TOPIC: CLOUD CLASSIFICATION

Transcription

1 INDIAN INSTITUTE OF TECHNOLOGY, DELHI DEPARTMENT OF ATMOSPHERIC SCIENCE ASL720: Satellite Meteorology and Remote Sensing TERM PAPER TOPIC: CLOUD CLASSIFICATION Group Members: Anil Kumar (2010ME10649) Mayank Choudhary (2010TT10927) Muktesh Jain (2010TT10932) Prateek Kulhari (2010TT10942)

2 INTRODUCTION There are many different types of cloud which can be identified visually in the atmosphere. These were first classified by Lamarck in 1802, and Howard in 1803 published a classification scheme which became the basis for modern cloud classification. The modern classification scheme used by the UK Met Office, with similar schemes used elsewhere, classifies clouds according to the altitude of cloud base, there being three altitude classes: low; mid-level and high. Within each altitude class additional classifications are defined based on four basic types and combinations thereof. These types are Cirrus (meaning hair like), Stratus (meaning layer), Cumulus (meaning pile) and Nimbus (meaning rain producing). Each main classification may be further subdivided to provide a means of identifying the many variations which are observed in the atmosphere. Additional to these main types there are a few other types of cloud including noctilucent, polar stratospheric and orographic clouds. A few examples of some of these are given below. Often several types of cloud will be present at different levels of the atmosphere at the same time. Low Level Cloud - Base is usually below 6500ft. Cumulus Cloud: These clouds usually form at altitudes between 1,000 and 5,000ft, though often temperature rises after formation lead to an increase in cloud base height. These clouds are generally formed by air rising as a result of surface heating and may occasionally produce light showers. Stratus Cloud: Usually forms between the surface and 2,000ft, but cloud base can be up to 4,000ft. Thick stratus can produce considerable precipitation, particularly in hilly or coastal regions, though in some cases this precipitation may be falling from higher clouds such as nimbostratus. While thick stratus will obscure the sun or moon, they are clearly visible through thin stratus.

3 Stratocumulus Cloud: This cloud often occurs at altitudes between 1,000 and 4,000ft, though sometimes may be higher. While not generally producing precipitation these clouds may produce drizzle, particularly in hilly or coastal areas, and may be thick enough to obscure the sun or moon. These clouds consist entirely of liquid drops and are often formed close to the top of the planetary boundary layer. Mid-Level Clouds Base is usually between 6,500ft and 20,000ft with some exceptions. Mid-level clouds typically form at temperatures between 0 and 40C depending on altitude and season, so may consist of warm or super-cooled droplets and ice particles. Altostratus Cloud: Cloud base ranges between 10,000 and 20,000ft. Thicker forms of these clouds often produce continuous light precipitation and hide the sun or moon, though thinner forms show the sun or moon with a ground glass appearance.

4 Altocumulus Cloud: This type of cloud typically occurs between 6,500 and 20,000ft and is generally broken in appearance, though can occasionally produce precipitation and be thick enough to hide the sun or moon. Nimbostratus Cloud: Cloud base ranges from the surface to 10,000ft. These clouds always hide the sun or moon, and normally produce continuous precipitation which is often moderate to heavy

5 High Level Clouds Base is usually above 20,000ft Cirrus Cloud: Base is typically between 20,000 and 40,000ft in the UK, and may be considerably higher in the tropics. Cirrus clouds do not produce precipitation which reaches the ground, though streaks of particles (known as fall streaks) are often observed below these clouds. Various halos and other optical effects may be produced by cirrus cloud. In some cases these clouds are also thick enough to hide the sun. Cirrus clouds typically form at temperatures below -40C and consist entirely of ice particles. Cirrostratus Cloud: A thin high level layer cloud, which often produce halos and through which the outline of the sun is generally visible. These clouds are often the first visible indication of an approaching weather front, and may progress.

6 DEEP CONVECTIVE CLOUDS: In contrast to both of the cloud categories previously discussed are deep convective clouds, typified by cumulonimbus clouds. A cumulonimbus cloud can be many kilometers thick, with a base near the Earth's surface and a top frequently reaching an altitude of 10 km (33,000 feet), and sometimes much higher. Because cumulonimbus cloud tops are high and cold, the energy radiated to outer space is lower than it would be without the cloud (the cloud greenhouse forcing is large). But because they also are very thick, they reflect much of the solar energy back to space (their cloud albedo forcing is also large); hence, with the reduced shortwave radiation to be absorbed, there is essentially no excess radiation to be trapped. As a consequence, overall, the cloud greenhouse and albedo forcing almost balance, and the overall effect of cumulonimbus clouds is neutral-neither warming nor cooling. As is indicated above different types of cloud are formed in different atmospheric conditions temperature humidity dynamics stability. From a microphysical point of view, clouds may be liquid, mixed phase, or ice and different microphysical processes are involved in each case.

7 BASIS FOR DETERMINING CLOUDS The Figure below shows different types of clouds for different ranges of Cloud Optical Depth and Cloud Top Pressure. We can use this graph to match the values of COD and CTP that we have obtained from Giovanni website. Basically, we are trying to analyze the difference between the clouds that in two contrasting regions, say one of heavy rainfall and other that receives small rainfall.

8 OBSERVATIONS: Average COD Value Range COD Jaisalmer (2003) Jaisalmer (2013) Mawsynram (2003) Mawsynram (2013) Jan Low Low Mid Low Feb Low Mid Mid Low Mar Low Mid Mid Low Apr Mid Mid Mid Mid May Mid Low Mid Mid Jun Low Mid Mid Mid Jul Mid Mid Mid Mid Aug Mid Low Mid Mid Sep Mid Mid Mid Mid Oct - - Mid Low Nov Low Low Mid Mid Dec Low Mid Low Mid Average CTP Value Range CTP Jaisalmer (2003) Jaisalmer (2013) Mawsynram (2003) Mawsynram (2013) Jan Mid High High High Feb Mid High High High Mar Low Low High High Apr Low Low Mid High May Mid Mid Low Low Jun Mid Mid Low Low Jul Low Low Low Low Aug Mid Low Low Low Sep High High Mid Low Oct - - Low Low Nov Low Low High Low Dec Low High High Low

9 Cumulus Altocumulus Cirrus Stratocumulus Altostratus Cirrostratus Stratus Nimbostratus Deep Convection Jaisalmer 2003 Jaisalmer 2013 Mawsynram 2003 Mawsynram 2013 Mawsynram 2013 Mawsynram 2003 Jaisalmer 2013 Jaisalmer Cumulus Altocumulus Cirrus Stratocumulus Altostratus Cirrostratus Stratus Nimbostratus Deep Convection

10 JAISALMER JAISALMER 2003 Dec-13 Nov-13 Oct-13 Sep-13 Aug-13 Jul-13 Jun-13 May-13 Apr-13 Mar-13 Feb-13 Jan Relative Cloudiness Cumulus Altocumulus Cirrus Stratocumulus Altostratus Cirrostratus Stratus Nimbostratus Deep Convection JAISALMER 2013 Dec-13 Nov-13 Oct-13 Sep-13 Aug-13 Jul-13 Jun-13 May-13 Apr-13 Mar-13 Feb-13 Jan Relative Cloudiness Cumulus Altocumulus Cirrus Stratocumulus Altostratus Cirrostratus Stratus Nimbostratus Deep Convection

11 MAWSYNRAM MAWSYNRAM Dec-03 Nov-03 Oct-03 Sep-03 Aug-03 Jul-03 Jun-03 May-03 Apr-03 Mar-03 Feb-03 Jan Relative Cloudiness Cumulus Altocumulus Cirrus Stratocumulus Altostratus Cirrostratus Stratus Nimbostratus Deep Convection MAWSYNRAM Axis Title Dec-13 Nov-13 Oct-13 Sep-13 Aug-13 Jul-13 Jun-13 May-13 Apr-13 Mar-13 Feb-13 Jan Relative Cloudiness Cumulus Altocumulus Cirrus Stratocumulus Altostratus Cirrostratus Stratus Nimbostratus Deep Convection

12 RESULTS: JAISALMER Region: Clouds exist mainly during the summer monsoon season only. In 2003, monsoon season was of a longer duration (June September) than in 2013 (late June mid-august) due to prevalence of El-Nino conditions in Moreover, in 2013, we mainly find Cirrostratus clouds (high clouds with medium optical depth). Whereas, in 2003, we found a relatively larger proportion of stratus and deep convective clouds (in monsoon season) as well, which are high optical depth clouds, which bring in more rainfall. MAWSYNRAM Region: Clouds exist during both the summer, as well as the winter monsoon season. In 2013, lesser clouds appeared in the region during the months of Feb, March and Nov (the months between the two monsoon seasons). Whereas, in 2003, due to El-Nino conditions there was no such drop in cloudiness in the region. It can also be seen, that the Deep Convective clouds were found in much more abundance in the El-Nino year, which resulted in much higher rainfall in 2003, compared to 2013, where the rainfall was predominantly due to the Cirrostratus clouds. We can clearly see that, in India, whether it is the desert region or the high rainfall receiving region, we primarily find medium optical depth clouds. Cirrostratus clouds are the most common in both regions; these are high clouds that are brought into the regions due to the monsoon winds. We also observe that we find much lesser amounts of low clouds in the desert region of Jaisalmer, since low clouds are basically formed due to evaporation in that particular region, which is almost negligible in the desert region. Although, in the monsoon season, we do find Stratocumulus clouds which are formed due the evaporation of water received from rainfall itself. The Mawsynram region also has a very good winter monsoon season, where the clouds are brought-in from China and associated regions. Since, Mawsynram is located close to Bay of Bengal and Brahmaputra, sufficient evaporation from these water bodies also facilitates rainfall throughout the year in the region.

13 REFERENCES: [1] [2] en.wikipedia.org/wiki/list_of_cloud_types [3] [4] ASL720 Module