(see seasonal variations)

rainfall estimates

Warm rain

Warm rain over tropical oceans can be from stratocumulus (Austin et al. 1995), or well-developed trade wind cumuli (Malkus, 1954, 1955; Baker 1993), and usually are in the form of shallow, isolated convective showers (Schumacher and Houze 2003). To study rain initiation, cloud processing and large scale conditions of these warm rain systems, there have been several field campaigns targeting warm rain systems over oceans, such as the field campaign near Barbados, West indies in 1963 (Simpson et al., 1967), Atlantic Trade Wind Experiment (ATEX; Augstein et al. 1972), the Barbados Oceanographic and Meteorological Experiment ( BOMEX; Nitta and Esbensen 1974), The East Pacific Investigation of Climate (EPIC, Bretherton et al., 2004), and recently the Rain In shallow Cumulus over the Ocean experiment (RICO; Rauber et al. 2007). Using 9 years of TRMM observations, some statistics of warm rainfall have been refined (Liu and Zipser, 2009).

Over tropical land, the total amount of rainfall from warm PFs is small. Most warm rainfall occurs over tropical oceans, including the east Pacific ITCZ, eastern part of the South Pacific Convergence Zone (SPCZ), central North Pacific, South Indian Ocean, and several coastal regions, such as the east coasts of Madagascar, Brazil, Costa Rica, Philippines and Caribbean islands (see picture).

Some statistics of mean unconditional rainfall, contribution of raining area and rainfall from the differently defined warm PFs and the pixels with VIRS TB11 > 273 K, and the pixels with TMI 85 GHz PCT > 250 K over 20oS-20oN land and ocean:

 

 

Land

 

 

Ocean

 

 

Rainfall mm/month

rain area
%

Rainfall
%

Rainfall mm/month

rain area
%

Rainfall
%

PFs with min TB11 >273 K

2.2

4.0

2.3

9.5

16.4

10.0

PFs with max storm height < 4.5 km

3.0

6.3

3.0

11.0

20.0

11.6

PFs with min 85 GHz PCT > 250 K

18.3

32.0

18.6

25.9

43.2

27.2

Pixels with VIRS TB11 >273 K

7.4

12.2

7.5

19.2

30.2

20.2

Pixels with TMI 85 GHz PCT > 250 K

58.8

77.5

59.7

61.1

83.4

64.3

Warm rain with large horizontal coverage

Generally, non-drizzle warm rain systems are shallow, isolated and of small size. However, warm precipitation systems with large size do exist over the tropics (Rauber et al. 2007). For example, two precipitation systems were observed from the Tropical Rainfall Measuring Mission (TRMM, Kummerow et al. 1998) satellite, with mesoscale size and near surface reflectivity greater than 40 dBZ, but low radar echo top below the typical freezing level (4.5 km) over the tropics. The minimum infrared brightness temperatures inside these systems are above 273 K.

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