which are sometimes devastating to life and property. As explained in the last lecture, the planet earth receives energy from the Sun, and the partition of this energy happens in such a manner that there is a complete balance between the energy received and the energy used up by various terrestrial and atmospheric processes. Moreover, in the budget calculation it is necessary to consider the spherical shape of the planet earth. It is now a common knowledge that accurate weather can be predicted using the physical laws of nature. The understanding of weather thus relates to predicting three-dimensional spatial and temporal changes that result from heating of the atmosphere, moisture distribution and air motions on the global and local scales. Continuous observations from land and space-borne platforms on weather, first by national services; and subsequently, dissemination of such data on the global network, viz., the World Weather Watch (WWW) of the World Meteorological Organization (WMO) have enabled the major forecast centres to produce regularly short-, medium- and long-range reliable weather forecasts from numerical models which use vast computer resources. The “art of weather forecasting” has received appreciation widely in recent years mainly because satellites put in space for constant weather watch produce enormous volumes of data of uniform quality that all go into models to accomplish successful numerical weather predictions. From a statistics of different platform data used in the forecast system of European Centre for Medium- range Weather Forecasts (ECMWF), it is estimated that more than 500,000 daily data points are assimilated in the analysis, which is regularly used in producing an operational 10-day (medium-range) high accuracy useful forecast. The majority of the data used in this prediction are from remote-sensing platforms. Further, the data usage at other major prediction centres is of the same order. We may recall that the term weather refers to a synoptic state of the atmosphere that is experienced by a location on the earth and it is uniquely described (or determined) by temperature, humidity, rainfall, wind speed and cloud cover which continuously evolve from an interplay of meteorological processes. Whereas climate refers to an average state of the atmosphere over longer time periods and it 1 was practically referred to as an entity independent of time. The perception has changed since and the 30-year average fields of meteorological variables are now used to represent long-term conditions and delineation of variations in the climatic averages that are necessary for climate change studies. Shortest time scales are observed in atmospheric variables, and ocean variables show variations on longer time scales ranging from months to thousands of years. However, the events like cyclones in the ocean induce strong air-sea interaction, which results in strong ocean surface cooling arising due to strong cooling and the lifting of the thermocline locally. We shall now describe some of the laws, which led to the understanding of meteorological and oceanographic phenomena. One such law in meteorology was established in 1857, the year of revolt by The Great Indian Sepoy Mangal Pandey. The Dutch meteorologist C.H.D. Buys-Ballot formulated this meteorological law empirically, and it is known after his name. The Buys-Ballot’s law is extensively used in the analysis of weather charts and synoptic forecasting. However, William Ferrel (1854) already derived theoretically the relationship between wind direction and the pressure distribution. Ferrel’s theoretical relationship impeccably distinguishes the laboratory flows from those dominated by earth’s rotation. Consider the case of a fluid flow through a conduit as an example of the laboratory flow. The movement of fluid in the conduit is down the pressure gradient; that is, the directions of velocity and of the negative pressure gradient coincide because the fluid flows in the tube from high pressure to low pressure. But in the geophysical fluid flows dominated by rotation of the earth, the direction of flow is perpendicular to the direction of the pressure gradient. 2.2 Buys-Ballot’s law It describes the relationship between the wind direction and the pressure distribution. It states that if you stand in the direction of wind blowing from your back in the Northern Hemisphere, then the low pressure is located on the left and the high pressure on the right. In the Southern Hemisphere, the low pressure is located on the right and the high pressure on the left while looking downstream. This illustrates that the direction of the wind is perpendicular to pressure gradient force (Fig. 2.1a), which in turn implies that wind blows parallel to the isobars. However, in the equatorial region the law is not applicable due to weak Coriolis effect. In effect, it is true for the free atmosphere (above 1.5 km or 850 hPa), but near to the surface the wind turn towards low pressure (Fig.2.1b) due to frictional effects of the ground. Thus the law modifies near the surface in the following manner. If you stand in Northern Hemisphere with your face looking downstream (i.e. wind is blowing from your back), and turn clockwise by an angle of about 30o, the lower pressure will be to your left and the high pressure on your right; in Southern Hemisphere, if you stand with your back to the wind (i.e. wind is blowing from your back), and turn counter clockwise by about 30o, the lower pressure will be to your right and the high pressure on your left. Since meteorology deals with atmospheric motions, the wind systems over any part of globe could be interpreted in the light of the Buys-Ballot’s Law. This law thus helps in understanding the weather systems and their movement from meteorological (synoptic) charts of different pressure levels. The synoptic charts depict the state of the atmosphere at synoptic hours which are produced daily by weather services of all countries for 00 GMT, 06 GMT, 12 GMT and 18 GMT (the synoptic hours). 2 2.3 Geostrophic flow: In a circular pipe the flow is down the pressure gradient, i.e. fluid flows from high pressure to low pressure. On a rotating planet the Coriolis force (CF) balances the pressure gradient force (PGF). The Coriolis force (apparent force arises due to earth’s rotation) is always directed to the right relative to the direction of the geostrophic flow in the northern hemisphere, and to the left relative to the direction of flow in the southern hemisphere. That is, Coriolis force is always perpendicular to the direction of the geostrophic flow (Fig 2.1) and wind blows parallel to isobar, which is also the essence of the Buys Ballot Law. Coriolis force balancing the pressure gradient force, one may observe in the free atmosphere above 850 hPa (1500 m) and in the interior of the ocean below the Ekman layer of depth 30 m. Indeed there is an Ekman layer at the bottom of the ocean also, which is about 35 m thick. So, between these two Ekman layers in the ocean, currents are geostrophic. Geostrophic currents are therefore frictionless. In oceanography, currents are always expressed with reference to the oceanic floor (rotating with the earth) where current velocity is zero and effect of rotation is accounted for in the balances of forces. (a) Free atmosphere (850 hPa) (b) Surface map p Isobar 0 + p δ p PGF δ p0 p0 Pressure gadient force (PGF) + δ p > 0 0 Vg Low pressure (L) p L δ p > 0 Low pressure (L) L p0 −δ p Isobar p p Isobar l 0 −δ 0V iona 30 ict V Geostrophic wind (V ) Fr ce g g for p 0 −δ CF p0 Isobar p p Isobar p0 p0 p0 High pressure (H) δ − High pressure (H) 0 Coriolis force (CF) p H H Vg Northern Hemisphere Isobar Northern Hemisphere Fig. 2.1 (a) Wind direction at 850 hPa under geostrophic balance: wind blows parallel to isobars; (b) Turning of the wind at surface under the action of frictional force. Note that balance of frictional force, pressure gradient force (PGF) and Coriolis force (CF) happens in such a manner that the Coriolis force is always perpendicular to the direction of flow and always acts on the right to the flow direction in the northern hemisphere. In Northern Hemisphere: Surface map shows winds spiraling clockwise out from a high pressure region (divergence); spiraling counter clockwise into low pressure (convergence). In Southern Hemisphere: the sense of wind is opposite; that is, clockwise rotation of wind in low pressure and counter clockwise rotation of wind in high pressure.