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T-C. Yeh
,
R. T. Wetherald
, and
S. Manabe

Abstract

This paper describes the results from a set of numerical experiments which stimulate the effect of a large-scale removal of snow cover in middle and high latitudes during the early spring season. This is done through use of a simplified general circulation model with a limited computational domain and idealized geography.

It is found that removal of snow cover reduces the water available to the soil through snowmelt and decreases soil moisture in this region during the following seasons. Furthermore, it also reduces surface albedo in this region and increases absorption of insolation by the ground surface. This, in turn, heats the ground surface and allows more evaporation to occur. However, the change of evaporation is relatively small owing to the low values of surface temperature in high latitudes. Therefore, the negative anomaly of soil moisture induced initially by the removal of snow cover persists for the entire spring and summer seasons.

The removal of snow cover also affects the thermal and dynamical structure of the atmosphere. It is found that the increase of surface temperature extends into the upper troposphere thereby reducing both meridional temperature gradient and zonal wind in high latitudes.

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T-C. Yeh
,
R. T. Wetherald
, and
S. Manabe

Abstract

This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model, with a limited computational domain and idealized geography.

The soil at three latitude bands, namely 30°N–60°N, 0–30°N, and 15°S–15°N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, it the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist moisture anomaly for the 30°N–60°N cast which is located under the middle latitude rainbelt tends to persist longer than the corresponding anomaly for the 0–30°N case.

Although both the 30°N–60°N and 15°S–15°N latitude regions occur under rainbelts, the soil moisture anomaly for the 15°S–15°N case does not persist as long as it does for the 30°N–60°N case. This is because in the 15°S–15°N case, a much greater fraction of the increased precipitation is lost from the hydrologic cycle due to runoff there as compared with the 30°N-60°N case.

The above changes of the hydrological processes also cause corresponding changes of the thermal state of the atmosphere such as a cooling of the surface due to increased evaporation. This results in change of the mean zonal circulation through the thermal wind relationship. It is found that irrigation in the tropical region weakens the upward branch of the Hadley circulation in the vicinity of the tropical rainbelt.

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C-P. Chang
,
T-C. Yeh
, and
J. M. Chen

Abstract

The island of Taiwan is situated in the main path of western North Pacific typhoons. Its dominant central mountain range (CMR), with a hoizontal scale comparable to the radius of a typhoon, often produces significant distortions in the typhoon circulation. A 20-year dataset from 22 surface stations is used to describe the effects of the Taiwan terrain on the surface structure of typhoons.

Empirical orthogonal function analysis on the pressure field is used to identify the primary structure modes. The first mode is a uniform-sign anomaly pattern portraying the decrease in pressure as a typhoon is approaching. The second mode represents the strong terrain-induced west-east pressure gradient that is normal to the main axis of the CMR. The third mode results mainly from the west-cast pressure gradient arising from the relative location of the typhoon center to the east or west of Taiwan, but it also contains a weak south-north pressure gradient that can he attributed to the terrain. A regression technique is then used to determine the surface wind, temperature, relative humidity, and hourly rainfall associated with each pressure mode. In all cases, them fields are consistent, showing the effects of the terrain blocking or deflection and their consequent ascending and descending motions.

The relative importance of each mode depends strongly on the location of the typhoon center. No dependence on the direction or speed of motion is discernible when all cases are considered. When different, persistently smooth tracks are identified, the variations due to motion direction can be recognized because the terrain effect is affected by the mean steering flow. Only two types of smooth tracks that represent clearly different steering flows intersect in an area. At the intersection, a subsequent difference in storm structure over Taiwan exists that can be explained by the difference in the steering flows associated with the two track types.

The leeside secondary low that was often observed on the west coast of Taiwan is found to consist of at least two basic modes. It develops only when the typhoon center is in southeastern Taiwan or an ocean area to the east-southeast. The observed scale of this low is significantly smaller than that which can be produced by an interaction of the mean steering flow and the CMR. This smaller scale is due to a local buildup of the surface pressure south of the lee vortex, which results from the against-mountain return flow of the cyclonic circulation.

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H. Riehl
,
T. C. Yeh
, and
N. E. La seur

Abstract

Changes of the general circulation, especially the strength of the upper westerlies, over 28 months are investigated quantitatively.The role of the high troposphere, represented by the 300-mb level, is stressed and it is sought to determine whether the tropics have any measurable influence on middle-latitude weather.

Computations cover the latitude belt 10°N to 70°N, at least for part of the periods studied. To bring out the fluctuations clearly, they are expressed in terms of deviations from seasonal and mean latitudinal gradients. These deviations amount to over 20 per cent of the mean circulation, even in the belt of strongest westerlies. The concern is not with very small values whose accuracy would be questionable.

When the latitudinal distribution of deviations, especially those of momentum of the westerlies, is plotted against time, definite long-term trends appear. Bands of high and low westerly momentum gradually traverse the latitude circles, sometimes from north to south, but usually from south to north. Momentum maxima and minima hardly ever form in middle latitudes. They approach the jet-stream region from equator or pole and pass across it, continuing on their poleward or equatorward course.

A synoptic example shows the effect of a traveling momentum maximum on the northern hemisphere during several stages of its motion. Coincident weather developments, such as typhoon formations and long-wave motion, are noted.

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L. Peng
,
S.-T. Wang
,
S.-L. Shieh
,
M.-D. Cheng
, and
T.-C. Yeh

Abstract

Surface tracks of some cross-Taiwan tropical cyclones were discontinuous as a result of the blockage of the north-northeast–south-southwest-oriented Central Mountain Range (CMR). This paper tries to identify the variables that may be used to diagnose track continuity in advance. The track records of 131 westbound cross-Taiwan tropical cyclones between 1897 and 2009 are examined. It is found that the track continuity of a westbound cross-Taiwan tropical cyclone depends mostly upon the landfall location (YLF), the approaching direction (ANG), and the maximum wind (VMX) of the cyclone. According to the empirical probability of track continuity estimated from the data, the dependence on YLF, which is nonlinear and remarkably asymmetric with respect to the midpoint of the east coast, may be well approximated by a quadratic function of YLF. The nonlinearity and asymmetry can be interpreted in terms of the length scale of the CMR and the north–south antisymmetry of the cyclonic flow. The estimated dependence of track continuity on cyclone intensity and size may be approximated by a linear function of VMX. The estimated dependence of track continuity on ANG may be approximated by a single term of the modified variable DIR (=|ANG − 110|, where 110 is the direction, in degrees, perpendicular to the CMR’s long axis).

Using the 64 tracks between 1944 and 1996 as the training sample, a logistic regression equation model, built in terms of YLF, YLF square, DIR, and VMX gives an overall accuracy score of 89%. As to the probability estimates of individual tracks, 49 of the 64 tracks have estimated probabilities outside the (0.5 − 0.127, 0.5 + 0.127) RMS error range and are correctly classified. A prediction test using another set of 67 tracks not included in the model-training sample, scores a success rate of 82%. As to the probability predictions for individual tracks, 49 of the 67 tracks have predicted probabilities outside the RMS error range and are correctly predicted. These results confirm the appropriateness of the model and moreover demonstrate that the three parameters, YLF, DIR, and VMX, primarily control the surface track continuity of a westbound tropical cyclone crossing Taiwan.

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L. L. Stowe
,
C. G. Wellemeyer
,
H. Y. M. Yeh
,
T. F. Eck
, and
The Nimbus-7 CLOUD DATA PROCecessing TEAM

Abstract

Data from the Temperature Humidity Infrared Radiometer (THIR) and the Total Ozone Mapping Spectrometer (TOMS), both aboard the Nimbus-7 satellite, are used to determine cloudiness parameters for the globe. The 11.5 μm THIR radiances and the 0.36 μm and 0.38 μm TOMS reflectivities, along with concurrent surface temperature data from the Air Force 3-D nephanalysis, are the primary data sources. They are processed by an algorithm that determines total cloud amount, cloud amount in three altitude categories, cirrus cloud, deep convective cloud, warm cloud, and the radiance of radiation emitted by the clouds. and the underlying surface. The algorithm is of the bispectral threshold type, which yields two independent estimates of total cloud, one from the infrared algorithm and one from the UV reflectivity algorithm. For the daytime observations (local noon at the equator), these two independent estimates are combined to determine a composite estimate, while at night (local midnight at the equator), only the infrared threshold algorithm is used in the estimate. Quantitative validation of total cloud amount was performed by comparing the algorithm results with estimates derived by an analyst interpreting geosynchronous satellite (GOES) images, along with auxiliary meteorological data. It has been concluded that the systematic errors of the Nimbus-7 total cloud amount algorithm relative to the analyst are less than 10%, and that the random errors of daily estimates range between 7% and 16%, day or night. These empirical results are consistent with results from a theoretical sensitivity study. Qualitative validation has also been performed by making comparisons with GOES visible and infrared images for specific days. Results indicate that the TOMS cloud estimates improve the IR algorithm estimates of low cloud amount and provide for the identification of cirrus and deep convective cloud, but cloud amounts over humid tropical regions tend to be overestimated even with the use of TOMS. These results suggest that the spatial and temporal characteristics of daily and monthly averaged global cloud cover, including cirrus acid deep convective cloud types, which are presented in Part II, are generally well represented by the Nimbus-7 dataset, which covers a six-year period from April 1979 to March 1985.

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